http://www.pnas.org/cgi/content/full/103/39/14288
Global temperature change -- Hansen et al. 103 (39): 14288 --
Proceedings of the National Academy of Sciences
Published online before print September 25, 2006, 10.1073/pnas.0606291103
PHYSICAL SCIENCES / ENVIRONMENTAL SCIENCES
Global temperature change
James Hansen*,,, Makiko Sato*,, Reto Ruedy*,, Ken Lo*,, David W.
Lea¶, and Martin Medina-Elizade¶
*National Aeronautics and Space Administration Goddard Institute for
Space Studies, Columbia University Earth Institute, and Sigma Space
Partners, Inc., 2880 Broadway, New York, NY 10025; and ¶Department of
Earth Science, University of California, Santa Barbara, CA 93106
Abstract
Global surface temperature has increased 0.2°C per decade in the past
30 years, similar to the warming rate predicted in the 1980s in
initial global climate model simulations with transient greenhouse
gas changes. Warming is larger in the Western Equatorial Pacific than
in the Eastern Equatorial Pacific over the past century, and we
suggest that the increased West-East temperature gradient may have
increased the likelihood of strong El Niños, such as those of 1983
and 1998. Comparison of measured sea surface temperatures in the
Western Pacific with paleoclimate data suggests that this critical
ocean region, and probably the planet as a whole, is approximately as
warm now as at the Holocene maximum and within 1°C of the maximum
temperature of the past million years. We conclude that global
warming of more than 1°C, relative to 2000, will constitute
"dangerous" climate change as judged from likely effects on sea level
and extermination of species.
climate change | El Niños | global warming | sea level | species extinctions
Global temperature is a popular metric for summarizing the state of
global climate. Climate effects are felt locally, but the global
distribution of climate response to many global climate forcings is
reasonably congruent in climate models (1), suggesting that the
global metric is surprisingly useful. We will argue further,
consistent with earlier discussion (2, 3), that measurements in the
Western Pacific and Indian Oceans provide a good indication of global
temperature change.
We first update our analysis of surface temperature change based on
instrumental data and compare observed temperature change with
predictions of global climate change made in the 1980s. We then
examine current temperature anomalies in the tropical Pacific Ocean
and discuss their possible significance. Finally, we compare
paleoclimate and recent data, using the Earth's history to estimate
the magnitude of global warming that is likely to constitute
dangerous human-made climate change.
Modern Global Temperature Change
Global surface temperature in more than a century of instrumental
data is recorded in the Goddard Institute for Space Studies analysis
for 2005. Our analysis, summarized in Fig. 1, uses documented
procedures for data over land (4), satellite measurements of sea
surface temperature (SST) since 1982 (5), and a ship-based analysis
for earlier years (6). Estimated 2 error (95% confidence) in
comparing nearby years of global temperature (Fig. 1A), such as 1998
and 2005, decreases from 0.1°C at the beginning of the 20th century
to 0.05°C in recent decades (4). Error sources include incomplete
station coverage, quantified by sampling a model-generated data set
with realistic variability at actual station locations (7), and
partly subjective estimates of data quality problems (8). The
estimated uncertainty of global mean temperature implies that we can
only state that 2005 was probably the warmest year.
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Fig. 1. Surface temperature anomalies relative to 1951-1980
from surface air measurements at meteorological stations and ship and
satellite SST measurements. (A) Global annual mean anomalies. (B)
Temperature anomaly for the first half decade of the 21st century.
The map of temperature anomalies for the first half-decade of the
21st century (Fig. 1B), relative to 1951-1980 climatology, shows that
current warmth is nearly ubiquitous, generally larger over land than
over ocean, and largest at high latitudes in the Northern Hemisphere.
Our ranking of 2005 as the warmest year depends on the positive polar
anomalies, especially the unusual Arctic warmth. In calculating the
global mean, we give full weight to all regions based on area.
Meteorological stations are sparse in the Arctic, but the estimated
strong warm anomaly there in 2005 is consistent with record low sea
ice concentration and Arctic temperature anomalies inferred from
infrared satellite data (9).
Our analysis includes estimated temperature anomalies up to 1,200 km
from the nearest measurement station (7). Resulting spatial
extrapolations and interpolations of temperature anomalies usually
are meaningful for seasonal and longer time scales at middle and high
latitudes, where the spatial scale of anomalies is set by Rossby
waves (7). Thus, we believe that the unusual Arctic warmth of 2005 is
real. Other characteristics of our analysis method are summarized in
Supporting Text, which is published as supporting information on the
PNAS web site.
Independent analysis by the National Climate Data Center
(www.ncdc.noaa.gov/oa/climate/research/2005/ann/global.html), using a
"teleconnection" approach to fill in data sparse regions, also finds
2005 to be the warmest year. The joint analysis of the University of
East Anglia and the Hadley Centre
(www.met-office.gov.uk/research/hadleycentre/obsdata/globaltemperature
.html) also yields high global temperature for 2005, but a few
hundredths of a degree cooler than in 1998.
Record, or near record, warmth in 2005 is notable, because global
temperature did not receive a boost from an El Niño in 2005. The
temperature in 1998, on the contrary, was lifted 0.2°C above the
trend line by a "super El Niño" (see below), the strongest El Niño of
the past century.
Global warming is now 0.6°C in the past three decades and 0.8°C in
the past century. It is no longer correct to say "most global warming
occurred before 1940." A better summary is: slow global warming, with
large fluctuations, over the century up to 1975, followed by rapid
warming at a rate 0.2°C per decade. Global warming was 0.7°C between
the late 19th century (the earliest time at which global mean
temperature can be accurately defined) and 2000, and continued
warming in the first half decade of the 21st century is consistent
with the recent rate of +0.2°C per decade.
The conclusion that global warming is a real climate change, not an
artifact due to measurements in urban areas, is confirmed by surface
temperature change inferred from borehole temperature profiles at
remote locations, the rate of retreat of alpine glaciers around the
world, and progressively earlier breakup of ice on rivers and lakes
(10). The geographical distribution of warming (Fig. 1B) provides
further proof of real climate change. Largest warming is in remote
regions including high latitudes. Warming occurs over ocean areas,
far from direct human effects, with warming over ocean less than over
land, an expected result for a forced climate change because of the
ocean's great thermal inertia.
Early Climate Change Predictions. Manabe and Wetherald (11) made the
first global climate model (GCM) calculations of warming due to
instant doubling of atmospheric CO2. The first GCM calculations with
transient greenhouse gas (GHG) amounts, allowing comparison with
observations, were those of Hansen et al. (12). It has been asserted
that these calculations, presented in congressional testimony in 1988
(13), turned out to be "wrong by 300%" (14). That assertion, posited
in a popular novel, warrants assessment because the author's views on
global warming have been welcomed in testimony to the United States
Senate (15) and in a meeting with the President of the United States
(16), at a time when the Earth may be nearing a point of dangerous
human-made interference with climate (17).
The congressional testimony in 1988 (13) included a graph (Fig. 2) of
simulated global temperature for three scenarios (A, B, and C) and
maps of simulated temperature change for scenario B. The three
scenarios were used to bracket likely possibilities. Scenario A was
described as "on the high side of reality," because it assumed rapid
exponential growth of GHGs and it included no large volcanic
eruptions during the next half century. Scenario C was described as
"a more drastic curtailment of emissions than has generally been
imagined," specifically GHGs were assumed to stop increasing after
2000. Intermediate scenario B was described as "the most plausible."
Scenario B has continued moderate increase in the rate of GHG
emissions and includes three large volcanic eruptions sprinkled
through the 50-year period after 1988, one of them in the 1990s.
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Fig. 2. Global surface temperature computed for scenarios A,
B, and C (12), compared with two analyses of observational data. The
0.5°C and 1°C temperature levels, relative to 1951-1980, were
estimated (12) to be maximum global temperatures in the Holocene and
the prior interglacial period, respectively.
Real-world GHG climate forcing (17) so far has followed a course
closest to scenario B. The real world even had one large volcanic
eruption in the 1990s, Mount Pinatubo in 1991, whereas scenario B
placed a volcano in 1995.
Fig. 2 compares simulations and observations. The red curve, as in
ref. 12, is the updated Goddard Institute for Space Studies
observational analysis based on meteorological stations. The black
curve is the land-ocean global temperature index from Fig. 1, which
uses SST changes for ocean areas (5, 6). The land-ocean temperature
has more complete coverage of ocean areas and yields slightly smaller
long-term temperature change, because warming on average is less over
ocean than over land (Fig. 1B).
Temperature change from climate models, including that reported in
1988 (12), usually refers to temperature of surface air over both
land and ocean. Surface air temperature change in a warming climate
is slightly larger than the SST change (4), especially in regions of
sea ice. Therefore, the best temperature observation for comparison
with climate models probably falls between the meteorological station
surface air analysis and the land-ocean temperature index.
Observed warming (Fig. 2) is comparable to that simulated for
scenarios B and C, and smaller than that for scenario A. Following
refs. 18 and 14, let us assess "predictions" by comparing simulated
and observed temperature change from 1988 to the most recent year.
Modeled 1988-2005 temperature changes are 0.59, 0.33, and 0.40°C,
respectively, for scenarios A, B, and C. Observed temperature change
is 0.32°C and 0.36°C for the land-ocean index and meteorological
station analyses, respectively.
Warming rates in the model are 0.35, 0.19, and 0.24°C per decade for
scenarios A, B. and C, and 0.19 and 0.21°C per decade for the
observational analyses. Forcings in scenarios B and C are nearly the
same up to 2000, so the different responses provide one measure of
unforced variability in the model. Because of this chaotic
variability, a 17-year period is too brief for precise assessment of
model predictions, but distinction among scenarios and comparison
with the real world will become clearer within a decade.
Close agreement of observed temperature change with simulations for
the most realistic climate forcing (scenario B) is accidental, given
the large unforced variability in both model and real world. Indeed,
moderate overestimate of global warming is likely because the
sensitivity of the model used (12), 4.2°C for doubled CO2, is larger
than our current estimate for actual climate sensitivity, which is 3
± 1°C for doubled CO2, based mainly on paleoclimate data (17). More
complete analyses should include other climate forcings and cover
longer periods. Nevertheless, it is apparent that the first transient
climate simulations (12) proved to be quite accurate, certainly not
"wrong by 300%" (14). The assertion of 300% error may have been based
on an earlier arbitrary comparison of 1988-1997 observed temperature
change with only scenario A (18). Observed warming was slight in that
9-year period, which is too brief for meaningful comparison.
Super El Niños. The 1983 and 1998 El Niños were successively labeled
"El Niño of the century," because the warming in the Eastern
Equatorial Pacific (EEP) was unprecedented in 100 years (Fig. 3). We
suggest that warming of the Western Equatorial Pacific (WEP), and the
absence of comparable warming in the EEP, has increased the
likelihood of such "super El Niños."
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Fig. 3. Comparison of SST in West and East Equatorial Pacific
Ocean. (A) SST in 2001-2005 relative to 1870-1900, from concatenation
of two data sets (5, 6), as described in the text. (B) SSTs (12-month
running means) in WEP and EEP relative to 1870-1900 means.
In the "normal" (La Niña) phase of El Niño Southern Oscillation the
east-to-west trade winds push warm equatorial surface water to the
west such that the warmest SSTs are located in the WEP near
Indonesia. In this normal state, the thermocline is shallow in the
EEP, where upwelling of cold deep water occurs, and deep in the WEP
(figure 2 of ref. 20). Associated with this tropical SST gradient is
a longitudinal circulation pattern in the atmosphere, the Walker
cell, with rising air and heavy rainfall in the WEP and sinking air
and drier conditions in the EEP. The Walker circulation enhances
upwelling of cold water in the East Pacific, causing a powerful
positive feedback, the Bjerknes (21) feedback, which tends to
maintain the La Niña phase, as the SST gradient and resulting higher
pressure in the EEP support east-to-west trade winds.
This normal state is occasionally upset when, by chance, the
east-to-west trade winds slacken, allowing warm water piled up in the
west to slosh back toward South America. If the fluctuation is large
enough, the Walker circulation breaks down and the Bjerknes feedback
loses power. As the east-to-west winds weaken, the Bjerknes feedback
works in reverse, and warm waters move more strongly toward South
America, reducing the thermocline tilt and cutting off upwelling of
cold water along the South American coast. In this way, a classical
El Niño is born.
Theory does not provide a clear answer about the effect of global
warming on El Niños (19, 20). Most climate models yield either a
tendency toward a more El Niño-like state or no clear change (22). It
has been hypothesized that, during the early Pliocene, when the Earth
was 3°C warmer than today, a permanent El Niño condition existed (23).
We suggest, on empirical grounds, that a near-term global warming
effect is an increased likelihood of strong El Niños. Fig. 1B shows
an absence of warming in recent years relative to 1951-1980 in the
equatorial upwelling region off the coast of South America. This is
also true relative to the earliest period of SST data, 1870-1900
(Fig. 3A). Fig. 7, which is published as supporting information on
the PNAS web site, finds a similar result for linear trends of SSTs.
The trend of temperature minima in the East Pacific, more relevant
for our purpose, also shows no equatorial warming in the East Pacific.
The absence of warming in the EEP suggests that upwelling water there
is not yet affected much by global warming. Warming in the WEP, on
the other hand, is 0.5-1°C (Fig. 3). We suggest that increased
temperature difference between the near-equatorial WEP and EEP allows
the possibility of increased temperature swing from a La Niña phase
to El Niño, and that this is a consequence of global warming
affecting the WEP surface sooner than it affects the deeper ocean.
Fig. 3B compares SST anomalies (12-month running means) in the WEP
and EEP at sites (marked by circles in Fig. 3A) of paleoclimate data
discussed below. Absolute temperatures at these sites are provided in
Fig. 8, which is published as supporting information on the PNAS web
site. Even though these sites do not have the largest warming in the
WEP or largest cooling in the EEP, Fig. 3B reveals warming of the WEP
relative to the EEP [135-year changes, based on linear trends, are
+0.27°C (WEP) and -0.01°C (EEP)].
The 1983 and 1998 El Niños in Fig. 3B are notably stronger than
earlier El Niños. This may be partly an artifact of sparse early data
or the location of data sites, e.g., the late 1870s El Niño is
relatively stronger if averages are taken over Niño 3 or a 5° x 10°
box. Nevertheless, "super El Niños" clearly were more abundant in the
last quarter of the 20th century than earlier in the century.
Global warming is expected to slow the mean tropical circulation
(24-26), including the Walker cell. Sea level pressure data suggest a
slowdown of the longitudinal wind by 3.5% in the past century (26). A
relaxed longitudinal wind should reduce the WEP-EEP temperature
difference on the broad latitudinal scale (10°N to 15°S) of the
atmospheric Walker cell. Observed SST anomalies are consistent with
this expectation, because the cooling in the EEP relative to WEP
decreases at latitudes away from the narrower region strongly
affected by upwelling off the coast of Peru (Fig. 3A). Averaged over
10°N to 15°S, observed warming is as great in the EEP as in the WEP
(see also Fig. 7).
We make no suggestion about changes of El Niño frequency, and we note
that an abnormally warm WEP does not assure a strong El Niño. The
origin and nature of El Niños is affected by chaotic ocean and
atmosphere variations, the season of the driving anomaly, the state
of the thermocline, and other factors, assuring that there will
always be great variability of strength among El Niños.
Will increased contrast between near-equatorial WEP and EEP SSTs be
maintained or even increase with more global warming? The WEP should
respond relatively rapidly to increasing GHGs. In the EEP, to the
extent that upwelling water has not been exposed to the surface in
recent decades, little warming is expected, and the contrast between
WEP and EEP may remain large or increase in coming decades.
Thus, we suggest that the global warming effect on El Niños is
analogous to an inferred global warming effect on tropical storms
(27). The effect on frequency of either phenomenon is unclear,
depending on many factors, but the intensity of the most powerful
events is likely to increase as GHGs increase. In this case, slowing
the growth rate of GHGs should diminish the probability of both super
El Niños and the most intense tropical storms.
Estimating Dangerous Climate Change
Modern vs. Paleo Temperatures. Modern SST measurements (5, 6) are
compared with proxy paleoclimate temperature (28) in the WEP (Ocean
Drilling Program Hole 806B, 0°19' N, 159°22' E; site circled in Fig.
3A) in Fig. 4A. Modern data are from ships and buoys for 1870-1981
(6) and later from satellites (5). In concatenation of satellite and
ship data, as shown in Fig. 8A, the satellite data are adjusted down
slightly so that the 1982-1992 mean matches the mean ship data for
that period.
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Fig. 4. Comparison of modern surface temperature measurements
with paleoclimate proxy data in the WEP (28) (A), EEP (3, 30, 31)
(B), Indian Ocean (40) (C), and Vostok Antarctica (41) (D).
The paleoclimate SST, based on Mg content of foraminifera shells,
provides accuracy to 1°C (29). Thus we cannot be sure that we have
precisely aligned the paleo and modern temperature scales. Accepting
paleo and modern temperatures at face value implies a WEP 1870 SST in
the middle of its Holocene range. Shifting the scale to align the
1870 SST with the lowest Holocene value raises the paleo curve by
0.5°C. Even in that case, the 2001-2005 WEP SST is at least as great
as any Holocene proxy temperature at that location. Coarse temporal
resolution of the Holocene data, 1,000 years, may mask brief warmer
excursions, but cores with higher resolution (29) suggest that peak
Holocene WEP SSTs were not more than 1°C warmer than in the late
Holocene, before modern warming. It seems safe to assume that the SST
will not decline this century, given continued increases of GHGs, so
in a practical sense the WEP temperature is at or near its highest
level in the Holocene. Fig. 5, including WEP data for the past 1.35
million years, shows that the current WEP SST is within 1°C of the
warmest interglacials in that period.
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Fig. 5. Modern sea surface temperatures (5, 6) in the WEP
compared with paleoclimate proxy data (28). Modern data are the
5-year running mean, while the paleoclimate data has a resolution of
the order of 1,000 years.
The Tropical Pacific is a primary driver of the global atmosphere and
ocean. The tropical Pacific atmosphere-ocean system is the main
source of heat transported by both the Pacific and Atlantic Oceans
(2). Heat and water vapor fluxes to the atmosphere in the Pacific
also have a profound effect on the global atmosphere, as demonstrated
by El Niño Southern Oscillation climate variations. As a result,
warming of the Pacific has worldwide repercussions. Even distant
local effects, such as thinning of ice shelves, are affected on
decade-to-century time scales by subtropical Pacific waters that are
subducted and mixed with Antarctic Intermediate Water and thus with
the Antarctic Circumpolar Current.
The WEP exhibits little seasonal or interannual variability of SST,
typically <1°C, so its temperature changes are likely to reflect
large scale processes, such as GHG warming, as opposed to small scale
processes, such as local upwelling. Thus, record Holocene WEP
temperature suggests that global temperature may also be at its
highest level. Correlation of local and global temperature change for
1880-2005 (Fig. 9, which is published as supporting information on
the PNAS web site) confirms strong positive correlation of global and
WEP temperatures, and an even stronger correlation of global and
Indian Ocean temperatures.
The Indian Ocean, due to rapid warming in the past 3-4 decades, is
now warmer than at any time in the Holocene, independent of any
plausible shift of the modern temperature scale relative to the
paleoclimate data (Fig. 4C). In contrast, the EEP (Fig. 4B) and
perhaps Central Antarctica (Vostok, Fig. 4D) warmed less in the past
century and are probably cooler than their Holocene peak values.
However, as shown in Figs. 1B and 3A, those are exceptional regions.
Most of the world and the global mean have warmed as much as the WEP
and Indian Oceans. We infer that global temperature today is probably
at or near its highest level in the Holocene.
Fig. 5 shows that recent warming of the WEP has brought its
temperature within <1°C of its maximum in the past million years.
There is strong evidence that the WEP SST during the penultimate
interglacial period, marine isotope stage (MIS) 5e, exceeded the WEP
SST in the Holocene by 1-2°C (30, 31). This evidence is consistent
with data in Figs. 4 and 5 and with our conclusion that the Earth is
now within 1°C of its maximum temperature in the past million years,
because recent warming has lifted the current temperature out of the
prior Holocene range.
Criteria for Dangerous Warming. The United Nations Framework
Convention on Climate Change (www.unfccc.int) has the objective "to
achieve stabilization of GHG concentrations" at a level preventing
"dangerous anthropogenic interference" (DAI) with climate, but
climate change constituting DAI is undefined. We suggest that global
temperature is a useful metric to assess proximity to DAI, because,
with knowledge of the Earth's history, global temperature can be
related to principal dangers that the Earth faces.
We propose that two foci in defining DAI should be sea level and
extinction of species, because of their potential tragic consequences
and practical irreversibility on human time scales. In considering
these topics, we find it useful to contrast two distinct scenarios
abbreviated as "business-as-usual" (BAU) and the "alternative
scenario" (AS).
BAU has growth of climate forcings as in intermediate or strong
Intergovernmental Panel on Climate Change scenarios, such as A1B or
A2 (10). CO2 emissions in BAU scenarios continue to grow at 2% per
year in the first half of this century, and non-CO2 positive forcings
such as CH4, N2O, O3, and black carbon (BC) aerosols also continue to
grow (10). BAU, with nominal climate sensitivity (3 ± 1°C for doubled
CO2), yields global warming (above year 2000 level) of at least 2-3°C
by 2100 (10, 17).
AS has declining CO2 emissions and an absolute decrease of non-CO2
climate forcings, chosen such that, with nominal climate sensitivity,
global warming (above year 2000) remains <1°C. For example, one
specific combination of forcings has CO2 peaking at 475 ppm in 2100
and a decrease of CH4, O3, and BC sufficient to balance positive
forcing from increase of N2O and decrease of sulfate aerosols. If
CH4, O3, and BC do not decrease, the CO2 cap in AS must be lower.
Sea level implications of BAU and AS scenarios can be considered in
two parts: equilibrium (long-term) sea level change and ice sheet
response time. Global warming <1°C in AS keeps temperatures near the
peak of the warmest interglacial periods of the past million years.
Sea level may have been a few meters higher than today in some of
those periods (10). In contrast, sea level was 25-35 m higher the
last time that the Earth was 2-3°C warmer than today, i.e., during
the Middle Pliocene about three million years ago (32).
Ice sheet response time can be investigated from paleoclimate
evidence, but inferences are limited by imprecise dating of climate
and sea level changes and by the slow pace of weak paleoclimate
forcings compared with stronger rapidly increasing human-made
forcings. Sea level rise lagged tropical temperature by a few
thousand years in some cases (28), but in others, such as Meltwater
Pulse 1A 14,000 years ago (33), sea level rise and tropical
temperature increase were nearly synchronous. Intergovernmental Panel
on Climate Change (10) assumes negligible contribution to 2100 sea
level change from loss of Greenland and Antarctic ice, but that
conclusion is implausible (17, 34). BAU warming of 2-3°C would bathe
most of Greenland and West Antarctic in melt-water during lengthened
melt seasons. Multiple positive feedbacks, including reduced surface
albedo, loss of buttressing ice shelves, dynamical response of ice
streams to increased melt-water, and lowered ice surface altitude
would assure a large fraction of the equilibrium ice sheet response
within a few centuries, at most (34).
Sea level rise could be substantial even in the AS, 1 m per century,
and cause problems for humanity due to high population in coastal
areas (10). However, AS problems would be dwarfed by the disastrous
BAU, which could yield sea level rise of several meters per century
with eventual rise of tens of meters, enough to transform global
coastlines.
Extinction of animal and plant species presents a picture analogous
to that for sea level. Extinctions are already occurring as a result
of various stresses, mostly human-made, including climate change
(35). Plant and animal distributions are a reflection of the regional
climates to which they are adapted. Thus, plants and animals attempt
to migrate in response to climate change, but their paths may be
blocked by human-constructed obstacles or natural barriers such as
coastlines.
A study of 1,700 biological species (36) found poleward migration of
6 km per decade and vertical migration in alpine regions of 6 m per
decade in the second half of the 20th century, within a factor of two
of the average poleward migration rate of surface isotherms (Fig. 6A)
during 1950-1995. More rapid warming in 1975-2005 yields an average
isotherm migration rate of 40 km per decade in the Northern
Hemisphere (Fig. 6B), exceeding known paleoclimate rates of change.
Some species are less mobile than others, and ecosystems involve
interactions among species, so such rates of climate change, along
with habitat loss and fragmentation, new invasive species, and other
stresses are expected to have severe impact on species survival (37).
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Fig. 6. Poleward migration rate of isotherms in surface
observations (A and B) and in climate model simulations (17) for
2000-2100 for Intergovernmental Panel on Climate Change scenario A2
(10) and an alternative scenario of forcings that keeps global
warming after 2000 less than 1°C (17) (C and D). Numbers in upper
right are global means excluding the tropical band.
The total distance of isotherm migration, as well as migration rate,
affects species survival. Extinction is likely if the migration
distance exceeds the size of the natural habitat or remaining habitat
fragment. Fig. 6 shows that the 21st century migration distance for a
BAU scenario (600 km) greatly exceeds the average migration distance
for the AS (100 km).
It has been estimated (38) that a BAU global warming of 3°C over the
21st century could eliminate a majority (60%) of species on the
planet. That projection is not inconsistent with mid-century BAU
effects in another study (37) or scenario sensitivity of stress
effects (35). Moreover, in the Earth's history several mass
extinctions of 50-90% of species have accompanied global temperature
changes of 5°C (39).
We infer that even AS climate change, which would slow warming to
<0.1°C per decade over the century, would contribute to species loss
that is already occurring due to a variety of stresses. However,
species loss under BAU has the potential to be truly disastrous,
conceivably with a majority of today's plants and animals headed
toward extermination.
Discussion
The pattern of global warming (Fig. 1B) has assumed expected
characteristics, with high latitude amplification and larger warming
over land than over ocean, as GHGs have become the dominant climate
forcing in recent decades. This pattern results mainly from the
ice-snow albedo feedback and the response times of ocean and land.
In assessing the level of global warming that constitutes DAI, we
must bear in mind that estimated climate sensitivity of 3 ± 1°C for
doubled CO2, based mainly on paleoclimate data but consistent with
models, refers to a case in which sea ice, snow, water vapor, and
clouds are included as feedbacks, but ice sheet area, vegetation
cover, and non-H2O GHGs are treated as forcings or fixed boundary
conditions. On long time scales, and as the present global warming
increases, these latter quantities can change and thus they need to
be included as feedbacks. Indeed, climate becomes very sensitive on
the ice-age time scale, as feedbacks, specifically ice sheet area and
GHGs, account for practically the entire global temperature change
(17).
Vegetation cover is already expanding poleward in the Northern
Hemisphere causing a positive climate feedback (42). Global warming
could result in release of large amounts of GHGs, e.g., from melting
permafrost or destabilized methane clathrates on continental shelves
(43). Some of the largest warmings in the Earth's history and mass
extinctions may be associated with such GHG releases (39, 43).
Although such disastrous GHG releases may require many centuries, our
ignorance of GHG climate feedbacks demands caution in estimating
requirements to avoid DAI.
The AS is based on the rationale that positive feedbacks such as GHG
release, as well as sea level rise, will be limited if global
temperature stays within the range of recent interglacial periods.
Ice core data reveal a positive GHG feedback, GHG changes lagging
temperature change, but the feedback magnitude is moderate (CO2, +20
ppm per °C; CH4, +50 ppb per °C) even if the entire observed gas
change is a feedback (44). However, paleo data do not constrain the
magnitude of feedbacks under BAU warming, which is far outside the
range of interglacial temperatures.
Such feedbacks enhance the dichotomy between AS and BAU scenarios. If
global warming is not limited to <1°C, feedbacks may add to BAU
emissions, making a "different planet" (17), including eventual
ice-free Arctic, almost inevitable. The AS requires concerted efforts
to both slow CO2 emissions and reduce atmospheric amounts of CH4, O3,
and BC (17, 34). Achievement of the AS should limit positive climate
feedbacks. However, continuation of BAU growth of CO2 emissions (2%
per year) through 2015 yields +35% CO2 emissions relative to 2000 CO2
emissions and +40% CO2 emissions relative to AS 2015 CO2 emissions.
Given the long life of CO2 and the impact of feedbacks on the
plausibility of CH4 reductions, another decade of BAU emissions
probably makes the AS infeasible.
Inference of imminent dangerous climate change may stimulate
discussion of "engineering fixes" to reduce global warming (45, 46).
The notion of such a "fix" is itself dangerous if it diminishes
efforts to reduce CO2 emissions, yet it also would be irresponsible
not to consider all ways to minimize climate change. Considering the
evidence that aerosol effects on clouds cause a large negative
forcing (10), we suggest that seeding of clouds by ships plying
selected ocean regions deserves investigation. However, given that a
large portion of human-made CO2 will remain in the air for many
centuries, sensible policies must focus on devising energy strategies
that greatly reduce CO2 emissions.
Acknowledgements
We thank Ralph Cicerone for reviewing our submitted paper; Mark
Bowen, Mark Cane, Adam Chambers, Bob Grumbine, Mickey Glantz, Isaac
Held, Bruce Johansen, Margaret Kneller, Chuck Kutscher, Ehrhard
Raschke, Joe Romm, Bill Ruddiman, Gus Speth, Harry van Loon, Gabriel
Vecchi, Michael Wright, and Steve Zebiac for comments on a draft
manuscript; Darnell Cain for technical assistance; and National
Aeronautics and Space Administration Earth Science managers Jack
Kaye, Don Anderson, and Eric Lindstrom and Hal Harvey of the Hewlett
Foundation for research support.
Footnotes
Abbreviations: SST, sea surface temperature; GHG, greenhouse gas;
EEP, Eastern Equatorial Pacific; WEP, Western Equatorial Pacific;
DAI, dangerous antrhopogenic interference; BAU, business as usual;
AS, alternative scenario; BC, black carbon.
To whom correspondence should be addressed: E-mail: [EMAIL PROTECTED]
Freely available online through the PNAS open access option.
Contributed by James Hansen, July 31, 2006
Author contributions: D.W.L. and M.M.-E. contributed data; J.H.,
M.S., R.R., K.L., D.W.L., and M.M.-E. analyzed data; and J.H. wrote
the paper.
The authors declare no conflict of interest.
© 2006 by The National Academy of Sciences of the USA
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