Fermat's Library | Assessing “Dangerous Climate Change”: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature annotated/explained version.

It is worth noting that global fossil fuel emissions [were flat in ...

It is worth showing the updated figure that has data through 2016: ...

A separate estimate calculates that we’ve already burned through ov...

This is sometimes referred to as the “global warming pause,” which ...

CO2 concentrations are now [firmly above 400ppm](https://www.esrl.n...

That reducing coal consumption will result in a slight “bounceback”...

The great barrier reef just suffered a major bleach event, [2 years...

This was a pretty stark finding at the time, and still continues to...

This is a massively important point – one that hasn’t been focused ...

This is a truly staggering figure. Even in the Great recession, glo...

This is a staggering number – far higher than the total national de...

It’s important to note that there are still no commercially viable ...

Most global climate scenarios that model a 2- or sub-2 degree warmi...

As I explained in an above comment, the world might be closer to sl...

As a recent [Vox analysis shows](http://www.vox.com/energy-and-envi...

It is important to note that the Federal subsidy for wind energy pr...

It will not necessarily be clear in the moment if we have hit a par...

Needless to say, this treaty occurred, though most notably it does ...

Review

Assessing ‘‘Dangerous Climate Change’’: Required

Reduction of Carbon Emissions to Protect Young People,

Future Generations and Nature

James Hansen

*, Pushker Kharecha

1,2

, Makiko Sato

, Valerie Masson-Delmotte

, Frank Ackerman

David J. Beerling

, Paul J. Hearty

, Ove Hoegh-Guldberg

, Shi-Ling Hsu

, Camille Parmesan

9,10

Johan Rockstrom

, Eelco J. Rohling

12,13

, Jeffrey Sachs

, Pete Smith

, Konrad Steffen

Lise Van Susteren

, Karina von Schuckmann

, James C. Zachos

1 Earth Institute, Columbia University, New York, New York, United States of America, 2 Goddard Institute for Space Studies, NASA, New York, New York, United States of

America, 3 Institut Pierre Simon Laplace, Laboratoire des Sciences du Climat et de l’Environnement (CEA-CNRS-UVSQ), Gif-sur-Yvette, France, 4 Synapse Energy Economics,

Cambridge, Massachusetts, United States of America, 5 Department of Animal and Plant Sciences, University of Sheffield, Sheffield, South Yorkshire, United Kingdom,

6 Department of Environmental Studies, University of North Carolina, Wilmington, North Carolina, United States of America, 7 Global Change Institute, University of

Queensland, St. Lucia, Queensland, Australia, 8 College of Law, Florida State University, Tallahassee, Florida, United States of America, 9 Marine Institute, Plymouth

University, Plymouth, Devon, United Kingdom, 10 Integrative Biology, University of Texas, Austin, Texas, United States of America, 11 Stockholm Resilience Center,

Stockholm University, Stockholm, Sweden, 12 School of Ocean and Earth Science, University of Southampton, Southampton, Hampshire, United Kingdom, 13 Research

School of Earth Sciences, Australian National University, Canberra, ACT, Australia, 14 University of Aberdeen, Aberdeen, Scotland, United Kingdom, 15 Swiss Federal

Institute of Technology, Swiss Federal Research Institute WSL, Zurich, Switzerland, 16 Center for Health and the Global Environment, Advisory Board, Harvard School of

Public Health, Boston, Massachusetts, United States of America, 17 L’Institut Francais de Recherche pour l’Exploitation de la Mer, Ifremer, Toulon, France, 18 Earth and

Planetary Science, University of California, Santa Cruz, CA, United States of America

Abstract: We assess climate impacts of global warming

using ongoing observations and paleoclimate data. We

use Earth’s measured energy imbalance, paleoclimate

data, and simple representations of the global carbon

cycle and temperature to define emission reductions

needed to stabilize climate and avoid potentially disas-

trous impacts on today’s young people, future genera-

tions, and nature. A cumulative industrial-era limit of

,500 GtC fossil fuel emissions and 100 GtC storage in the

biosphere and soil would keep climate close to the

Holocene range to which humanity and other species are

adapted. Cumulative emissions of ,1000 GtC, sometimes

associated with 2uC global warming, would spur ‘‘slow’’

feedbacks and eventual warming of 3–4uC with disastrous

consequences. Rapid emissions reduction is required to

restore Earth’s energy balance and avoid ocean heat

uptake that would practically guarantee irreversible

effects. Continuation of high fossil fuel emissions, given

current knowledge of the consequences, would be an act

of extraordinary witting intergenerational injustice. Re-

sponsible policymaking requires a rising price on carbon

emissions that would preclude emissions from most

remaining coal and unconventional fossil fuels and phase

down emissions from conventional fossil fuels.

Introduction

Humans are now the main cause of changes of Earth’s

atmospheric composition and thus the drive for future climate

change [1]. The principal climate forcing, defined as an imposed

change of planetary energy balance [1–2], is increasing carbon

dioxide (CO

) from fossil fuel emissions, much of which will

remain in the atmosphere for millennia [1,3]. The climate

response to this forcing and society’s response to climate change

are complicated by the system’s inertia, mainly due to the ocean

and the ice sheets on Greenland and Antarctica together with the

long residence time of fossil fuel carbon in the climate system. The

inertia causes climate to appear to respond slowly to this human-

made forcing, but further long-lasting responses can be locked in.

More than 170 nations have agreed on the need to limit fossil

fuel emissions to avoid dangerous human-made climate change, as

formalized in the 1992 Framework Convention on Climate

Change [6]. However, the stark reality is that global emissions

have accelerated (Fig. 1) and new efforts are underway to

massively expand fossil fuel extraction [7–9] by drilling to

increasing ocean depths and into the Arctic, squeezing oil from

tar sands and tar shale, hydro-fracking to expand extraction of

natural gas, developing exploitation of methane hydrates, and

mining of coal via mountaintop removal and mechanized long-

wall mining. The growth rate of fossil fuel emissions increased

from 1.5%/year during 1980–2000 to 3%/year in 2000–2012,

mainly because of increased coal use [4–5].

The Framework Convention [6] does not define a dangerous

level for global warming or an emissions limit for fossil fuels. The

Citation: Hansen J, Kharecha P, Sato M, Masson-Delmotte V, Ackerman F,

et al. (2013) Assessing ‘‘Dangerous Climate Change’’: Required Reduction of

Carbon Emissions to Protect Young People, Future Generations and Nature. PLoS

ONE 8(12): e81648. doi:10.1371/journal.pone.0081648

Editor: Juan A. An

el, University of Oxford, United Kingdom

Published December 3, 2013

This is an open-access article, free of all copyright, and may be freely reproduced,

distributed, transmitted, modified, built upon, or otherwise used by anyone for

any lawful purpose. The work is made available under the Cre ative Commons CC0

public domain dedication.

Funding: Funding came from: NASA Climate Research Funding, Gifts to

Columbia University from H.F. (‘‘Gerry’’) Lenfest, private philanthropist (no web

site, but see http://en.wikipedia.org/wiki/H._F._Lenfest), Jim Miller, Lee Wasser-

man (Rockefeller Family Fund) (http://www.rffund.org/), Flora Family Foundation

(http://www.florafamily.org/), Jeremy Grantham, ClimateWorks and the Energy

Foundation provided support for Hansen’s Climate Science, Awareness and

Solutions program at Columbia University to complete this research and

publication. The funders had no role in study design, data collection and analysis,

decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.

* E-mail: jimehansen@gmail.com

PLOS ONE | www.plosone.org 1 December 2013 | Volume 8 | Issue 12 | e81648

European Union in 1996 proposed to limit global warming to 2uC

relative to pre-industrial times [10], based partly on evidence that

many ecosystems are at risk with larger climate change. The 2uC

target was reaffirmed in the 2009 ‘‘Copenhagen Accord’’

emerging from the 15th Conference of the Parties of the

Framework Convention [11], with specific language ‘‘We agree

that deep cuts in global emissions are required according to

science, as documented in the IPCC Fourth Assessment Report

with a view to reduce global emissions so as to hold the increase in

global temperature below 2 degrees Celsius…’’.

A global warming target is converted to a fossil fuel emissions

target with the help of global climate-carbon-cycle models, which

reveal that eventual warming depends on cumulative carbon

emissions, not on the temporal history of emissions [12]. The

emission limit depends on climate sensitivity, but central estimates

[12–13], including those in the upcoming Fifth Assessment of the

Intergovernmental Panel on Climate Change [14], are that a 2uC

global warming limit implies a cumulative carbon emissions limit

of the order of 1000 GtC. In comparing carbon emissions, note

that some authors emphasize the sum of fossil fuel and

deforestation carbon. We bookkeep fossil fuel and deforestation

carbon separately, because the larger fossil fuel term is known

more accurately and this carbon stays in the climate system for

hundreds of thousands of years. Thus fossil fuel carbon is the

crucial human input that must be limited. Deforestation carbon is

more uncertain and potentially can be offset on the century time

scale by storage in the biosphere, including the soil, via

reforestation and improved agricultural and forestry practices.

There are sufficient fossil fuel resources to readily supply 1000

GtC, as fossil fuel emissions to date (370 GtC) are only a small

fraction of potential emissions from known reserves and potentially

recoverable resources (Fig. 2). Although there are uncertainties in

reserves and resources, ongoing fossil fuel subsidies and continuing

technological advances ensure that more and more of these fuels

will be economically recoverable. As we will show, Earth’s

paleoclimate record makes it clear that the CO

produced by

burning all or most of these fossil fuels would lead to a very

different planet than the one that humanity knows.

Our evaluation of a fossil fuel emissions limit is not based on

climate models but rather on observational evidence of global

climate change as a function of global temperature and on the fact

that climate stabilization requires long-term planetary energy

balance. We use measured global temperature and Earth’s

measured energy imbalance to determine the atmospheric CO

level required to stabilize climate at today’s global temperature,

which is near the upper end of the global temperature range in the

current interglacial period (the Holocene). We then examine

climate impacts during the past few decades of global warming

and in paleoclimate records including the Eemian period,

concluding that there are already clear indications of undesirable

impacts at the current level of warming and that 2uC warming

would have major deleterious consequences. We use simple

representations of the carbon cycle and global temperature,

consistent with observations, to simulate transient global temper-

ature and assess carbon emission scenarios that could keep global

climate near the Holocene range. Finally, we discuss likely over-

shooting of target emissions, the potential for carbon extraction

from the atmosphere, and implications for energy and economic

policies, as well as intergenerational justice.

Global Temperature and Earth’s Energy Balance

Global temperature and Earth’s energy imbalance provide our

most useful measuring sticks for quantifying global climate change

and the changes of global climate forcings that would be required

to stabilize global climate. Thus we must first quantify knowledge

of these quantities.

Temperature

Temperature change in the past century (Fig. 3; update of figures

in [16]) includes unforced variability and forced climate change.

The long-term global warming trend is predominantly a forced

climate change caused by increased human-made atmospheric

gases, mainly CO

[1]. Increase of ‘‘greenhouse’’ gases such as CO

has little effect on incoming sunlight but makes the atmosphere

more opaque at infrared wavelengths, causing infrared (heat)

radiation to space to emerge from higher, colder levels, which thus

reduces infrared radiation to space. The resulting planetary energy

imbalance, absorbed solar energy exceeding heat emitted to space,

causes Earth to warm. Observations, discussed below, confirm that

Earth is now substantially out of energy balance, so the long-term

warming will continue.

Figure 1. CO

annual emissions from fossil fuel use and cement manufacture, based on data of British Petroleum [4] concatenated

with data of Boden et al. [5]. (A) is log scale and (B) is linear.

doi:10.1371/journal.pone.0081648.g001

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Global temperature appears to have leveled off since 1998 (Fig.

3a). That plateau is partly an illusion due to the 1998 global

temperature spike caused by the El Nin˜o of the century that year.

The 11-year (132-month) running mean temperature (Fig. 3b)

shows only a moderate decline of the warming rate. The 11-year

averaging period minimizes the effect of variability due to the 10–

12 year periodicity of solar irradiance as well as irregular El Nin˜o/

La Nin˜a warming/cooling in the tropical Pacific Ocean. The

current solar cycle has weaker irradiance than the several prior

solar cycles, but the decreased irradiance can only partially

account for the decreased warming rate [17]. Variability of the El

Nin˜o/La Nin˜a cycle, described as a Pacific Decadal Oscillation,

largely accounts for the temporary decrease of warming [18], as

we discuss further below in conjunction with global temperature

simulations.

Assessments of dangerous climate change have focused on

estimating a permissible level of global warming. The Intergov-

ernmental Panel on Climate Change [1,19] summarized broad-

based assessments with a ‘‘burning embers’’ diagram, which

indicated that major problems begin with global warming of 2–

3uC. A probabilistic analysis [20], still partly subjective, found a

median ‘‘dangerous’’ threshold of 2.8uC, with 95% confidence

that the dangerous threshold was 1.5uC or higher. These

assessments were relative to global temperature in year 1990, so

add 0.6uC to these values to obtain the warming relative to 1880–

1920, which is the base period we use in this paper for

preindustrial time. The conclusion that humanity could tolerate

global warming up to a few degrees Celsius meshed with common

sense. After all, people readily tolerate much larger regional and

seasonal climate variations.

Figure 2. Fossil fuel CO

emissions and carbon content (1 ppm atmospheric CO

2.12 GtC). Estimates of reserves (profitable to extract

at current prices) and resources (potentially recoverable with advanced technology and/or at higher prices) are the mean of estimates of Energy

Information Administration (EIA) [7], German Advisory Council (GAC) [8], and Global Energy Assessment (GEA) [9]. GEA [9] suggests the possibility of

.15,000 GtC unconventional gas. Error estimates (vertical lines) are from GEA and probably underestimate the total uncertainty. We convert energy

content to carbon content using emission factors of Table 4.2 of [15] for coal, gas and conventional oil, and, also following [15], emission factor of

unconventional oil is approximated as being the same as for coal. Total emissions through 2012, including gas flaring and cement manufacture, are

384 GtC; fossil fuel emissions alone are ,370 GtC.

doi:10.1371/journal.pone.0081648.g002

Figure 3. Global surface temperature relative to 1880–1920 mean. B shows the 5 and 11 year means. Figures are updates of [16] using data

through August 2013.

doi:10.1371/journal.pone.0081648.g003

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The fallacy of this logic emerged recently as numerous impacts

of ongoing global warming emerged and as paleoclimate

implications for climate sensitivity became apparent. Arctic sea

ice end-of-summer minimum area, although variable from year to

year, has plummeted by more than a third in the past few decades,

at a faster rate than in most models [21], with the sea ice thickness

declining a factor of four faster than simulated in IPCC climate

models [22]. The Greenland and Antarctic ice sheets began to

shed ice at a rate, now several hundred cubic kilometers per year,

which is continuing to accelerate [23–25]. Mountain glaciers are

receding rapidly all around the world [26–29] with effects on

seasonal freshwater availability of major rivers [30–32]. The hot

dry subtropical climate belts have expanded as the troposphere has

warmed and the stratosphere cooled [33–36], contributing to

increases in the area and intensity of drought [37] and wildfires

[38]. The abundance of reef-building corals is decreasing at a rate

of 0.5–2%/year, at least in part due to ocean warming and

possibly ocean acidification caused by rising dissolved CO

[39–

41]. More than half of all wild species have shown significant

changes in where they live and in the timing of major life events

[42–44]. Mega-heatwaves, such as those in Europe in 2003, the

Moscow area in 2010, Texas and Oklahoma in 2011, Greenland

in 2012, and Australia in 2013 have become more widespread

with the increase demonstrably linked to global warming [45–47].

These growing climate impacts, many more rapid than

anticipated and occurring while global warming is less than 1uC,

imply that society should reassess what constitutes a ‘‘dangerous

level’’ of global warming. Earth’s paleoclimate history provides a

valuable tool for that purpose.

Paleoclimate Temperature

Major progress in quantitative understanding of climate change

has occurred recently by use of the combination of data from high

resolution ice cores covering time scales of order several hundred

thousand years [48–49] and ocean cores for time scales of order

one hundred million years [50]. Quantitative insights on global

temperature sensitivity to external forcings [51–52] and sea level

sensitivity to global temperature [52–53] are crucial to our

analyses. Paleoclimate data also provide quantitative information

about how nominally slow feedback processes amplify climate

sensitivity [51–52,54–56], which also is important to our analyses.

Earth’s surface temperature prior to instrumental measurements

is estimated via proxy data. We will refer to the surface

temperature record in Fig. 4 of a recent paper [52]. Global mean

temperature during the Eemian interglacial period (120,000 years

ago) is constrained to be 2uC warmer than our pre-industrial

(1880–1920) level based on several studies of Eemian climate [52].

The concatenation of modern and instrumental records [52] is

based on an estimate that global temperature in the first decade of

the 21st century (+0.8uC relative to 1880–1920) exceeded the

Holocene mean by 0.2560.25uC. That estimate was based in part

on the fact that sea level is now rising 3.2 mm/yr (3.2 m/

millennium) [57], an order of magnitude faster than the rate

during the prior several thousand years, with rapid change of ice

sheet mass balance over the past few decades [23] and Greenland

and Antarctica now losing mass at accelerating rates [23–24]. This

concatenation, which has global temperature 13.9uC in the base

period 1951–1980, has the first decade of the 21st century slightly

(,0.1uC) warmer than the early Holocene maximum. A recent

reconstruction from proxy temperature data [55] concluded that

global temperature declined about 0.7uC between the Holocene

maximum and a pre-industrial minimum before recent warming

brought temperature back near the Holocene maximum, which is

consistent with our analysis.

Climate oscillations evident in Fig. 4 of Hansen et al. [52] were

instigated by perturbations of Earth’s orbit and spin axis tilt

relative to the orbital plane, which alter the geographical and

seasonal distribution of sunlight on Earth [58]. These forcings

change slowly, with periods between 20,000 and 400,000 years,

and thus climate is able to stay in quasi-equilibrium with these

forcings. Slow insolation changes initiated the climate oscillations,

but the mechanisms that caused the climate changes to be so large

were two powerful amplifying feedbacks: the planet’s surface

albedo (its reflectivity, literally its whiteness) and atmospheric CO

amount. As the planet warms, ice and snow melt, causing the

surface to be darker, absorb more sunlight and warm further. As

the ocean and soil become warmer they release CO

and other

greenhouse gases, causing further warming. Together with fast

feedbacks processes, via changes of water vapor, clouds, and the

vertical temperature profile, these slow amplifying feedbacks were

responsible for almost the entire glacial-to-interglacial temperature

change [59–62].

The albedo and CO

feedbacks amplified weak orbital forcings,

the feedbacks necessarily changing slowly over millennia, at the

pace of orbital changes. Today, however, CO

is under the control

of humans as fossil fuel emissions overwhelm natural changes.

Atmospheric CO

has increased rapidly to a level not seen for at

least 3 million years [56,63]. Global warming induced by

increasing CO

will cause ice to melt and hence sea level to rise

as the global volume of ice moves toward the quasi-equilibrium

amount that exists for a given global temperature [53]. As ice

melts and ice area decreases, the albedo feedback will amplify

global warming.

Earth, because of the climate system’s inertia, has not yet fully

responded to human-made changes of atmospheric composition.

The ocean’s thermal inertia, which delays some global warming

for decades and even centuries, is accounted for in global climate

models and its effect is confirmed via measurements of Earth’s

energy balance (see next section). In addition there are slow

climate feedbacks, such as changes of ice sheet size, that occur

mainly over centuries and millennia. Slow feedbacks have little

effect on the immediate planetary energy balance, instead coming

into play in response to temperature change. The slow feedbacks

are difficult to model, but paleoclimate data and observations of

ongoing changes help provide quantification.

Earth’s Energy Imbalance

At a time of climate stability, Earth radiates as much energy to

space as it absorbs from sunlight. Today Earth is out of balance

because increasing atmospheric gases such as CO

reduce Earth’s

heat radiation to space, thus causing an energy imbalance, as there

is less energy going out than coming in. This imbalance causes

Earth to warm and move back toward energy balance. The

warming and restoration of energy balance take time, however,

because of Earth’s thermal inertia, which is due mainly to the

global ocean.

Earth warmed about 0.8uC in the past century. That warming

increased Earth’s radiation to space, thus reducing Earth’s energy

imbalance. The remaining energy imbalance helps us assess how

much additional warming is still ‘‘in the pipeline’’. Of course

increasing CO

is only one of the factors affecting Earth’s energy

balance, even though it is the largest climate forcing. Other

forcings include changes of aerosols, solar irradiance, and Earth’s

surface albedo.

Determination of the state of Earth’s climate therefore requires

measuring the energy imbalance. This is a challenge, because the

imbalance is expected to be only about 1 W/m

or less, so

accuracy approaching 0.1 W/m

is needed. The most promising

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approach is to measure the rate of changing heat content of the

ocean, atmosphere, land, and ice [64]. Measurement of ocean heat

content is the most critical observation, as nearly 90 percent of the

energy surplus is stored in the ocean [64–65].

Observed Energy Imbalance

Nations of the world have launched a cooperative program to

measure changing ocean heat content, distributing more than

3000 Argo floats around the world ocean, with each float

repeatedly diving to a depth of 2 km and back [66]. Ocean

coverage by floats reached 90% by 2005 [66], with the gaps

mainly in sea ice regions, yielding the potential for an accurate

energy balance assessment, provided that several systematic

measurement biases exposed in the past decade are minimized

[67–69].

Argo data reveal that in 2005–2010 the ocean’s upper 2000 m

gained heat at a rate equal to 0.41 W/m

averaged over Earth’s

surface [70]. Smaller contributions to planetary energy imbalance

are from heat gain by the deeper ocean (+0.10 W/m

), energy

used in net melting of ice (+0.05 W/m

), and energy taken up by

warming continents (+0.02 W/m

). Data sources for these

estimates and uncertainties are provided elsewhere [64]. The

resulting net planetary energy imbalance for the six years 2005–

2010 is +0.5860.15 W/m

The positive energy imbalance in 2005–2010 confirms that the

effect of solar variability on climate is much less than the effect of

human-made greenhouse gases. If the sun were the dominant

forcing, the planet would have a negative energy balance in 2005–

2010, when solar irradiance was at its lowest level in the period of

accurate data, i.e., since the 1970s [64,71]. Even though much of

the greenhouse gas forcing has been expended in causing observed

0.8uC global warming, the residual positive forcing overwhelms

the negative solar forcing. The full amplitude of solar cycle forcing

is about 0.25 W/m

[64,71], but the reduction of solar forcing due

to the present weak solar cycle is about half that magnitude as we

illustrate below, so the energy imbalance measured during solar

minimum (0.58 W/m

) suggests an average imbalance over the

solar cycle of about 0.7 W/m

Earth’s measured energy imbalance has been used to infer the

climate forcing by aerosols, with two independent analyses yielding

a forcing in the past decade of about 21.5 W/m

[64,72],

including the direct aerosol forcing and indirect effects via induced

cloud changes. Given this large (negative) aerosol forcing, precise

monitoring of changing aerosols is needed [73]. Public reaction to

increasingly bad air quality in developing regions [74] may lead to

future aerosol reductions, at least on a regional basis. Increase of

Earth’s energy imbalance from reduction of particulate air

pollution, which is needed for the sake of human health, can be

minimized via an emphasis on reducing absorbing black soot [75],

but the potential to constrain the net increase of climate forcing by

focusing on black soot is limited [76].

Energy Imbalance Implications for CO

Target

Earth’s energy imbalance is the most vital number character-

izing the state of Earth’s climate. It informs us about the global

temperature change ‘‘in the pipeline’’ without further change of

climate forcings and it defines how much greenhouse gases must

be reduced to restore Earth’s energy balance, which, at least to a

good approximation, must be the requirement for stabilizing

global climate. The measured energy imbalance accounts for all

natural and human-made climate forcings, including changes of

atmospheric aerosols and Earth’s surface albedo.

If Earth’s mean energy imbalance today is +0.5 W/m

,CO

must be reduced from the current level of 395 ppm (global-mean

annual-mean in mid-2013) to about 360 ppm to increase Earth’s

heat radiation to space by 0.5 W/m

and restore energy balance.

If Earth’s energy imbalance is 0.75 W/m

,CO

must be reduced

to about 345 ppm to restore energy balance [64,75].

The measured energy imbalance indicates that an initial CO

target ‘‘,350 ppm’’ would be appropriate, if the aim is to stabilize

climate without further global warming. That target is consistent

with an earlier analysis [54]. Additional support for that target is

provided by our analyses of ongoing climate change and

paleoclimate, in later parts of our paper. Specification now of a

target more precise than ,350 ppm is difficult and

unnecessary, because of uncertain future changes of forcings

including other gases, aerosols and surface albedo. More precise

assessments will become available during the time that it takes to

turn around CO

growth and approach the initial 350 ppm target.

Below we find the decreasing emissions scenario that would

achieve the 350 ppm target within the present century. Specifically,

we want to know the annual percentage rate at which emissions

must be reduced to reach this target, and the dependence of this rate

upon the date at which reductions are initiated. This approach is

complementary to the approach of estimating cumulative emissions

allowed to achieve a given limit on global warming [12].

Figure 4. Decay of atmospheric CO

perturbations. (A) Instantaneous injection or extraction of CO

with initial conditions at equilibrium. (B)

Fossil fuel emissions terminate at the end of 2015, 2030, or 2050 and land use emissions terminate after 2015 in all three cases, i.e., thereafter there is

no net deforestation.

doi:10.1371/journal.pone.0081648.g004

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If the only human-made climate forcing were changes of

atmospheric CO

, the appropriate CO

target might be close to

the pre-industrial CO

amount [53]. However, there are other

human forcings, including aerosols, the effect of aerosols on

clouds, non-CO

greenhouse gases, and changes of surface albedo

that will not disappear even if fossil fuel burning is phased out.

Aerosol forcings are substantially a result of fossil fuel burning

[1,76], but the net aerosol forcing is a sensitive function of various

aerosol sources [76]. The indirect aerosol effect on clouds is non-

linear [1,76] such that it has been suggested that even the modest

aerosol amounts added by pre-industrial humans to an otherwise

pristine atmosphere may have caused a significant climate forcing

[59]. Thus continued precise monitoring of Earth’s radiation

imbalance is probably the best way to assess and adjust the

appropriate CO

target.

Ironically, future reductions of particulate air pollution may

exacerbate global warming by reducing the cooling effect of

reflective aerosols. However, a concerted effort to reduce non-CO

forcings by methane, tropospheric ozone, other trace gases, and

black soot might counteract the warming from a decline in

reflective aerosols [54,75]. Our calculations below of future global

temperature assume such compensation, as a first approximation.

To the extent that goal is not achieved, adjustments must be made

in the CO

target or future warming may exceed calculated values.

Climate Impacts

Determination of the dangerous level of global warming

inherently is partly subjective, but we must be as quantitative as

possible. Early estimates for dangerous global warming based on

the ‘‘burning embers’’ approach [1,19–20] have been recognized

as probably being too conservative [77]. A target of limiting

warming to 2uC has been widely adopted, as discussed above. We

suspect, however, that this may be a case of inching toward a

better answer. If our suspicion is correct, then that gradual

approach is itself very dangerous, because of the climate system’s

inertia. It will become exceedingly difficult to keep warming below

a target smaller than 2uC, if high emissions continue much longer.

We consider several important climate impacts and use

evidence from current observations to assess the effect of 0.8uC

warming and paleoclimate data for the effect of larger warming,

especially the Eemian period, which had global mean temperature

about +2uC relative to pre-industrial time. Impacts of special

interest are sea level rise and species extermination, because they

are practically irreversible, and others important to humankind.

Sea Level

The prior interglacial period, the Eemian, was at most ,2uC

warmer than 1880–1920 (Fig. 3). Sea level reached heights several

meters above today’s level [78–80], probably with instances of sea

level change of the order of 1 m/century [81–83]. Geologic

shoreline evidence has been interpreted as indicating a rapid sea

level rise of a few meters late in the Eemian to a peak about 9

meters above present, suggesting the possibility that a critical

stability threshold was crossed that caused polar ice sheet collapse

[84–85], although there remains debate within the research

community about this specific history and interpretation. The

large Eemian sea level excursions imply that substantial ice sheet

melting occurred when the world was little warmer than today.

During the early Pliocene, which was only ,3uC warmer than

the Holocene, sea level attained heights as much as 15–25 meters

higher than today [53,86–89]. Such sea level rise suggests that

parts of East Antarctica must be vulnerable to eventual melting

with global temperature increase of a few degrees Celsius. Indeed,

satellite gravity data and radar altimetry reveal that the Totten

Glacier of East Antarctica, which fronts a large ice mass grounded

below sea level, is now losing mass [90].

Greenland ice core data suggest that the Greenland ice sheet

response to Eemian warmth was limited [91], but the fifth IPCC

assessment [14] concludes that Greenland very likely contributed

between 1.4 and 4.3 m to the higher sea level of the Eemian. The

West Antarctic ice sheet is probably more susceptible to rapid

change, because much of it rests on bedrock well below sea level

[92–93]. Thus the entire 3–4 meters of global sea level contained

in that ice sheet may be vulnerable to rapid disintegration,

although arguments for stability of even this marine ice sheet have

been made [94]. However, Earth’s history reveals sea level

changes of as much as a few meters per century, even though the

natural climate forcings changed much more slowly than the

present human-made forcing.

Expected human-caused sea level rise is controversial in part

because predictions focus on sea level at a specific time, 2100. Sea

level on a given date is inherently difficult to predict, as it depends

on how rapidly non-linear ice sheet disintegration begins. Focus on

a single date also encourages people to take the estimated result as

an indication of what humanity faces, thus failing to emphasize

that the likely rate of sea level rise immediately after 2100 will be

much larger than within the 21

century, especially if CO

emissions continue to increase.

Recent estimates of sea level rise by 2100 have been of the order

of 1 m [95–96], which is higher than earlier assessments [26], but

these estimates still in part assume linear relations between

warming and sea level rise. It has been argued [97–98] that

continued business-as-usual CO

emissions are likely to spur a

nonlinear response with multi-meter sea level rise this century.

Greenland and Antarctica have been losing mass at rapidly

increasing rates during the period of accurate satellite data [23];

the data are suggestive of exponential increase, but the records are

too short to be conclusive. The area on Greenland with summer

melt has increased markedly, with 97% of Greenland experiencing

melt in 2012 [99].

The important point is that the uncertainty is not about whether

continued rapid CO

emissions would cause large sea level rise,

submerging global coastlines – it is about how soon the large

changes would begin. The carbon from fossil fuel burning will

remain in and affect the climate system for many millennia,

ensuring that over time sea level rise of many meters will occur –

tens of meters if most of the fossil fuels are burned [53]. That order

of sea level rise would result in the loss of hundreds of historical

coastal cities worldwide with incalculable economic consequences,

create hundreds of millions of global warming refugees from

highly-populated low-lying areas, and thus likely cause major

international conflicts.

Shifting Climate Zone s

Theory and climate models indicate that the tropical overturn-

ing (Hadley) atmospheric circulation expands poleward with

global warming [33]. There is evidence in satellite and radiosonde

data and in observational data for poleward expansion of the

tropical circulation by as much as a few degrees of latitude since

the 1970s [34–35], but natural variability may have contributed to

that expansion [36]. Change in the overturning circulation likely

contributes to expansion of subtropical conditions and increased

aridity in the southern United States [30,100], the Mediterranean

region, South America, southern Africa, Madagascar, and

southern Australia. Increased aridity and temperature contribute

to increased forest fires that burn hotter and are more destructive

[38].

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Despite large year-to-year variability of temperature, decadal

averages reveal isotherms (lines of a given average temperature)

moving poleward at a typical rate of the order of 100 km/decade

in the past three decades [101], although the range shifts for

specific species follow more complex patterns [102]. This rapid

shifting of climate zones far exceeds natural rates of change.

Movement has been in the same direction (poleward, and upward

in elevation) since about 1975. Wild species have responded to

climate change, with three-quarters of marine species shifting their

ranges poleward as much as 1000 km [44,103] and more than half

of terrestrial species shifting ranges poleward as much as 600 km

and upward as much as 400 m [104].

Humans may adapt to shifting climate zones better than many

species. However, political borders can interfere with human

migration, and indigenous ways of life already have been adversely

affected [26]. Impacts are apparent in the Arctic, with melting

tundra, reduced sea ice, and increased shoreline erosion. Effects of

shifting climate zones also may be important for indigenous

Americans who possess specific designated land areas, as well as

other cultures with long-standing traditions in South America,

Africa, Asia and Australia.

Human Extermination of Species

Biodiversity is affected by many agents including overharvest-

ing, introduction of exotic species, land use changes, nitrogen

fertilization, and direct effects of increased atmospheric CO

plant ecophysiology [43]. However, an overriding role of climate

change is exposed by diverse effects of rapid warming on animals,

plants, and insects in the past three decades.

A sudden widespread decline of frogs, with extinction of entire

mountain-restricted species attributed to global warming [105–

106], provided a dramatic awakening. There are multiple causes

of the detailed processes involved in global amphibian declines and

extinctions [107–108], but global warming is a key contributor

and portends a planetary-scale mass extinction in the making

unless action is taken to stabilize climate while also fighting

biodiversity’s other threats [109].

Mountain-restricted and polar-restricted species are particularly

vulnerable. As isotherms move up the mountainside and poleward,

so does the climate zone in which a given species can survive. If

global warming continues unabated, many of these species will be

effectively pushed off the planet. There are already reductions in

the population and health of Arctic species in the southern parts of

the Arctic, Antarctic species in the northern parts of the Antarctic,

and alpine species worldwide [43].

A critical factor for survival of some Arctic species is retention of

all-year sea ice. Continued growth of fossil fuel emissions will cause

loss of all Arctic summer sea ice within several decades. In

contrast, the scenario in Fig. 5A, with global warming peaking just

over 1uC and then declining slowly, should allow summer sea ice

to survive and then gradually increase to levels representative of

recent decades.

The threat to species survival is not limited to mountain and

polar species. Plant and animal distributions reflect the regional

climates to which they are adapted. Although species attempt to

migrate in response to climate change, their paths may be blocked

by human-constructed obstacles or natural barriers such as coast

lines and mountain ranges. As the shift of climate zones [110]

becomes comparable to the range of some species, less mobile

species can be driven to extinction. Because of extensive species

interdependencies, this can lead to mass extinctions.

Rising sea level poses a threat to a large number of uniquely

evolved endemic fauna living on islands in marine-dominated

ecosystems, with those living on low lying islands being especially

vulnerable. Evolutionary history on Bermuda offers numerous

examples of the direct and indirect impact of changing sea level on

evolutionary processes [111–112], with a number of taxa being

extirpated due to habitat changes, greater competition, and island

inundation [113]. Similarly, on Aldahabra Island in the Indian

Ocean, land tortoises were exterminated during sea level high

stands [114]. Vulnerabilities would be magnified by the speed of

human-made climate change and the potentially large sea level

rise [115].

IPCC [26] reviewed studies relevant to estimating eventual

extinctions. They estimate that if global warming exceeds 1.6uC

above preindustrial, 9–31 percent of species will be committed to

extinction. With global warming of 2.9uC, an estimated 21–52

percent of species will be committed to extinction. A compre-

hensive study of biodiversity indicators over the past decade [116]

reveals that, despite some local success in increasing extent of

protected areas, overall indicators of pressures on biodiversity

including that due to climate change are continuing to increase

and indicators of the state of biodiversity are continuing to

decline.

Mass extinctions occurred several times in Earth’s history [117–

118], often in conjunction with rapid climate change. New species

evolved over millions of years, but those time scales are almost

beyond human comprehension. If we drive many species to

extinction we will leave a more desolate, monotonous planet for

our children, grandchildren, and more generations than we can

imagine. We will also undermine ecosystem functions (e.g.,

pollination which is critical for food production) and ecosystem

resilience (when losing keystone species in food chains), as well as

reduce functional diversity (critical for the ability of ecosystems to

respond to shocks and stress) and genetic diversity that plays an

important role for development of new medicines, materials, and

sources of energy.

Coral Reef Ecosystems

Coral reefs are the most biologically diverse marine ecosystem,

often described as the rainforests of the ocean. Over a million

species, most not yet described [119], are estimated to populate

coral reef ecosystems generating crucial ecosystem services for at

least 500 million people in tropical coastal areas. These ecosystems

are highly vulnerable to the combined effects of ocean acidification

and warming.

Acidification arises as the ocean absorbs CO

, producing

carbonic acid [120], thus making the ocean more corrosive to the

calcium carbonate shells (exoskeletons) of many marine organ-

isms. Geochemical records show that ocean pH is already outside

its range of the past several million years [121–122]. Warming

causes coral bleaching, as overheated coral expel symbiotic algae

and become vulnerable to disease and mortality [123]. Coral

bleaching and slowing of coral calcification already are causing

mass mortalities, increased coral disease, and reduced reef

carbonate accretion, thus disrupting coral reef ecosystem health

[40,124].

Local human-made stresses add to the global warming and

acidification effects, all of these driving a contraction of 1–2% per

year in the abundance of reef-building corals [39]. Loss of the

three-dimensional coral reef frameworks has consequences for all

the species that depend on them. Loss of these frameworks also has

consequences for the important roles that coral reefs play in

supporting fisheries and protecting coastlines from wave stress.

Consequences of lost coral reefs can be economically devastating

for many nations, especially in combination with other impacts

such as sea level rise and intensification of storms.

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Climate Extremes

Changes in the frequency and magnitude of climate extremes,

of both moisture and temperature, are affected by climate trends

as well as changing variability. Extremes of the hydrologic cycle

are expected to intensify in a warmer world. A warmer

atmosphere holds more moisture, so precipitation can be heavier

and cause more extreme flooding. Higher temperatures, on the

other hand, increase evaporation and can intensify droughts when

they occur, as can expansion of the subtropics, as discussed above.

Global models for the 21st century find an increased variability of

precipitation minus evaporation [P-E] in most of the world,

especially near the equator and at high latitudes [125]. Some

models also show an intensification of droughts in the Sahel,

driven by increasing greenhouse gases [126].

Observations of ocean salinity patterns for the past 50 years

reveal an intensification of [P-E] patterns as predicted by models,

but at an even faster rate. Precipitation observations over land

show the expected general increase of precipitation poleward of

the subtropics and decrease at lower latitudes [1,26]. An increase

of intense precipitation events has been found on much of the

world’s land area [127–129]. Evidence for widespread drought

intensification is less clear and inherently difficult to confirm with

available data because of the increase of time-integrated precip-

itation at most locations other than the subtropics. Data analyses

have found an increase of drought intensity at many locations

[130–131] The magnitude of change depends on the drought

index employed [132], but soil moisture provides a good means to

separate the effect of shifting seasonal precipitation and confirms

an overall drought intensification [37].

Global warming of ,0.6uC since the 1970s (Fig. 3) has already

caused a notable increase in the occurrence of extreme summer heat

[46]. The likelihood of occurrence or the fractional area covered by

3-standard-deviation hot anomalies, relative to a base period (1951–

1980) that was still within the range of Holocene climate, has

increased by more than a factor of ten. Large areas around Moscow,

the Mediterranean region, the United States and Australia have

experienced such extreme anomalies in the past three years. Heat

waves lasting for weeks have a devastating impact on human health:

the European heat wave of summer 2003 caused over 70,000 excess

deaths [133]. This heat record for Europe was surpassed already in

2010 [134]. The number of extreme heat waves has increased

several-fold due to global warming [45–46,135] and will increase

further if temperatures continue to rise.

Human Health

Impacts of climate change cause widespread harm to human

health, with children often suffering the most. Food shortages,

polluted air, contaminated or scarce supplies of water, an

expanding area of vectors causing infectious diseases, and more

intensely allergenic plants are among the harmful impacts [26].

More extreme weather events cause physical and psychological

harm. World health experts have concluded with ‘‘very high

confidence’’ that climate change already contributes to the global

burden of disease and premature death [26].

IPCC [26] projects the following trends, if global warming

continue to increase, where only trends assigned very high

confidence or high confidence are included: (i) increased

malnutrition and consequent disorders, including those related

to child growth and development, (ii) increased death, disease and

injuries from heat waves, floods, storms, fires and droughts, (iii)

increased cardio-respiratory morbidity and mortality associated

with ground-level ozone. While IPCC also projects fewer deaths

from cold, this positive effect is far outweighed by the negative

ones.

Growing awareness of the consequences of human-caused

climate change triggers anxiety and feelings of helplessness [136–

137]. Children, already susceptible to age-related insecurities, face

additional destabilizing insecurities from questions about how they

will cope with future climate change [138–139]. Exposure to

media ensures that children cannot escape hearing that their

future and that of other species is at stake, and that the window of

opportunity to avoid dramatic climate impacts is closing. The

psychological health of our children is a priority, but denial of the

truth exposes our children to even greater risk.

Health impacts of climate change are in addition to direct

effects of air and water pollution. A clear illustration of direct

effects of fossil fuels on human health was provided by an

inadvertent experiment in China during the 1950–1980 period of

central planning, when free coal for winter heating was provided

to North China but not to the rest of the country. Analysis of the

impact was made [140] using the most comprehensive data file

ever compiled on mortality and air pollution in any developing

country. A principal conclusion was that the 500 million residents

of North China experienced during the 1990s a loss of more than

2.5 billion life years owing to the added air pollution, and an

average reduction in life expectancy of 5.5 years. The degree of air

pollution in China exceeded that in most of the world, yet

Figure 5. Atmospheric CO

if fossil fuel emissions reduced. (A) 6% or 2% annual cut begins in 2013 and 100 GtC reforestation drawdown

occurs in 2031–2080, (B) effect of delaying onset of emission reduction.

doi:10.1371/journal.pone.0081648.g005

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assessments of total health effects must also include other fossil fuel

caused air and water pollutants, as discussed in the following

section on ecology and the environment.

The Text S1 has further discussion of health impacts of climate

change.

Ecology and the Environment

The ecological impact of fossil fuel mining increases as the

largest, easiest to access, resources are depleted [141]. A constant

fossil fuel production rate requires increasing energy input, but

also use of more land, water, and diluents, with the production of

more waste [142]. The increasing ecological and environmental

impact of a given amount of useful fossil fuel energy is a relevant

consideration in assessing alternative energy strategies.

Coal mining has progressively changed from predominantly

underground mining to surface mining [143], including moun-

taintop removal with valley fill, which is now widespread in the

Appalachian ecoregion in the United States. Forest cover and

topsoil are removed, explosives are used to break up rocks to

access coal, and the excess rock is pushed into adjacent valleys,

where it buries existing streams. Burial of headwater streams

causes loss of ecosystems that are important for nutrient cycling

and production of organic matter for downstream food webs

[144]. The surface alterations lead to greater storm runoff [145]

with likely impact on downstream flooding. Water emerging from

valley fills contain toxic solutes that have been linked to declines in

watershed biodiversity [146]. Even with mine-site reclamation

intended to restore pre-mined surface conditions, mine-derived

chemical constituents are found in domestic well water [147].

Reclaimed areas, compared with unmined areas, are found to

have increased soil density with decreased organic and nutrient

content, and with reduced water infiltration rates [148].

Reclaimed areas have been found to produce little if any regrowth

of woody vegetation even after 15 years [149], and, although this

deficiency might be addressed via more effective reclamation

methods, there remains a likely significant loss of carbon storage

[149].

Oil mining has an increasing ecological footprint per unit

delivered energy because of the decreasing size of new fields and

their increased geographical dispersion; transit distances are

greater and wells are deeper, thus requiring more energy input

[145]. Useful quantitative measures of the increasing ecological

impacts are provided by the history of oil development in Alberta,

Canada for production of both conventional oil and tar sands

development. The area of land required per barrel of produced oil

increased by a factor of 12 between 1955 and 2006 [150] leading

to ecosystem fragmentation by roads and pipelines needed to

support the wells [151]. Additional escalation of the mining impact

occurs as conventional oil mining is supplanted by tar sands

development, with mining and land disturbance from the latter

producing land use-related greenhouse gas emissions as much as

23 times greater than conventional oil production per unit area

[152], but with substantial variability and uncertainty [152–153].

Much of the tar sands bitumen is extracted through surface mining

that removes the ‘‘overburden’’ (i.e., boreal forest ecosystems) and

tar sand from large areas to a depth up to 100 m, with ecological

impacts downstream and in the mined area [154]. Although

mined areas are supposed to be reclaimed, as in the case of

mountaintop removal, there is no expectation that the ecological

value of reclaimed areas will be equivalent to predevelopment

condition [141,155]. Landscape changes due to tar sands mining

and reclamation cause a large loss of peatland and stored carbon,

while also significantly reducing carbon sequestration potential

[156]. Lake sediment cores document increased chemical

pollution of ecosystems during the past several decades traceable

to tar sands development [157] and snow and water samples

indicate that recent levels of numerous pollutants exceeded local

and national criteria for protection of aquatic organisms [158].

Gas mining by unconventional means has rapidly expanded in

recent years, without commensurate understanding of the

ecological, environmental and human health consequences

[159]. The predominant approach is hydraulic fracturing (‘‘frack-

ing’’) of deep shale formations via injection of millions of gallons of

water, sand and toxic chemicals under pressure, thus liberating

methane [155,160]. A large fraction of the injected water returns

to the surface as wastewater containing high concentrations of

heavy metals, oils, greases and soluble organic compounds [161].

Management of this wastewater is a major technical challenge,

especially because the polluted waters can continue to backflow

from the wells for many years [161]. Numerous instances of

groundwater and river contamination have been cited [162]. High

levels of methane leakage from fracking have been found [163], as

well as nitrogen oxides and volatile organic compounds [159].

Methane leaks increase the climate impact of shale gas, but

whether the leaks are sufficient to significantly alter the climate

forcing by total natural gas development is uncertain [164].

Overall, environmental and ecologic threats posed by unconven-

tional gas extraction are uncertain because of limited research,

however evidence for groundwater pollution on both local and

river basin scales is a major concern [165].

Today, with cumulative carbon emissions ,370 GtC from all

fossil fuels, we are at a point of severely escalating ecological and

environmental impacts from fossil fuel use and fossil fuel mining,

as is apparent from the mountaintop removal for coal, tar sands

extraction of oil, and fracking for gas. The ecological and

environmental implications of scenarios with carbon emissions of

1000 GtC or greater, as discussed below, would be profound and

should influence considerations of appropriate energy strategies.

Summary: Climate Impacts

Climate impacts accompanying global warming of 2uC or more

would be highly deleterious. Already there are numerous

indications of substantial effects in response to warming of the

past few decades. That warming has brought global temperature

close to if not slightly above the prior range of the Holocene. We

conclude that an appropriate target would be to keep global

temperature at a level within or close to the Holocene range.

Global warming of 2uC would be well outside the Holocene range

and far into the dangerous range.

Transient Climate Change

We must quantitatively relate fossil fuel emissions to global

temperature in order to assess how rapidly fossil fuel emissions

must be phased down to stay under a given temperature limit.

Thus we must deal with both a transient carbon cycle and

transient global climate change.

Global climate fluctuates stochastically and also responds to

natural and human-made climate forcings [1,166]. Forcings,

measured in W/m

averaged over the globe, are imposed

perturbations of Earth’s energy balance caused by changing

forcing agents such as solar irradiance and human-made

greenhouse gases (GHGs). CO

accounts for more than 80% of

the added GHG forcing in the past 15 years [64,167] and, if fossil

fuel emissions continue at a high level, CO

will be the dominant

driver of future global temperature change.

We first define our method of calculating atmospheric CO

as a

function of fossil fuel emissions. We then define our assumptions

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about the potential for drawing down atmospheric CO

via

reforestation and increase of soil carbon, and we define fossil fuel

emission reduction scenarios that we employ in our study. Finally

we describe all forcings employed in our calculations of global

temperature and the method used to simulate global temperature.

Carbon Cycle and Atmospheric CO

The carbon cycle defines the fate of CO

injected into the air by

fossil fuel burning [1,168] as the additional CO

distributes itself

over time among surface carbon reservoirs: the atmosphere,

ocean, soil, and biosphere. We use the dynamic-sink pulse-

response function version of the well-tested Bern carbon cycle

model [169], as described elsewhere [54,170].

Specifically, we solve equations 3–6, 16–17, A.2.2, and A.3 of

Joos et al. [169] using the same parameters and assumptions

therein, except that initial (1850) atmospheric CO

is assumed to

be 285.2 ppm [167]. Historical fossil fuel CO

emissions are from

Boden et al. [5]. This Bern model incorporates non-linear ocean

chemistry feedbacks and CO

fertilization of the terrestrial

biosphere, but it omits climate-carbon feedbacks, e.g., assuming

static global climate and ocean circulation. Therefore our results

should be regarded as conservative, especially for scenarios with

large emissions.

A pulse of CO

injected into the air decays by half in about 25

years as CO

is taken up by the ocean, biosphere and soil, but

nearly one-fifth is still in the atmosphere after 500 years (Fig. 4A).

Eventually, over hundreds of millennia, weathering of rocks will

deposit all of this initial CO

pulse on the ocean floor as carbonate

sediments [168].

Under equilibrium conditions a negative CO

pulse, i.e.,

artificial extraction and storage of some CO

amount, decays at

about the same rate as a positive pulse (Fig. 4A). Thus if it is

decided in the future that CO

must be extracted from the air and

removed from the carbon cycle (e.g., by storing it underground or

in carbonate bricks), the impact on atmospheric CO

amount will

diminish in time. This occurs because carbon is exchanged among

the surface carbon reservoirs as they move toward an equilibrium

distribution, and thus, e.g., CO

out-gassing by the ocean can

offset some of the artificial drawdown. The CO

extraction

required to reach a given target atmospheric CO

level therefore

depends on the prior emission history and target timeframe, but

the amount that must be extracted substantially exceeds the net

reduction of the atmospheric CO

level that will be achieved. We

clarify this matter below by means of specific scenarios for capture

of CO

It is instructive to see how fast atmospheric CO

declines if fossil

fuel emissions are instantly terminated (Fig. 4B). Halting emissions

in 2015 causes CO

to decline to 350 ppm at century’s end (Fig.

4B). A 20 year delay in halting emissions has CO

returning to

350 ppm at about 2300. With a 40 year delay, CO

does not

return to 350 ppm until after 3000. These results show how

difficult it is to get back to 350 ppm if emissions continue to grow

for even a few decades.

These results emphasize the urgency of initiating emissions reduction [171].

As discussed above, keeping global climate close to the Holocene

range requires a long-term atmospheric CO

level of about

350 ppm or less, with other climate forcings similar to today’s

levels. If emissions reduction had begun in 2005, reduction at

3.5%/year would have achieved 350 ppm at 2100. Now the

requirement is at least 6%/year. Delay of emissions reductions

until 2020 requires a reduction rate of 15%/year to achieve

350 ppm in 2100. If we assume only 50 GtC reforestation, and

begin emissions reduction in 2013, the required reduction rate

becomes about 9%/year.

Reforestation and Soil Carbon

Of course fossil fuel emissions will not suddenly terminate.

Nevertheless, it is not impossible to return CO

to 350 ppm this

century. Reforestation and increase of soil carbon can help draw

down atmospheric CO

. Fossil fuels account for ,80% of the CO

increase from preindustrial time, with land use/deforestation

accounting for 20% [1,170,172–173]. Net deforestation to date is

estimated to be 100 GtC (gigatons of carbon) with 650%

uncertainty [172].

Complete restoration of deforested areas is unrealistic, yet 100

GtC carbon drawdown is conceivable because: (1) the human-

enhanced atmospheric CO

level increases carbon uptake by some

vegetation and soils, (2) improved agricultural practices can

convert agriculture from a CO

ource into a CO

sink [174], (3)

biomass-burning power plants with CO

capture and storage can

contribute to CO

drawdown.

Forest and soil storage of 100 GtC is challenging, but has other

benefits. Reforestation has been successful in diverse places [175].

Minimum tillage with biological nutrient recycling, as opposed to

plowing and chemical fertilizers, could sequester 0.4–1.2 GtC/year

[176] while conserving water in soils, building agricultural resilience

to climate change, and increasing productivity especially in

smallholder rain-fed agriculture, thereby reducing expansion of

agriculture into forested ecosystems [177–178]. Net tropical defor-

estation may have decreased in the past decade [179], but because of

extensive deforestation in earlier decades [170,172–173,180–181]

there is a large amount of land suitable for reforestation [182].

Use of bioenergy to draw down CO

should employ feedstocks

from residues, wastes, and dedicated energy crops that do not

compete with food crops, thus avoiding loss of natural ecosystems and

cropland [183–185]. Reforestation competes with agricultural land

use; land needs could decline by reducing use of animal products, as

livestock now consume more than half of all crops [186].

Our reforestation scenarios assume that today’s net deforesta-

tion rate (,1 GtC/year; see [54]) will stay constant until 2020,

then linearly decrease to zero by 2030, followed by sinusoidal 100

GtC biospheric carbon storage over 2031–2080. Alternative

timings do not alter conclusions about the potential to achieve a

given CO

level such as 350 ppm.

Emission Reduction Scenarios

A 6%/year decrease of fossil fuel emissions beginning in 2013,

with 100 GtC reforestation, achieves a CO

decline to 350 ppm

near the end of this century (Fig. 5A). Cumulative fossil fuel

emissions in this scenario are ,129 GtC from 2013 to 2050, with

an additional 14 GtC by 2100. If our assumed land use changes

occur a decade earlier, CO

returns to 350 ppm several years

earlier; however that has negligible effect on the maximum global

temperature calculated below.

Delaying fossil fuel emission cuts until 2020 (with 2%/year

emissions growth in 2012–2020) causes CO

to remain above

350 ppm (with associated impacts on climate) until 2300 (Fig. 5B).

If reductions are delayed until 2030 or 2050, CO

remains above

350 ppm or 400 ppm, respectively, until well after 2500.

We conclude that it is urgent that large, long-term emission

reductions begin soon. Even if a 6%/year reduction rate and 500

GtC are not achieved, it makes a huge difference when reductions

begin. There is no practical justification for why emissions

necessarily must even approach 1000 GtC.

Climate Forcings

Atmospheric CO

and other GHGs have been well-measured

for the past half century, allowing accurate calculation of their

climate forcing. The growth rate of the GHG forcing has declined

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moderately since its peak values in the 1980s, as the growth rate of

and chlorofluorocarbons has slowed [187]. Annual changes

of CO

are highly correlated with the El Nin˜o cycle (Fig. 6). Two

strong La Nin˜ as in the past five years have depressed CO

growth

as well as the global warming rate (Fig. 3). The CO

growth rate

and warming rate can be expected to increase as we move into the

next El Nin˜o, with the CO

growth already reaching 3 ppm/year

in mid-2013 [188]. The CO

climate forcing does not increase as

rapidly as the CO

amount because of partial saturation of CO

absorption bands [75]. The GHG forcing is now increasing at a

rate of almost 0.4 W/m

per decade [187].

Solar irradiance variations are sometimes assumed to be the

most likely natural driver of climate change. Solar irradiance has

been measured from satellites since the late 1970s (Fig. 7). These

data are from a composite of several satellite-measured time series.

Data through 28 February 2003 are from [189] and Physikalisch

Meteorologisches Observatorium Davos, World Radiation Center.

Subsequent update is from University of Colorado Solar Radiation

& Climate Experiment (SORCE). Data sets are concatenated by

matching the means over the first 12 months of SORCE data.

Monthly sunspot numbers (Fig. 7) support the conclusion that the

solar irradiance in the current solar cycle is significantly lower than

in the three preceding solar cycles. Amplification of the direct solar

forcing is conceivable, e.g., through effects on ozone or

atmospheric condensation nuclei, but empirical data place a

factor of two upper limit on the amplification, with the most likely

forcing in the range 100–120% of the directly measured solar

irradiance change [64].

Recent reduced solar irradiance (Fig. 7) may have decreased the

forcing over the past decade by about half of the full amplitude of

measured irradiance variability, thus yielding a negative forcing of,

say, 2 0.12 W/m

. This compares with a decadal increase of the

GHG forcing that is positive and about three times larger in

magnitude. Thus the solar forcing is not negligible and might

partially account for the slowdown in global warming in the past

decade [17]. However, we must (1) compare the solar forcing with

the net of other forcings, which enhances the importance of solar

change, because the net forcing is smaller than the GHG forcing,

and (2) consider forcing changes on longer time scales, which

greatly diminishes the importance of solar change, because solar

variability is mainly oscillatory.

Human-made tropospheric aerosols, which arise largely from

fossil fuel use, cause a substantial negative forcing. As noted above,

two independent analyses [64,72] yield a total (direct plus indirect)

aerosol forcing in the past decade of about 21.5 W/m

, half the

magnitude of the GHG forcing and opposite in sign. That

empirical aerosol forcing assessment for the past decade is

consistent with the climate forcings scenario (Fig. 8) that we use

for the past century in the present and prior studies [64,190].

Supplementary Table S1 specifies the historical forcings and Table

S2 gives several scenarios for future forcings.

Future Climate Forcings

Future global temperature change should depend mainly on

atmospheric CO

, at least if fossil fuel emissions remain high. Thus

to provide the clearest picture of the CO

effect, we approximate

the net future change of human-made non-CO

forcings as zero

and we exclude future changes of natural climate forcings, such as

solar irradiance and volcanic aerosols. Here we discuss possible

effects of these approximations.

Uncertainties in non-CO

forcings concern principally solar,

aerosol and other GHG forcings. Judging from the sunspot

numbers (Fig. 7B and [191]) for the past four centuries, the current

solar cycle is almost as weak as the Dalton Minimum of the late

18th century. Conceivably irradiance could decline further to the

level of the Maunder Minimum of the late 17th century [192–

193]. For our simulation we choose an intermediate path between

recovery to the level before the current solar cycle and decline to a

still lower level. Specifically, we keep solar irradiance fixed at the

reduced level of 2010, which is probably not too far off in either

direction. Irradiance in 2010 is about 0.1 W/m

less than the

mean of the prior three solar cycles, a decrease of forcing that

Figure 6. Annual increase of CO

based on data from the NOAA Earth System Research Laboratory [188]. Prior to 1981 the CO

change

is based on only Mauna Loa, Hawaii. Temperature changes in lower diagram are 12-month running means for the globe and Nin

o3.4 area [16].

doi:10.1371/journal.pone.0081648.g006

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would be restored by the CO

increase within 3–4 years at its

current growth rate. Extensive simulations [17,194] confirm that

the effect of solar variability is small compared with GHGs if CO

emissions continue at a high level. However, solar forcing can

affect the magnitude and detection of near-term warming. Also, if

rapidly declining GHG emissions are achieved, changes of solar

forcing will become relatively more important.

Aerosols present a larger uncertainty. Expectations of decreases

in large source regions such as China [195] may be counteracted

by aerosol increases other places as global population continues to

increase. Our assumption of unchanging human-made aerosols

could be substantially off in either direction. For the sake of

interpreting on-going and future climate change it is highly

desirable to obtain precise monitoring of the global aerosol forcing

[73].

Non-CO

GHG forcing has continued to increase at a slow rate

since 1995 (Fig. 6 in [64]). A desire to constrain climate change

may help reduce emissions of these gases in the future. However, it

will be difficult to prevent or fully offset positive forcing from

increasing N

O, as its largest source is associated with food

production and the world’s population is continuing to rise.

On the other hand, we are also probably underestimating a

negative aerosol forcing, e.g., because we have not included future

volcanic aerosols. Given the absence of large volcanic eruptions in

the past two decades (the last one being Mount Pinatubo in 1991),

multiple volcanic eruptions would cause a cooling tendency [196]

and reduce heat storage in the ocean [197].

Overall, we expect the errors due to our simple approximation

of non-CO

forcings to be partially off-setting. Specifically, we

have likely underestimated a positive forcing by non-CO

GHGs,

while also likely underestimating a negative aerosol forcing.

Figure 7. Solar irradiance and sunspot number in the era of satellite data (see text). Left scale is the energy passing through an area

perpendicular to Sun-Earth line. Averaged over Earth’s surface the absorbed solar energy is ,240 W/m

, so the full amplitude of measured solar

variability is ,0.25 W/m

doi:10.1371/journal.pone.0081648.g007

Figure 8. Climate forcings employed in our six main scenarios. Forcings through 2010 are as in [64].

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Note that uncertainty in forcings is partly obviated via the focus

on Earth’s energy imbalance in our analysis. The planet’s energy

imbalance is an integrative quantity that is especially useful for a

case in which some of the forcings are uncertain or unmeasured.

Earth’s measured energy imbalance includes the effects of all

forcings, whether they are measured or not.

Simulations of Futu re Global Temperature

We calculate global temperature change for a given CO

scenario using a climate response function (Table S3) that

accurately replicates results from a global climate model with

sensitivity 3uC for doubled CO

[64]. A best estimate of climate

sensitivity close to 3uC for doubled CO

has been inferred from

paleoclimate data [51–52]. This empirical climate sensitivity is

generally consistent with that of global climate models [1], but the

empirical approach makes the inferred high sensitivity more

certain and the quantitative evaluation more precise. Because this

climate sensitivity is derived from empirical data on how Earth

responded to past changes of boundary conditions, including

atmospheric composition, our conclusions about limits on fossil

fuel emissions can be regarded as largely independent of climate

models.

The detailed temporal and geographical response of the climate

system to the rapid human-made change of climate forcings is not

well-constrained by empirical data, because there is no faithful

paleoclimate analog. Thus climate models necessarily play an

important role in assessing practical implications of climate

change. Nevertheless, it is possible to draw important conclusions

with transparent computations. A simple response function

(Green’s function) calculation [64] yields an estimate of global

mean temperature change in response to a specified time series for

global climate forcing. This approach accounts for the delayed

response of the climate system caused by the large thermal inertia

of the ocean, yielding a global mean temporal response in close

accord with that obtained from global climate models.

Tables S1 and S2 in Supporting Information give the forcings

we employ and Table S3 gives the climate response function for

our Green’s function calculation, defined by equation 2 of [64].

The Green’s function is driven by the net forcing, which, with the

response function, is sufficient information for our results to be

reproduced. However, we also include the individual forcings in

Table S1, in case researchers wish to replace specific forcings or

use them for other purposes.

Simulated global temperature (Fig. 9) is for CO

scenarios of

Fig. 5. Peak global warming is ,1.1uC, declining to less than 1uC

by mid-century, if CO

emissions are reduced 6%/year beginning

in 2013. In contrast, warming reaches 1.5uC and stays above 1uC

until after 2400 if emissions continue to increase until 2030, even

though fossil fuel emissions are phased out rapidly (5%/year) after

2030 and 100 GtC reforestation occurs during 2030–2080. If

emissions continue to increase until 2050, simulated warming

exceeds 2uC well into the 22

century.

Increased global temperature persists for many centuries after

the climate forcing declines, because of the thermal inertia of the

ocean [198]. Some temperature reduction is possible if the climate

forcing is reduced rapidly, before heat has penetrated into the

deeper ocean. Cooling by a few tenths of a degree in Fig. 9 is a

result mainly of the 100 GtC biospheric uptake of CO

during

2030–2080. Note the longevity of the warming, especially if

emissions reduction is as slow as 2%/year, which might be

considered to be a rapid rate of reduction.

The temporal response of the real world to the human-made

climate forcing could be more complex than suggested by a simple

response function calculation, especially if rapid emissions growth

continues, yielding an unprecedented climate forcing scenario. For

example, if ice sheet mass loss becomes rapid, it is conceivable that

the cold fresh water added to the ocean could cause regional

surface cooling [199], perhaps even at a point when sea level rise

has only reached a level of the order of a meter [200]. However,

any uncertainty in the surface thermal response this century due to

such phenomena has little effect on our estimate of the dangerous

level of emissions. The long lifetime of the fossil fuel carbon in the

climate system and the persistence of ocean warming for millennia

[201] provide sufficient time for the climate system to achieve full

response to the fast feedback processes included in the 3uC climate

sensitivity.

Indeed, the long lifetime of fossil fuel carbon in the climate

system and persistence of the ocean warming ensure that ‘‘slow’’

feedbacks, such as ice sheet disintegration, changes of the global

vegetation distribution, melting of permafrost, and possible release

of methane from methane hydrates on continental shelves, would

also have time to come into play. Given the unprecedented

rapidity of the human-made climate forcing, it is difficult to

establish how soon slow feedbacks will become important, but

clearly slow feedbacks should be considered in assessing the

‘‘dangerous’’ level of global warming, as discussed in the next

section.

Danger of Initiating Uncontrollable Climate

Change

Our calculated global warming as a function of CO

amount is

based on equilibrium climate sensitivity 3uC for doubled CO

That is the central climate sensitivity estimate from climate models

[1], and it is consistent with climate sensitivity inferred from

Earth’s climate history [51–52]. However, this climate sensitivity

includes only the effects of fast feedbacks of the climate system,

such as water vapor, clouds, aerosols, and sea ice. Slow feedbacks,

such as change of ice sheet area and climate-driven changes of

greenhouse gases, are not included.

Slow Climate Feedbacks and Irreversible Climate Change

Excluding slow feedbacks was appropriate for simulations of the

past century, because we know the ice sheets were stable then and

our climate simulations used observed greenhouse gas amounts

that included any contribution from slow feedbacks. However, we

must include slow feedbacks in projections of warming for the 21

century and beyond. Slow feedbacks are important because they

affect climate sensitivity and because their instigation is related to

the danger of passing ‘‘points of no return’’, beyond which

irreversible consequences become inevitable, out of humanity’s

control.

Antarctic and Greenland ice sheets present the danger of

change with consequences that are irreversible on time scales

important to society [1]. These ice sheets required millennia to

grow to their present sizes. If ice sheet disintegration reaches a

point such that the dynamics and momentum of the process take

over, at that point reducing greenhouse gases may be unable to

prevent major ice sheet mass loss, sea level rise of many meters,

and worldwide loss of coastal cities – a consequence that is

irreversible for practical purposes. Interactions between the ocean

and ice sheets are particularly important in determining ice sheet

changes, as a warming ocean can melt the ice shelves, the tongues

of ice that extend from the ice sheets into the ocean and buttress

the large land-based ice sheets [92,202–203]. Paleoclimate data for

sea level change indicate that sea level changed at rates of the

order of a meter per century [81–83], even at times when the

forcings driving climate change were far weaker than the human-

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made forcing. Thus, because ocean warming is persistent for

centuries, there is a danger that large irreversible change could be

initiated by excessive ocean warming.

Paleoclimate data are not as helpful for defining the likely rate of

sea level rise in coming decades, because there is no known case of

growth of a positive (warming) climate forcing as rapid as the

anthropogenic change. The potential for unstable ice sheet

disintegration is controversial, with opinion varying from likely

stability of even the (marine) West Antarctic ice sheet [94] to likely

rapid non-linear response extending up to multi-meter sea level

rise [97–98]. Data for the modern rate of annual ice sheet mass

changes indicate an accelerating rate of mass loss consistent with a

mass loss doubling time of a decade or less (Fig. 10). However, we

do not know the functional form of ice sheet response to a large

persistent climate forcing. Longer records are needed for empirical

assessment of this ostensibly nonlinear behavior.

Greenhouse gas amounts in the atmosphere, most importantly

and CH

, change in response to climate change, i.e., as a

feedback, in addition to the immediate gas changes from human-

caused emissions. As the ocean warms, for example, it releases

to the atmosphere, with one principal mechanism being the

simple fact that the solubility of CO

decreases as the water

temperature rises [204]. We also include in the category of slow

feedbacks the global warming spikes, or ‘‘hyperthermals’’, that

have occurred a number of times in Earth’s history during the

course of slower global warming trends. The mechanisms behind

these hyperthermals are poorly understood, as discussed below,

but they are characterized by the injection into the surface climate

system of a large amount of carbon in the form of CH

and/or

on the time scale of a millennium [205–207]. The average

rate of injection of carbon into the climate system during these

hyperthermals was slower than the present human-made injection

of fossil fuel carbon, yet it was faster than the time scale for

removal of carbon from the surface reservoirs via the weathering

process [3,208], which is tens to hundreds of thousands of years.

Methane hydrates – methane molecules trapped in frozen water

molecule cages in tundra and on continental shelves – and organic

matter such as peat locked in frozen soils (permafrost) are likely

mechanisms in the past hyperthermals, and they provide another

climate feedback with the potential to amplify global warming if

large scale thawing occurs [209–210]. Paleoclimate data reveal

instances of rapid global warming, as much as 5–6uC, as a sudden

additional warming spike during a longer period of gradual

warming [see Text S1]. The candidates for the carbon injected

into the climate system during those warmings are methane

hydrates on continental shelves destabilized by sea floor warming

[211] and carbon released from frozen soils [212]. As for the

present, there are reports of methane release from thawing

permafrost on land [213] and from sea-bed methane hydrate

deposits [214], but amounts so far are small and the data are

snapshots that do not prove that there is as yet a temporal increase

of emissions.

Figure 9. Simulated global temperature relative to 1880–1920 mean for CO

scenarios of Figure 5.

doi:10.1371/journal.pone.0081648.g009

Figure 10. Annual Greenland and West Antarctic ice mass changes as estimated via alternative methods. Data were read from Figure 4

of Shepherd et al. [23] and averaged over the available records.

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There is a possibility of rapid methane hydrate or permafrost

emissions in response to warming, but that risk is largely

unquantified [215]. The time needed to destabilize large methane

hydrate deposits in deep sediments is likely millennia [215].

Smaller but still large methane hydrate amounts below shallow

waters as in the Arctic Ocean are more vulnerable; the methane

may oxidize to CO

in the water, but it will still add to the long-

term burden of CO

in the carbon cycle. Terrestrial permafrost

emissions of CH

and CO

likely can occur on a time scale of a

few decades to several centuries if global warming continues [215].

These time scales are within the lifetime of anthropogenic CO

and thus these feedbacks must be considered in estimating the

dangerous level of global warming. Because human-made

warming is more rapid than natural long-term warmings in the

past, there is concern that methane hydrate or peat feedbacks

could be more rapid than the feedbacks that exist in the

paleoclimate record.

Climate model studies and empirical analyses of paleoclimate

data can provide estimates of the amplification of climate

sensitivity caused by slow feedbacks, excluding the singular

mechanisms that caused the hyperthermal events. Model studies

for climate change between the Holocene and the Pliocene, when

Earth was about 3uC warmer, find that slow feedbacks due to

changes of ice sheets and vegetation cover amplified the fast

feedback climate response by 30–50% [216]. These same slow

feedbacks are estimated to amplify climate sensitivity by almost a

factor of two for the climate change between the Holocene and the

nearly ice-free climate state that existed 35 million years ago [54].

Implication for Carbon Emissions Target

Evidence presented under Climate Impacts above makes clear

that 2uC global warming would have consequences that can be

described as disastrous. Multiple studies [12,198,201] show that

the warming would be very long lasting. The paleoclimate record

and changes underway in the Arctic and on the Greenland and

Antarctic ice sheets with only today’s warming imply that sea level

rise of several meters could be expected. Increased climate

extremes, already apparent at 0.8uC warming [46], would be

more severe. Coral reefs and associated species, already stressed

with current conditions [40], would be decimated by increased

acidification, temperature and sea level rise. More generally,

humanity and nature, the modern world as we know it, is adapted

to the Holocene climate that has existed more than 10,000 years.

Warming of 1uC relative to 1880–1920 keeps global temperature

close to the Holocene range, but warming of 2uC, to at least the

Eemian level, could cause major dislocations for civilization.

However, distinctions between pathways aimed at ,1uC and

2uC warming are much greater and more fundamental than the

numbers 1uC and 2uC themselves might suggest. These funda-

mental distinctions make scenarios with 2uC or more global

warming far more dangerous; so dangerous, we suggest, that

aiming for the 2uC pathway would be foolhardy.

First, most climate simulations, including ours above and those

of IPCC [1], do not include slow feedbacks such as reduction of ice

sheet size with global warming or release of greenhouse gases from

thawing tundra. These exclusions are reasonable for a ,1uC

scenario, because global temperature barely rises out of the

Holocene range and then begins to subside. In contrast, global

warming of 2uC or more is likely to bring slow feedbacks into play.

Indeed, it is slow feedbacks that cause long-term climate sensitivity

to be high in the empirical paleoclimate record [51–52]. The

lifetime of fossil fuel CO

in the climate system is so long that it

must be assumed that these slow feedbacks will occur if

temperature rises well above the Holocene range.

Second, scenarios with 2uC or more warming necessarily imply

expansion of fossil fuels into sources that are harder to get at,

requiring greater energy using extraction techniques that are

increasingly invasive, destructive and polluting. Fossil fuel

emissions through 2012 total ,370 GtC (Fig. 2). If subsequent

emissions decrease 6%/year, additional emissions are ,130 GtC,

for a total ,500 GtC fossil fuel emissions. This 130 GtC can be

obtained mainly from the easily extracted conventional oil and gas

reserves (Fig. 2), with coal use rapidly phased out and unconven-

tional fossil fuels left in the ground. In contrast, 2uC scenarios have

total emissions of the order of 1000 GtC. The required additional

fossil fuels will involve exploitation of tar sands, tar shale,

hydrofracking for oil and gas, coal mining, drilling in the Arctic,

Amazon, deep ocean, and other remote regions, and possibly

exploitation of methane hydrates. Thus 2uC scenarios result in

more CO

per unit useable energy, release of substantial CH

via

the mining process and gas transportation, and release of CO

and

other gases via destruction of forest ‘‘overburden’’ to extract

subterranean fossil fuels.

Third, with our ,1uC scenario it is more likely that the

biosphere and soil will be able to sequester a substantial portion of

the anthropogenic fossil fuel CO

carbon than in the case of 2uC

or more global warming. Empirical data for the CO

‘‘airborne

fraction’’, the ratio of observed atmospheric CO

increase divided

by fossil fuel CO

emissions, show that almost half of the emissions

is being taken up by surface (terrestrial and ocean) carbon

reservoirs [187], despite a substantial but poorly measured

contribution of anthropogenic land use (deforestation and

agriculture) to airborne CO

[179,216]. Indeed, uptake of CO

by surface reservoirs has at least kept pace with the rapid growth of

emissions [187]. Increased uptake in the past decade may be a

consequence of a reduced rate of deforestation [217] and

fertilization of the biosphere by atmospheric CO

and nitrogen

deposition [187]. With the stable climate of the ,1uC scenario it is

plausible that major efforts in reforestation and improved

agricultural practices [15,173,175–177], with appropriate support

provided to developing countries, could take up an amount of

carbon comparable to the 100 GtC in our ,1uC scenario. On the

other hand, with warming of 2uC or more, carbon cycle feedbacks

are expected to lead to substantial additional atmospheric CO

[218–219], perhaps even making the Amazon rainforest a source

of CO

[219–220].

Fourth, a scenario that slows and then reverses global warming

makes it possible to reduce other greenhouse gases by reducing

their sources [75,221]. The most important of these gases is CH

whose reduction in turn reduces tropospheric O

and stratospheric

O. In contrast, chemistry modeling and paleoclimate records

[222] show that trace gases increase with global warming, making

it unlikely that overall atmospheric CH

will decrease even if a

decrease is achieved in anthropogenic CH

sources. Reduction of

the amount of atmospheric CH

and related gases is needed to

counterbalance expected forcing from increasing N

O and

decreasing sulfate aerosols.

Now let us compare the 1uC (500 GtC fossil fuel emissions) and

the 2uC (1000 GtC fossil fuel emissions) scenarios. Global

temperature in 2100 would be close to 1uC in the 500 GtC

scenario, and it is less than 1uC if 100 GtC uptake of carbon by the

biosphere and soil is achieved via improved agricultural and

forestry practices (Fig. 9). In contrast, the 1000 GtC scenario,

although nominally designed to yield a fast-feedback climate

response of , 2uC, would yield a larger eventual warming because

of slow feedbacks, probably at least 3uC.

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Danger of Uncontrollable Consequences

Inertia of the climate system reduces the near-term impact of

human-made climate forcings, but that inertia is not necessarily

our friend. One implication of the inertia is that climate impacts

‘‘in the pipeline’’ may be much greater than the impacts that we

presently observe. Slow climate feedbacks add further danger of

climate change running out of humanity’s control. The response

time of these slow feedbacks is uncertain, but there is evidence that

some of these feedbacks already are underway, at least to a minor

degree. Paleoclimate data show that on century and millennial

time scales the slow feedbacks are predominately amplifying

feedbacks.

The inertia of energy system infrastructure, i.e., the time

required to replace fossil fuel energy systems, will make it

exceedingly difficult to avoid a level of atmospheric CO

that

would eventually have highly undesirable consequences. The

danger of uncontrollable and irreversible consequences necessarily

raises the question of whether it is feasible to extract CO

from the

atmosphere on a large enough scale to affect climate change.

Carbon Extraction

We have shown that extraordinarily rapid emission reductions

are needed to stay close to the 1uC scenario. In absence of

extraordinary actions, it is likely that growing climate disruptions

will lead to a surge of interest in ‘‘geo-engineering’’ designed to

minimize human-made climate change [223]. Such efforts must

remove atmospheric CO

, if they are to address direct CO

effects

such as ocean acidification as well as climate change. Schemes

such as adding sulfuric acid aerosols to the stratosphere to reflect

sunlight [224], an attempt to mask one pollutant with another, is a

temporary band-aid for a problem that will last for millennia;

besides it fails to address ocean acidification and may have other

unintended consequences [225].

Potential for Carbon Extraction

At present there are no proven technologies capable of large-

scale air capture of CO

. It has been suggested that, with strong

research and development support and industrial scale pilot

projects sustained over decades, costs as low as ,

$500/tC may be

achievable [226]. Thermodynamic constraints [227] suggest that

this cost estimate may be low. An assessment by the American

Physical Society [228] argues that the lowest currently achievable

cost, using existing approaches, is much greater (

$600/tCO

$2200/tC).

The cost of capturing 50 ppm of CO

,at$500/tC (,$135/

tCO

), is , $50 trillion (1 ppm CO

is ,2.12 GtC), but more than

$200 trillion for the price estimate of the American Physical

Society study. Moreover, the resulting atmospheric CO

reduction

will ultimately be less than 50 ppm for the reasons discussed

above. For example, let us consider the scenario of Fig. 5B in

which emissions continue to increase until 2030 before decreasing

at 5%/year – this scenario yields atmospheric CO

of 410 ppm in

2100. Using our carbon cycle model we calculate that if we extract

100 ppm of CO

from the air over the period 2030–2100

(10/7 ppm per year), say storing that CO

in carbonate bricks, the

atmospheric CO

amount in 2100 will be reduced 52 ppm to

358 ppm, i.e., the reduction of airborne CO

is about half of the

amount extracted from the air and stored. The estimated cost of

this 52 ppm CO

reduction is $100–400 trillion.

The cost of CO

capture and storage conceivably may decline

in the future. Yet the practicality of carrying out such a program

with alacrity in response to a climate emergency is dubious. Thus

it may be appropriate to add a CO

removal cost to the current

price of fossil fuels, which would both reduce ongoing emissions

and provide resources for future cleanup.

Responsibility for Carbon Extraction

We focus on fossil fuel carbon, because of its long lifetime in the

carbon cycle. Reversing the effects of deforestation is also

important and there will need to be incentives to achieve increased

carbon storage in the biosphere and soil, but the crucial

requirement now is to limit the amount of fossil fuel carbon in

the air.

The high cost of carbon extraction naturally raises the question

of responsibility for excess fossil fuel CO

in the air. China has the

largest CO

emissions today (Fig. 11A), but the global warming

effect is closely proportional to cumulative emissions [190]. The

United States is responsible for about one-quarter of cumulative

emissions, with China next at about 10% (Fig. 11B). Cumulative

responsibilities change rather slowly (compare Fig. 10 of 190).

Estimated per capita emissions (Fig. 12) are based on population

estimates for 2009–2011.

Various formulae might be devised to assign costs of CO

air

capture, should removal prove essential for maintaining acceptable

climate. For the sake of estimating the potential cost, let us assume

that it proves necessary to extract 100 ppm of CO

(yielding a

reduction of airborne CO

of about 50 ppm) and let us assign each

country the responsibility to clean up its fraction of cumulative

emissions. Assuming a cost of

$500/tC (,$135/tCO

) yields a cost

$28 trillion for the United States, about $90,000 per individual.

Costs would be slightly higher for a UK citizen, but less for other

nations (Fig. 12B).

Cost of CO

capture might decline, but the cost estimate used is

more than a factor of four smaller than estimated by the American

Physical Society [228] and 50 ppm is only a moderate reduction.

The cost should also include safe permanent disposal of the

captured CO

, which is a substantial mass. For the sake of scaling

the task, note that one GtC, made into carbonate bricks, would

produce the volume of ,3000 Empire State buildings or ,1200

Great Pyramids of Giza. Thus the 26 ppm assigned to the United

States, if made into carbonate bricks, would be equivalent to the

stone in 165,000 Empire State buildings or 66,000 Great Pyramids

of Giza. This is not intended as a practical suggestion: carbonate

bricks are not a good building material, and the transport and

construction costs would be additional.

The point of this graphic detail is to make clear the magnitude

of the cleanup task and potential costs, if fossil fuel emissions

continue unabated. More useful and economic ways of removing

may be devised with the incentive of a sufficient carbon price.

For example, a stream of pure CO

becomes available for capture

and storage if biomass is used as the fuel for power plants or as

feedstock for production of liquid hydrocarbon fuels. Such clean

energy schemes and improved agricultural and forestry practices

are likely to be more economic than direct air capture of CO

, but

they must be carefully designed to minimize undesirable impacts

and the amount of CO

that can be extracted on the time scale of

decades will be limited, thus emphasizing the need to limit the

magnitude of the cleanup task.

Policy Implications

Human-made climate change concerns physical sciences, but

leads to implications for policy and politics. Conclusions from the

physical sciences, such as the rapidity with which emissions must

be reduced to avoid obviously unacceptable consequences and the

long lag between emissions and consequences, lead to implications

in social sciences, including economics, law and ethics. Intergov-

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ernmental climate assessments [1,14] purposely are not policy

prescriptive. Yet there is also merit in analysis and discussion of the

full topic through the objective lens of science, i.e., ‘‘connecting the

dots’’ all the way to policy implications.

Energy and Carbon Pathways: A Fork in the Road

The industrial revolution began with wood being replaced by

coal as the primary energy source. Coal provided more

concentrated energy, and thus was more mobile and effective.

We show data for the United States (Fig. 13) because of the

availability of a long data record that includes wood [229]. More

limited global records yield a similar picture [Fig. 14], the largest

difference being global coal now at ,30% compared with ,20%

in the United States. Economic progress and wealth generation

were further spurred in the twentieth century by expansion into

liquid and gaseous fossil fuels, oil and gas being transported and

burned more readily than coal. Only in the latter part of the

twentieth century did it become clear that long-lived combustion

products from fossil fuels posed a global climate threat, as formally

acknowledged in the 1992 Framework Convention on Climate

Change [6]. However, efforts to slow emissions of the principal

atmospheric gas driving climate change, CO

, have been

ineffectual so far (Fig. 1).

Consequently, at present, as the most easily extracted oil and

gas reserves are being depleted, we stand at a fork in the road to

our energy and carbon future. Will we now feed our energy needs

by pursuing difficult to extract fossil fuels, or will we pursue energy

policies that phase out carbon emissions, moving on to the post

fossil fuel era as rapidly as practical?

This is not the first fork encountered. Most nations agreed to the

Framework Convention on Climate Change in 1992 [6]. Imagine

if a bloc of countries favoring action had agreed on a common

gradually rising carbon fee collected within each of country at

domestic mines and ports of entry. Such nations might place

equivalent border duties on products from nations not having a

carbon fee and they could rebate fees to their domestic industry for

export products to nations without an equivalent carbon fee. The

legality of such a border tax adjustment under international trade

law is untested, but is considered to be plausibly consistent with

trade principles [230]. As the carbon fee gradually rose and as

additional nations, for their own benefit, joined this bloc of

nations, development of carbon-free energies and energy efficiency

would have been spurred. If the carbon fee had begun in 1995, we

Figure 11. Fossil fuel CO

emissions. (A) 2012 emissions by source region, and (B) cumulative 1751–2012 emissions. Results are an update of Fig.

10 of [190] using data from [5].

doi:10.1371/journal.pone.0081648.g011

Figure 12. Per capita fossil fuel CO

emissions. Countries, regions and data sources are the same as in Fig. 11. Horizontal lines are the global

mean and multiples of the global mean.

doi:10.1371/journal.pone.0081648.g012

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calculate that global emissions would have needed to decline

2.1%/year to limit cumulative fossil fuel emissions to 500 GtC. A

start date of 2005 would have required a reduction of 3.5%/year

for the same result.

The task faced today is more difficult. Emissions reduction of

6%/year and 100 GtC storage in the biosphere and soils are

needed to get CO

back to 350 ppm, the approximate require-

ment for restoring the planet’s energy balance and stabilizing

climate this century. Such a pathway is exceedingly difficult to

achieve, given the current widespread absence of policies to drive

rapid movement to carbon-free energies and the lifetime of energy

infrastructure in place.

Yet we suggest that a pathway is still conceivable that could

restore planetary energy balance on the century time scale. That

path requires policies that spur technology development and

provide economic incentives for consumers and businesses such

that social tipping points are reached where consumers move

rapidly to energy conservation and low carbon energies. Moderate

overshoot of required atmospheric CO

levels can possibly be

counteracted via incentives for actions that more-or-less naturally

sequester carbon. Developed countries, responsible for most of the

excess CO

in the air, might finance extensive efforts in developing

countries to sequester carbon in the soil and in forest regrowth on

marginal lands as described above. Burning sustainably designed

biofuels in power plants, with the CO

captured and sequestered,

would also help draw down atmospheric CO

. This pathway

would need to be taken soon, as the magnitude of such carbon

extractions is likely limited and thus not a solution to unfettered

fossil fuel use.

The alternative pathway, which the world seems to be on now,

is continued extraction of all fossil fuels, including development of

unconventional fossil fuels such as tar sands, tar shale, hydro-

fracking to extract oil and gas, and exploitation of methane

hydrates. If that path (with 2%/year growth) continues for 20

years and is then followed by 3%/year emission reduction from

2033 to 2150, we find that fossil fuel emissions in 2150 would total

1022 GtC. Extraction of the excess CO

from the air in this case

would be very expensive and perhaps implausible, and warming of

the ocean and resulting climate impacts would be practically

irreversible.

Economic Implications: Need for a Carbon Fee

The implication is that the world must move rapidly to carbon-

free energies and energy efficiency, leaving most remaining fossil

fuels in the ground, if climate is to be kept close to the Holocene

range and climate disasters averted. Is rapid change possible?

Figure 13. United States energy consumption [229].

doi:10.1371/journal.pone.0081648.g013

Figure 14. World energy consumption for indicated fuels, which excludes wood [4].

doi:10.1371/journal.pone.0081648.g014

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The potential for rapid change can be shown by examples. A

basic requirement for phasing down fossil fuel emissions is

abundant carbon-free electricity, which is the most rapidly

growing form of energy and also has the potential to provide

energy for transportation and heating of buildings. In one decade

(1977–1987), France increased its nuclear power production 15-

fold, with the nuclear portion of its electricity increasing from 8%

to 70% [231]. In one decade (2001–2011) Germany increased the

non-hydroelectric renewable energy portion of its electricity from

4% to 19%, with fossil fuels decreasing from 63% to 61%

(hydroelectric decreased from 4% to 3% and nuclear power

decreased from 29% to 18%) [231].

Given the huge task of replacing fossil fuels, contributions are

surely required from energy efficiency, renewable energies, and

nuclear power, with the mix depending on local preferences.

Renewable energy and nuclear power have been limited in part by

technical challenges. Nuclear power faces persistent concerns

about safety, nuclear waste, and potential weapons proliferation,

despite past contributions to mortality prevention and climate

change mitigation [232]. Most renewable energies tap diffuse

intermittent sources often at a distance from the user population,

thus requiring large-scale energy storage and transport. Develop-

ing technologies can ameliorate these issues, as discussed below.

However, apparent cost is the constraint that prevents nuclear and

renewable energies from fully supplanting fossil fuel electricity

generation.

Transition to a post-fossil fuel world of clean energies will not

occur as long as fossil fuels appear to the investor and consumer to

be the cheapest energy. Fossil fuels are cheap only because they do

not pay their costs to society and receive large direct and indirect

subsidies [233]. Air and water pollution from fossil fuel extraction

and use have high costs in human health, food production, and

natural ecosystems, killing more than 1,000,000 people per year

and affecting the health of billions of people [232,234], with costs

borne by the public. Costs of climate change and ocean

acidification, already substantial and expected to grow consider-

ably [26,235], also are borne by the public, especially by young

people and future generations.

Thus the essential underlying policy, albeit not sufficient, is for

emissions of CO

to come with a price that allows these costs to be

internalized within the economics of energy use. Because so much

energy is used through expensive capital stock, the price should

rise in a predictable way to enable people and businesses to

efficiently adjust lifestyles and investments to minimize costs.

Reasons for preference of a carbon fee or tax over cap-and-trade

include the former’s simplicity and relative ease of becoming

global [236]. A near-global carbon tax might be achieved, e.g., via

a bi-lateral agreement between China and the United States, the

greatest emitters, with a border duty imposed on products from

nations without a carbon tax, which would provide a strong

incentive for other nations to impose an equivalent carbon tax.

The suggestion of a carbon fee collected from fossil fuel companies

with all revenues distributed to the public on a per capita basis

[237] has received at least limited support [238].

Economic analyses indicate that a carbon price fully incorpo-

rating environmental and climate damage would be high [239].

The cost of climate change is uncertain to a factor of 10 or more

and could be as high as ,

$1000/tCO

[235,240]. While the

imposition of such a high price on carbon emissions is outside the

realm of short-term political feasibility, a price of that magnitude is

not required to engender a large change in emissions trajectory.

An economic analysis indicates that a tax beginning at

$15/

tCO

and rising $10/tCO

each year would reduce emissions in

the U.S. by 30% within 10 years [241]. Such a reduction is more

than 10 times as great as the carbon content of tar sands oil carried

by the proposed Keystone XL pipeline (830,000 barrels/day)

[242]. Reduced oil demand would be nearly six times the pipeline

capacity [241], thus the carbon fee is far more effective than the

proposed pipeline.

A rising carbon fee is the sine qua non for fossil fuel phase out, but

not enough by itself. Investment is needed in RD&D (research,

development and demonstration) to help renewable energies and

nuclear power overcome obstacles limiting their contributions.

Intermittency of solar and wind power can be alleviated with

advances in energy storage, low-loss smart electric grids, and

electrical vehicles interacting with the grid. Most of today’s nuclear

power plants have half-century-old technology with light-water

reactors [243] utilizing less than 1% of the energy in the nuclear

fuel and leaving unused fuel as long-lived nuclear ‘‘waste’’

requiring sequestration for millennia. Modern light-water reactors

can employ convective cooling to eliminate the need for external

cooling in the event of an anomaly such as an earthquake.

However, the long-term future of nuclear power will employ ‘‘fast’’

reactors, which utilize ,99% of the nuclear fuel and can ‘‘burn’’

nuclear waste and excess weapons material [243]. It should be

possible to reduce the cost of nuclear power via modular standard

reactor design, but governments need to provide a regulatory

environment that supports timely construction of approved

designs. RD&D on carbon capture and storage (CCS) technology

is needed, especially given our conclusion that the current

atmospheric CO

level is already in the dangerous zone, but

continuing issues with CCS technology [7,244] make it inappro-

priate to construct fossil fuel power plants with a promise of future

retrofit for carbon capture. Governments should support energy

planning for housing and transportation, energy and carbon

efficiency requirements for buildings, vehicles and other manu-

factured products, and climate mitigation and adaptation in

undeveloped countries.

Economic efficiency would be improved by a rising carbon fee.

Energy efficiency and alternative low-carbon and no-carbon

energies should be allowed to compete on an equal footing,

without subsidies, and the public and business community should

be made aware that the fee will continually rise. The fee for

unconventional fossil fuels, such as oil from tar sands and gas from

hydrofracking, should include carbon released in mining and

refining processes, e.g., methane leakage in hydrofracking [245–

249]. If the carbon fee rises continually and predictably, the

resulting energy transformations should generate many jobs, a

welcome benefit for nations still suffering from long-standing

economic recession. Economic modeling shows that about 60% of

the public, especially low-income people, would receive more

money via a per capita 100% dispersal of the collected fee than

they would pay because of increased prices [241].

Fairness: Intergenerational Justice and Human Rights

Relevant fundamentals of climate science are clear. The

physical climate system has great inertia, which is due especially

to the thermal inertia of the ocean, the time required for ice sheets

to respond to global warming, and the longevity of fossil fuel CO

in the surface carbon reservoirs (atmosphere, ocean, and

biosphere). This inertia implies that there is additional climate

change ‘‘in the pipeline’’ even without further change of

atmospheric composition. Climate system inertia also means that,

if large-scale climate change is allowed to occur, it will be

exceedingly long-lived, lasting for many centuries.

One implication is the likelihood of intergenerational effects,

with young people and future generations inheriting a situation in

which grave consequences are assured, practically out of their

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control, but not of their doing. The possibility of such intergen-

erational injustice is not remote – it is at our doorstep now. We

have a planetary climate crisis that requires urgent change to our

energy and carbon pathway to avoid dangerous consequences for

young people and other life on Earth.

Yet governments and industry are rushing into expanded use of

fossil fuels, including unconventional fossil fuels such as tar sands,

tar shale, shale gas extracted by hydrofracking, and methane

hydrates. How can this course be unfolding despite knowledge of

climate consequences and evidence that a rising carbon price

would be economically efficient and reduce demand for fossil

fuels? A case has been made that the absence of effective

governmental leadership is related to the effect of special interests

on policy, as well as to public relations efforts by organizations that

profit from the public’s addiction to fossil fuels [237,250].

The judicial branch of governments may be less subject to

pressures from special financial interests than the executive and

legislative branches, and the courts are expected to protect the

rights of all people, including the less powerful. The concept that

the atmosphere is a public trust [251], that today’s adults must

deliver to their children and future generations an atmosphere as

beneficial as the one they received, is the basis for a lawsuit [252]

in which it is argued that the U.S. government is obligated to

protect the atmosphere from harmful greenhouse gases.

Independent of this specific lawsuit, we suggest that intergen-

erational justice in this matter derives from fundamental rights of

equality and justice. The Universal Declaration of Human Rights

[253] declares ‘‘All are equal before the law and are entitled

without any discrimination to equal protection of the law.’’

Further, to consider a specific example, the United States

Constitution provides all citizens ‘‘equal protection of the laws’’

and states that no person can be deprived of ‘‘life, liberty or

property without due process of law’’. These fundamental rights

are a basis for young people to expect fairness and justice in a

matter as essential as the condition of the planet they will inhabit.

We do not prescribe the legal arguments by which these rights can

be achieved, but we maintain that failure of governments to

effectively address climate change infringes on fundamental rights

of young people.

Ultimately, however, human-made climate change is more a

matter of morality than a legal issue. Broad public support is

probably needed to achieve the changes needed to phase out fossil

fuel emissions. As with the issue of slavery and civil rights, public

recognition of the moral dimensions of human-made climate

change may be needed to stir the public’s conscience to the point

of action.

A scenario is conceivable in which growing evidence of climate

change and recognition of implications for young people lead to

massive public support for action. Influential industry leaders,

aware of the moral issue, may join the campaign to phase out

emissions, with more business leaders becoming supportive as they

recognize the merits of a rising price on carbon. Given the relative

ease with which a flat carbon price can be made international

[236], a rapid global emissions phasedown is feasible. As fossil fuels

are made to pay their costs to society, energy efficiency and clean

energies may reach tipping points and begin to be rapidly adopted.

Our analysis shows that a set of actions exists with a good

chance of averting ‘‘dangerous’’ climate change, if the actions

begin now. However, we also show that time is running out.

Unless a human ‘‘tipping point’’ is reached soon, with implemen-

tation of effective policy actions, large irreversible climate changes

will become unavoidable. Our parent’s generation did not know

that their energy use would harm future generations and other life

on the planet. If we do not change our course, we can only pretend

that we did not know.

Discussion

We conclude that an appropriate target is to keep global

temperature within or close to the temperature range in the

Holocene, the interglacial period in which civilization developed.

With warming of 0.8uC in the past century, Earth is just emerging

from that range, implying that we need to restore the planet’s

energy balance and curb further warming. A limit of approx-

imately 500 GtC on cumulative fossil fuel emissions, accompanied

by a net storage of 100 GtC in the biosphere and soil, could keep

global temperature close to the Holocene range, assuming that the

net future forcing change from other factors is small. The longevity

of global warming (Fig. 9) and the implausibility of removing the

warming if it is once allowed to penetrate the deep ocean

emphasize the urgency of slowing emissions so as to stay close to

the 500 GtC target.

Fossil fuel emissions of 1000 GtC, sometimes associated with a

2uC global warming target, would be expected to cause large

climate change with disastrous consequences. The eventual

warming from 1000 GtC fossil fuel emissions likely would reach

well over 2uC, for several reasons. With such emissions and

temperature tendency, other trace greenhouse gases including

methane and nitrous oxide would be expected to increase, adding

to the effect of CO

. The global warming and shifting climate

zones would make it less likely that a substantial increase in forest

and soil carbon could be achieved. Paleoclimate data indicate that

slow feedbacks would substantially amplify the 2uC global

warming. It is clear that pushing global climate far outside the

Holocene range is inherently dangerous and foolhardy.

The fifth IPCC assessment Summary for Policymakers [14]

concludes that to achieve a 50% chance of keeping global

warming below 2uC equivalent CO

emissions should not exceed

1210 GtC, and after accounting for non-CO

climate forcings this

limit on CO

emissions becomes 840 GtC. The existing drafts of

the fifth IPCC assessment are not yet approved for comparison

and citation, but the IPCC assessment is consistent with studies of

Meinshausen et al. [254] and Allen et al. [13], hereafter M2009

and A2009, with which we can make comparisons. We will also

compare our conclusions with those of McKibben [255]. M2009

and A2009 appear together in the same journal with the two lead

authors on each paper being co-authors on the other paper.

McKibben [255], published in a popular magazine, uses

quantitative results of M2009 to conclude that most remaining

fossil fuel reserves must be left in the ground, if global warming this

century is to be kept below 2uC. McKibben [255] has been very

successful in drawing public attention to the urgency of rapidly

phasing down fossil fuel emissions.

M2009 use a simplified carbon cycle and climate model to make

a large ensemble of simulations in which principal uncertainties in

the carbon cycle, radiative forcings, and climate response are

allowed to vary, thus yielding a probability distribution for global

warming as a function of time throughout the 21st century. M2009

use this distribution to infer a limit on total (fossil fuel+net land use)

carbon emissions in the period 2000–2049 if global warming in the

21st century is to be kept below 2uC at some specified probability.

For example, they conclude that the limit on total 2000–2049

carbon emissions is 1440 GtCO

(393 GtC) to achieve a 50%

chance that 21st century global warming will not exceed 2uC.

A2009 also use a large ensemble of model runs, varying

uncertain parameters, and conclude that total (fossil fuel+net land

use) carbon emissions of 1000 GtC would most likely yield a peak

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-induced warming of 2uC, with 90% confidence that the peak

warming would be in the range 1.3–3.9uC. They note that their

results are consistent with those of M2009, as the A2009 scenarios

that yield 2uC warming have 400–500 GtC emissions during

2000–2049; M2009 find 393 GtC emissions for 2uC warming, but

M2009 included a net warming effect of non-CO

forcings, while

A2009 neglected non-CO

forcings.

McKibben [255] uses results of M2009 to infer allowable fossil

fuel emissions up to 2050 if there is to be an 80% chance that

maximum warming in the 21st century will not exceed 2uC above

the pre-industrial level. M2009 conclude that staying under this

2uC limit with 80% probability requires that 2000–2049 emissions

must be limited to 656 GtCO

(179 GtC) for 2007–2049.

McKibben [255] used this M2009 result to determine a remaining

carbon budget (at a time not specified exactly) of 565 GtCO

(154

GtC) if warming is to stay under 2uC. Let us update this analysis to

the present: fossil fuel emissions in 2007–2012 were 51 GtC [5], so,

assuming no net emissions from land use in these few years, the

M2009 study implies that the remaining budget at the beginning

of 2013 was 128 GtC.

Thus, coincidentally, the McKibben [255] approach via M2009

yields almost exactly the same remaining carbon budget (128 GtC)

as our analysis (130 GtC). However, our budget is that required to

limit warming to about 1uC (there is a temporary maximum

during this century at about 1.1–1.2uC, Fig. 9), while McKibben

[255] is allowing global warming to reach 2uC, which we have

concluded would be a disaster scenario! This apparently vast

difference arises from three major factors.

First, we assumed that reforestation and improved agricultural

and forestry practices can suck up the net land use carbon of the

past. We estimate net land use emissions as 100 GtC, while M2009

have land use emissions almost twice that large (,180 GtC). We

argue elsewhere (see section 14 in Supporting Information of [54])

that the commonly employed net land use estimates [256] are

about a factor of two larger than the

net land use carbon that is

most consistent with observed CO

history. However, we need not

resolve that long-standing controversy here. The point is that, to

make the M2009 study equivalent to ours, negative land use

emissions must be included in the 21st century equal to earlier

positive land use emissions.

Second, we have assumed that future net change of non-CO

forcings will be zero, while M2009 have included significant non-

forcings. In recent years non-CO

GHGs have provided

about 20% of the increase of total GHG climate forcing.

Third, our calculations are for a single fast-feedback equilibrium

climate sensitivity, 3uC for doubled CO

, which we infer from

paleoclimate data. M2009 use a range of climate sensitivities to

compute a probability distribution function for expected warming,

and then McKibben [255] selects the carbon emission limit that

keeps 80% of the probability distribution below 2uC.

The third factor is a matter of methodology, but one to be borne

in mind. Regarding the first two factors, it may be argued that our

scenario is optimistic. That is true, but both goals, extracting 100

GtC from the atmosphere via improved forestry and agricultural

practices (with possibly some assistance from CCS technology) and

limiting additional net change of non-CO

forcings to zero, are

feasible and probably much easier than the principal task of

limiting additional fossil fuel emissions to 130 GtC.

We noted above that reforestation and improving agricultural

and forestry practices that store more carbon in the soil make sense

for other reasons. Also that task is made easier by the excess CO

in the air today, which causes vegetation to take up CO

efficiently. Indeed, this may be the reason that net land use

emissions seem to be less than is often assumed.

As for the non-CO

forcings, it is noteworthy that greenhouse

gases controlled by the Montreal Protocol are now decreasing, and

recent agreement has been achieved to use the Montreal Protocol

to phase out production of some additional greenhouse gases even

though those gases do not affect the ozone layer. The most

important non-CO

forcing is methane, whose increases in turn

cause tropospheric ozone and stratospheric water vapor to

increase. Fossil fuel use is probably the largest source of methane

[1], so if fossil fuel use begins to be phased down, there is good

basis to anticipate that all three of these greenhouse gases could

decrease, because of the approximate 10-year lifetime of methane.

As for fossil fuel CO

emissions, considering the large, long-lived

fossil fuel infrastructure in place, the science is telling us that policy

should be set to reduce emissions as rapidly as possible. The most

fundamental implication is the need for an across-the-board rising

fee on fossil fuel emissions in order to allow true free market

competition from non-fossil energy sources. We note that

biospheric storage should not be allowed to offset further fossil

fuel emissions. Most fossil fuel carbon will remain in the climate

system more than 100,000 years, so it is essential to limit the

emission of fossil fuel carbon. It will be necessary to have incentives

to restore biospheric carbon, but these must be accompanied by

decreased fossil fuel emissions.

A crucial point to note is that the three tasks [limiting fossil fuel

emissions, limiting (and reversing) land use emissions,

limiting (and reversing) growth of non-CO

forcings] are

interactive and reinforcing. In mathematical terms, the problem

is non-linear. As one of these climate forcings increases, it increases

the others. The good news is that, as one of them decreases, it

tends to decrease the others. In order to bestow upon future

generations a planet like the one we received, we need to win on

all three counts, and by far the most important is rapid phasedown

of fossil fuel emissions.

It is distressing that, despite the clarity and imminence of the

danger of continued high fossil fuel emissions, governments

continue to allow and even encourage pursuit of ever more fossil

fuels. Recognition of this reality and perceptions of what is

‘‘politically feasible’’ may partially account for acceptance of

targets for global warming and carbon emissions that are well into

the range of ‘‘dangerous human-made interference’’ with climate.

Although there is merit in simply chronicling what is happening,

there is still opportunity for humanity to exercise free will. Thus

our objective is to define what the science indicates is needed, not

to assess political feasibility. Further, it is not obvious to us that

there are physical or economic limitations that prohibit fossil fuel

emission targets far lower than 1000 GtC, even targets closer to

500 GtC. Indeed, we suggest that rapid transition off fossil fuels

would have numerous near-term and long-term social benefits,

including improved human health and outstanding potential for

job creation.

A world summit on climate change will be held at United

Nations Headquarters in September 2014 as a preliminary to

negotiation of a new climate treaty in Paris in late 2015. If this

treaty is analogous to the 1997 Kyoto Protocol [257], based on

national targets for emission reductions and cap-and-trade-with-

offsets emissions trading mechanisms, climate deterioration and

gross intergenerational injustice will be practically guaranteed.

The palpable danger that such an approach is conceivable is

suggested by examination of proposed climate policies of even the

most forward-looking of nations. Norway, which along with the

other Scandinavian countries has been among the most ambitious

and successful of all nations in reducing its emissions, nevertheless

approves expanded oil drilling in the Arctic and development of

tar sands as a majority owner of Statoil [258–259]. Emissions

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foreseen by the Energy Perspectives of Statoil [259], if they occur,

would approach or exceed 1000 GtC and cause dramatic climate

change that would run out of control of future generations. If, in

contrast, leading nations agree in 2015 to have internal rising fees

on carbon with border duties on products from nations without a

carbon fee, a foundation would be established for phaseover to

carbon free energies and stable climate.

Supporting Information

Table S1

(ODS)

Table S2

(ODS)

Table S3

(ODS)

Text S1

(DOC)

Acknowledgments

We greatly appreciate the assistance of editor Juan A. An˜el in achieving

requisite form and clarity for publication. The paper is dedicated to Paul

Epstein, a fervent defender of the health of humans and the environment,

who graciously provided important inputs to this paper while battling late

stages of non-Hodgkin’s lymphoma. We thank David Archer, Inez Fung,

Charles Komanoff and two anonymous referees for perceptive helpful

reviews and Mark Chandler, Bishop Dansby, Ian Dunlop, Dian Gaffen

Seidel, Edward Greisch, Fred Hendrick, Tim Mock, Ana Prados, Stefan

Rahmstorf, Rob Socolow and George Stanford for helpful suggestions on a

draft of the paper.

Author Contributions

Conceived and designed the experiments: JH PK MS. Performed the

experiments: MS PK. Wrote the paper: JH. Wrote the first draft: JH. All

authors made numerous critiques and suggested specific wording and

references: JH PK MS VM-D FA DJB PJH OHG SLH CP JR EJR JS PS

KS LVS KvS JCZ. Especially: PK MS VM-D.

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Discussion

The great barrier reef just suffered a major bleach event, [2 years in a row:](http://e360.yale.edu/features/inside-look-at-catastrophic-bleaching-of-the-great-barrier-reef-2017-hughes) ![](http://i.imgur.com/LHU5ix1.png) This was a pretty stark finding at the time, and still continues to be the elephant in the room with respect to international climate policy. There is a balance that must be struck between the difficulty (it will be extremely difficult to achieve this, given the amount of warming already in the proverbial pipeline) of limiting warming to 1.5 degrees and the deleterious impacts of 2 degrees of warming. This is a truly staggering figure. Even in the Great recession, global emissions only decreased only by a few percent. This paper is saying that we need about triple this amount, every year. This is a massively important point – one that hasn’t been focused on enough. Basically, it’s saying we should never get to the point where we say “too late, nothing we can do to save ourselves.” It’s always better to start reducing emissions late than never. This is a staggering number – far higher than the total national debt, when multiplied out by the population ($90,000 * ~320 million) = ~$29 trillion CO2 concentrations are now [firmly above 400ppm](https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html). Needless to say, this treaty occurred, though most notably it does not require any country to reduce emissions. It only requires countries to “pledge” to do so. This is why [Hansen is not a fan of the agreement.](https://www.theguardian.com/environment/2015/dec/12/james-hansen-climate-change-paris-talks-fraud) That reducing coal consumption will result in a slight “bounceback” of warming increases the case to shift [from coal to non-emitting power generation sources, such as nuclear and renewables](https://www2.ucar.edu/atmosnews/news/5292/switching-coal-natural-gas-would-do-little-global-climate-study-indicates), rather than natural gas, as switching to natural gas would have lower climate benefits than would appear by only looking at the differences in CO2 emissions between coal and gas. This is a staggering number – far higher than the total national debt, when multiplied out by the population ($90,000 * ~320 million) = ~$29 trillion It will not necessarily be clear in the moment if we have hit a particular tipping point or not. This underscores the need to keep reducing greenhouse gas emissions without getting demoralized and thinking that nothing we can do will help. It’s important to note that there are still no commercially viable carbon capture and storage (CCS) projects that do this. There are some that use the captured CO2 to pump into oil well to stimulate more production (somewhat ironically increasing carbon extraction), but CCS has never been achieved for the sole purpose of capturing and storing CO2. It is worth showing the updated figure that has data through 2016: ![](http://i.imgur.com/SCLTVio.png) Most global climate scenarios that model a 2- or sub-2 degree warming scenario rely on this technology (especially in the latter half of this century), of which there is no operating example in the world today. As I explained in an above comment, the world might be closer to slowing (some say even peaking) emissions than we thought back in 2013. Few energy analysts that I know of predicted 2 straight years of flat emissions growth. The [Climate Action Tracker](http://climateactiontracker.org/global.html) group has an informative modelling of possible future temperature pathways. ![](http://i.imgur.com/cigT500.png) It is important to note that the Federal subsidy for wind energy production is slated to expire in 2020, and the Federal solar tax credit is expected to be reduced from the current 30 percent to 10 percent by 2022. As we approach the point at which solar and wind must stand on their own, there will be pressure from the environmental community to either extend the subsidies or pass a carbon tax. It is also important to note that the U.S. Department of Energy predicts [little wind growth](https://www.eia.gov/todayinenergy/detail.php?id=26492) after the tax credit expires, though solar is expected to keep on growing. It is worth noting that global fossil fuel emissions [were flat in 2016](https://www.iea.org/newsroom/news/2017/march/iea-finds-co2-emissions-flat-for-third-straight-year-even-as-global-economy-grew.html), the third year in a row this happened. It is also worth noting that emissions must come down, rather than remain flat, to avoid exceeding the global carbon budget. As a recent [Vox analysis shows](http://www.vox.com/energy-and-environment/2017/3/27/15043522/nuclear-power-future-innovation) , only nuclear energy thus far has achieved the type of carbon-free energy scale-up on any comparable scale as would be necessary to fight the climate crisis. Note that Hansen is quite pro nuclear, and often times comes at odds with the environmental activist community with his support (and their opposition) to nuclear. See [here](https://www.theguardian.com/environment/2015/dec/03/nuclear-power-paves-the-only-viable-path-forward-on-climate-change) (Hansen) and [here](https://thinkprogress.org/why-james-hansen-is-wrong-about-nuclear-power-44b486ed8a72) (critics) for more. It is important to note that Joe Romm, far more towards the pro-renewable side than nuclear, and one of the U.S.’s best minds on climate, says: “in the best-case scenario, nuclear power can play a modest, but important, role in avoiding catastrophic global warming if it can solve its various nagging problems without sacrificing safety.” ![](http://i.imgur.com/BZPTMwb.png) A separate estimate calculates that we’ve already burned through over 500 GtC (Pg, or “petagram” is the same as a Gigaton) – this estimate likely includes emissions from deforestation, which are about 100 GtC. Subtracting that roughly aligns these estimates. ![](http://i.imgur.com/Uyg6pNT.png) This is sometimes referred to as the “global warming pause,” which has since disappeared, with warming in recent years continuing. The data in Hansen et al.’s graph stop at 2013, right before a large amount of warming in 2014, 2015, and 2016. ![](http://i.imgur.com/JV7kWkE.png)

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