Climate sensitivity is a measure of how responsive the temperature of the climate system is to a change in the
radiative forcingIn climate science, radiative forcing is generally defined as the change in net irradiance between different layers of the atmosphere. Typically, radiative forcing is quantified at the tropopause in units of watts per square meter. A positive forcing tends to warm the system, while a negative...
. It is usually expressed as the temperature change associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere.
The
equilibrium climate sensitivity refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric (equivalent)
CO2Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
concentration (ΔT
x2). This value is estimated, by the
IPCC Fourth Assessment ReportClimate Change 2007, the Fourth Assessment Report of the United Nations Intergovernmental Panel on Climate Change , is the fourth in a series of reports intended to assess scientific, technical and socio-economic information concerning climate change, its potential effects, and options for...
(
AR4) as
likely to be in the range 2 to 4.5 °C with a best estimate of about 3 °C, and is very unlikely to be less than 1.5 °C. Values substantially higher than 4.5 °C cannot be excluded, but agreement of models with observations is not as good for those values. This is a slight change from the
IPCC Third Assessment ReportThe IPCC Third Assessment Report, Climate Change 2001, is an assessment of available scientific and socio-economic information on climate change by the IPCC. The IPCC was established in 1988 by the United Nations Environment Programme and the UN's World Meteorological Organization ".....
(
TAR), which said it was "likely to be in the range of 1.5 to 4.5 °C". More recent work continues to support a best-guess value around 3 °C.
A model estimate of equilibrium sensitivity thus requires a very long model integration. A measure requiring shorter integrations is the
transient climate sensitivity which is defined as the average temperature response over a twenty year period centered at CO
2 doubling in a transient simulation with CO
2 increasing at 1% per year. The transient sensitivity is lower than the equilibrium sensitivity, due to the "inertia" of ocean heat uptake. Fully equilibrating ocean temperatures would require integrations of thousands of model years.
An estimate of the equilibrium climate sensitivity may be made from combining the effective climate sensitivity with the known properties of the ocean reservoirs and the surface heat fluxes; this is the
effective climate sensitivity. This "may vary with forcing history and climate state".
A less commonly used concept, the
long-term sensitivity, can be defined which includes the effects of slower feedbacks.
Although climate sensitivity is usually used in the context of radiative forcing by CO
2, it is thought of as a general property of the climate system: the change in surface air temperature (ΔT
s) following a unit change in
radiative forcingIn climate science, radiative forcing is generally defined as the change in net irradiance between different layers of the atmosphere. Typically, radiative forcing is quantified at the tropopause in units of watts per square meter. A positive forcing tends to warm the system, while a negative...
(RF) and expressed in units of °C/(W/m
2). For this to be so, the measure must be independent of the nature of the forcing (e.g. from greenhouse gases or
solar variationSolar variation is the change in the amount of radiation emitted by the Sun and in its spectral distribution over years to millennia. These variations have periodic components, the main one being the approximately 11-year solar cycle . The changes also have aperiodic fluctuations...
); to first order this is indeed found to be so.
For a coupled atmosphere-ocean global climate model the climate sensitivity is an emergent property: it is not a model parameter, but rather a result of a combination of model physics and parameters. By contrast, simpler energy-balance models may have climate sensitivity as an explicit parameter.
The terms represented in the equation relate radiative forcing of any cause to linear changes in global surface temperature change.
It is also possible to estimate climate sensitivity from observations; however, this is difficult due to uncertainties in the forcing and temperature histories.
Climate sensitivity can be a useful summary of the sensitivity of the real climate, or of a given model climate. But it is not the same as the expected climate change at, say 2100: the
TARThe IPCC Third Assessment Report, Climate Change 2001, is an assessment of available scientific and socio-economic information on climate change by the IPCC. The IPCC was established in 1988 by the United Nations Environment Programme and the UN's World Meteorological Organization ".....
forecasts this to be an increase of 1.4 to 5.8 °C over 1990.
Essentials
CO
2 climate sensitivity has a component directly due to radiative forcing by CO
2 (or any other change in Earth's radiative balance), and a further contribution arising from
feedbackFeedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
s, positive and negative. "Without any feedbacks, a doubling of CO
2 (which amounts to a forcing of 3.7 W/m
2) would result in 1 °C global warming, which is easy to calculate and is undisputed. The remaining uncertainty is due entirely to feedbacks in the system, namely, the water vapor feedback, the
ice-albedo feedbackIce-albedo feedback is a positive feedback climate process where a change in the area of snow-covered land, ice caps, glaciers or sea ice alters the albedo. This change in albedo acts to reinforce the initial alteration in ice area...
, the
cloud feedbackCloud feedback is the coupling between cloudiness and surface air temperature in which a change in radiative forcing perturbs the surface air temperature, leading to a change in clouds, which could then amplify or diminish the initial temperature perturbation....
, and the lapse rate feedback"; addition of these feedbacks leads to a value of approximately 3 °C ± 1.5 °C.
Radiative forcing due to doubled CO2
In the 1979 NAS report (p. 7), the radiative forcing due to doubled CO
2 is estimated to be 4 W/m
2, as calculated (for example) in
RamanathanVeerabhadran Ramanathan is Victor Alderson Professor of Applied Ocean Sciences and director of the Center for Atmospheric Sciences at the Scripps Institution of Oceanography, University of California, San Diego. He has contributed to many areas of the atmospheric sciences including developments to...
et al. (1979). In 2001 the IPCC adopted the revised value of 3.7 W/m
2, the difference attributed to a "stratospheric temperature adjustment". More recently an intercomparison of radiative transfer codes (Collins et al., 2006) showed substantial discrepancies among climate models and between climate models and more exact radiation codes in the forcing attributed to doubled CO
2 even in cloud-free sky; presumably the differences would be even greater if forcing were evaluated in the presence of clouds because of differences in the treatment of clouds in different models. Undoubtedly the difference in forcing attributed to doubled CO
2 in different climate models contributes to differences in apparent sensitivities of the models, although this effect is thought to be small relative to the intrinsic differences in sensitivities of the models themselves
(Webb et al., 2006).
Sample calculation using industrial-age data
Rahmstorf (2008) provides an informal example of how climate sensitivity might be estimated empirically, from which the following is modified. Denote the sensitivity, i.e. the equilibrium increase in global mean temperature including the effects of feedbacks due to a sustained forcing by doubled CO
2 (taken as 3.7 W/m
2), as
x °C. If Earth were to experience an equilibrium temperature change of ΔT (°C) due to a sustained forcing of ΔF (W/m
2), then one might say that
x/(ΔT) = (3.7 W/m
2)/(ΔF), i.e. that
x = ΔT * (3.7 W/m
2)/ΔF. The global temperature increase since the beginning of the industrial period (taken as 1750) is about 0.8 °C, and the radiative forcing due to CO
2 and other long-lived greenhouse gases (mainly methane, nitrous oxide, and chlorofluorocarbons) emitted since that time is about 2.6 W/m
2. Neglecting other forcings and considering the temperature increase to be an equilibrium increase would lead to a sensitivity of about 1.1 °C. However, ΔF also contains contributions due to solar activity (+0.3 W/m
2), aerosols (-1 W/m
2), ozone (0.3 W/m
2) and other lesser influences, bringing the total forcing over the industrial period to 1.6 W/m
2 according to best estimate of the
IPCC AR4Climate Change 2007, the Fourth Assessment Report of the United Nations Intergovernmental Panel on Climate Change , is the fourth in a series of reports intended to assess scientific, technical and socio-economic information concerning climate change, its potential effects, and options for...
, albeit with substantial uncertainty. Additionally the fact that the climate system is not at equilibrium must be accounted for; this is done by subtracting the planetary heat uptake rate H from the forcing; i.e.,
x = ΔT * (3.7 W/m
2)/(ΔF-H). Taking planetary heat uptake rate as the rate of ocean heat uptake, estimated by the
IPCC AR4Climate Change 2007, the Fourth Assessment Report of the United Nations Intergovernmental Panel on Climate Change , is the fourth in a series of reports intended to assess scientific, technical and socio-economic information concerning climate change, its potential effects, and options for...
as 0.2 W/m
2, yields a value for
x of 2.1 °C. (All numbers are approximate and quite uncertain.)
Sample calculation using ice-age data
"... examine the change in temperature and solar forcing between glaciation (ice age) and
interglacialAn Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...
(no ice age) periods. The change in temperature, revealed in
ice coreAn ice core is a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. As the ice forms from the incremental build up of annual layers of snow, lower layers are older than upper, and an ice...
samples, is 5 °C, while the change in solar forcing is 7.1 W/m
2. The computed climate sensitivity is therefore 5/7.1 = 0.7 K(W/m
2)
−1. We can use this empirically derived climate sensitivity to predict the temperature rise from a forcing of 4 W/m
2, arising from a doubling of the atmospheric CO
2 from pre-industrial levels. The result is a predicted temperature increase of 3 °C."
Based on analysis of uncertainties in total forcing, in Antarctic cooling, and in the ratio of global to Antarctic cooling of the
last glacial maximumThe Last Glacial Maximum refers to a period in the Earth's climate history when ice sheets were at their maximum extension, between 26,500 and 19,000–20,000 years ago, marking the peak of the last glacial period. During this time, vast ice sheets covered much of North America, northern Europe and...
relative to the present, Ganopolski and Schneider von Deimling (2008) infer a range of 1.3 to 6.8 °C for climate sensitivity determined by this approach.
Three degrees as the consensus estimate
The standard modern estimate of climate sensitivity – 3 °C, plus or minus 1.5 °C – originates with a committee on anthropogenic global warming convened in 1979 by the
National Academy of SciencesThe National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...
and chaired by Jule Charney. Only two sets of models were available; one, due to
Syukuro Manabeis a Japanese meteorologist and climatologist who pioneered the use of computers to simulate global climate change and natural climate variations.-Scientific accomplishments:...
, exhibited a climate sensitivity of 2 °C, the other, due to James E. Hansen, exhibited a climate sensitivity of 4 °C. "According to Manabe,
Charney chose 0.5 °C as a not-unreasonable margin of error, subtracted it from Manabe’s number, and added it to Hansen’s. Thus was born the 1.5 °C-to-4.5 °C range of likely climate sensitivity that has appeared in every greenhouse assessment since..."
Chapter 4 of the "Charney report" compares the predictions of the models: "We conclude that the predictions ... are basically consistent and mutually supporting. The differences in model results are relatively small and may be accounted for by differences in model characteristics and simplifying assumptions."
Subsequent developments
In 2008 climatologist
Stefan RahmstorfStefan Rahmstorf is a German oceanographer and climatologist. Since 2000, he has been a Professor of Physics of the Oceans at Potsdam University. He received his Ph.D. in oceanography from Victoria University of Wellington...
wrote, regarding the Charney report's original range of uncertainty: "At that time, this range was on very shaky ground. Since then, many vastly improved models have been developed by a number of climate research centers around the world. Current state-of-the-art climate models span a range of 2.6–4.1 °C, most clustering around 3 °C."
A 2011 paper foreseen for
ScienceScience is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...
by Schmittner et al suggests that the rate of global warming from doubling of atmospheric carbon dioxide may be less than projected by the Intergovernmental Panel on Climate Change report in 2007. Sensitivity values would insfar move downwards to 1.7 - 2.6 C°.
Other estimates
Idso, (1998) reprinted in calculated based on eight natural experiments a λ of 0.1 °C/(Wm
−2) resulting in a climate sensitivity of only 0.4 °C for a doubling of the concentration of CO2 in the atmosphere.
Andronova and Schlesinger (2001) found that the climate sensitivity could lie between 1 and 10 °C, with a 54 percent likelihood that it lies outside the IPCC range. The exact range depends on which factors are most important during the instrumental period: "At present, the most likely scenario is one that includes anthropogenic sulfate aerosol forcing but not solar variation. Although the value of the climate sensitivity in that case is most uncertain, there is a 70 percent chance that it exceeds the maximum IPCC value. This is not good news." said Schlesinger.
Forest,
et al. (2002) using patterns of change and the MIT EMIC estimated a 95% confidence interval of 1.4–7.7 °C for the climate sensitivity, and a 30% probability that sensitivity was outside the 1.5 to 4.5 °C range.
Gregory,
et al. (2002) estimated a lower bound of 1.6 °C by estimating the change in Earth's radiation budget and comparing it to the global warming observed over the 20th century.
Shaviv (2005) carried out a similar analysis for 6 different time scales, ranging from the 11-yr solar cycle to the climate variations over geological time scales. He found a typical sensitivity of 0.54±0.12°K/(W m
−2) or 2.1 °C (ranging between 1.6 °C and 2.5 °C at 99% confidence) if there is no cosmic-ray climate connection, or a typical sensitivity of 0.35±0.09°K/(W m
−2) or 1.3 °C (between 0.99 °C and 2.5 °C at 99% confidence), if the
cosmic-ray climate linkSolar variation is the change in the amount of radiation emitted by the Sun and in its spectral distribution over years to millennia. These variations have periodic components, the main one being the approximately 11-year solar cycle . The changes also have aperiodic fluctuations...
is real. (Note Shaviv quotes a radiative forcing equivalent of 3.8Wm
−2. [ΔT
x2=3.8 Wm
−2 λ].)
Frame,
et al. (2005)] and
Allen et al. noted that the range of the confidence limits is dependent on the nature of the prior assumptions made.
Annan and Hargreaves (2006) presented an estimate that resulted from combining prior estimates based on analyses of paleoclimate, responses to volcanic eruptions, and the temperature change in response to forcings over the twentieth century. They also introduced a triad notation (L, C, H) to convey the probability distribution function (pdf) of the sensitivity, where the central value C indicates the maximum likelihood estimate in degrees Celsius and the outer values L and H represent the limits of the 95% confidence interval for a pdf, or 95% of the area under the curve for a likelihood function. In this notation their estimate of sensitivity was (1.7, 2.9, 4.9)°C.
Forster and Gregory (2006) presented a new independent estimate based on the slope of a plot of calculated greenhouse gas forcing minus top-of-atmosphere energy imbalance, as measured by satellite borne radiometers, versus global mean surface temperature. In the triad notation of Annan and Hargreaves their estimate of sensitivity was (1.0, 1.6, 4.1)°C.
Royer,
et al. (2007) determined climate sensitivity within a major part of the
PhanerozoicThe Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
. The range of values—1.5 °C minimum, 2.8 °C best estimate, and 6.2 °C maximum—is, given various uncertainties, consistent with sensitivities of current climate models and with other determinations.
See also
- Climate change feedback
Climate change feedback is important in the understanding of global warming because feedback processes may amplify or diminish the effect of each climate forcing, and so play an important part in determining the overall climate sensitivity...
- Glossary of climate change
This article serves as a glossary of climate change terms. It lists terms that are related to global warming.- 0-9 :* 100,000-year problem - a discrepancy between the climate response and the forcing from the amount of incoming solar radiation....
- Index of climate change articles
- Global warming controversy
Global warming controversy refers to a variety of disputes, significantly more pronounced in the popular media than in the scientific literature, regarding the nature, causes, and consequences of global warming...
- Runaway climate change
Runaway climate change describes a theoretical scenario in which the climate system passes a threshold or tipping point, after which internal positive feedback effects cause the climate to continue changing without further external forcings...
- Tipping point (climatology)
A climate tipping point is a point when global climate changes from one stable state to another stable state, in a similar manner to a wine glass tipping over. After the tipping point has been passed, a transition to a new state occurs...
External links
- Estimates of climate sensitivity, 1896-2006 (archived)
- Theoretical framework for modeling the climate sensitivity of a black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...
earth, a "gray body" earth, and a gray-body earth with feedbacks affecting albedoAlbedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
and emissivityThe emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...