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Review
Assessing ‘‘Dangerous Climate Change’’: Required
Reduction of Carbon Emissions to Protect Young People,
Future Generations and Nature
James Hansen
1
*, Pushker Kharecha
1,2
, Makiko Sato
1
, Valerie Masson-Delmotte
3
, Frank Ackerman
4
,
David J. Beerling
5
, Paul J. Hearty
6
, Ove Hoegh-Guldberg
7
, Shi-Ling Hsu
8
, Camille Parmesan
9,10
,
Johan Rockstrom
11
, Eelco J. Rohling
12,13
, Jeffrey Sachs
1
, Pete Smith
14
, Konrad Steffen
15
,
Lise Van Susteren
16
, Karina von Schuckmann
17
, James C. Zachos
18
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
2
) 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
2
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
2
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
2
[1]. Increase of ‘‘greenhouse’’ gases such as CO
2
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
2
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
Assessing Dangerous Climate Change
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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
2
emissions and carbon content (1 ppm atmospheric CO
2
,
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
Assessing Dangerous Climate Change
PLOS ONE | www.plosone.org 3 December 2013 | Volume 8 | Issue 12 | e81648
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
2
[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
2
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
2
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
2
feedbacks amplified weak orbital forcings,
the feedbacks necessarily changing slowly over millennia, at the
pace of orbital changes. Today, however, CO
2
is under the control
of humans as fossil fuel emissions overwhelm natural changes.
Atmospheric CO
2
has increased rapidly to a level not seen for at
least 3 million years [56,63]. Global warming induced by
increasing CO
2
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
2
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
2
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
2
or less, so
accuracy approaching 0.1 W/m
2
is needed. The most promising
Assessing Dangerous Climate Change
PLOS ONE | www.plosone.org 4 December 2013 | Volume 8 | Issue 12 | e81648
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
2
averaged over Earth’s
surface [70]. Smaller contributions to planetary energy imbalance
are from heat gain by the deeper ocean (+0.10 W/m
2
), energy
used in net melting of ice (+0.05 W/m
2
), and energy taken up by
warming continents (+0.02 W/m
2
). 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
2
.
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
2
[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
2
) suggests an average imbalance over the
solar cycle of about 0.7 W/m
2
.
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
2
[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
2
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
2
,CO
2
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
2
and restore energy balance.
If Earth’s energy imbalance is 0.75 W/m
2
,CO
2
must be reduced
to about 345 ppm to restore energy balance [64,75].
The measured energy imbalance indicates that an initial CO
2
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
CO
2
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
2
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
2
perturbations. (A) Instantaneous injection or extraction of CO
2
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
2
, the appropriate CO
2
target might be close to
the pre-industrial CO
2
amount [53]. However, there are other
human forcings, including aerosols, the effect of aerosols on
clouds, non-CO
2
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
2
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
2
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
2
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
st
century, especially if CO
2
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
2
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
2
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
2
on
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
2
, 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
PLOS ONE | www.plosone.org 7 December 2013 | Volume 8 | Issue 12 | e81648