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.

Assessing Dangerous Climate Change

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