# Climate sensitivity

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Climate sensitivity

Climate sensitivity is a measure of how responsive the temperature of the climate system is to a change in the radiative forcing. 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) CO2 concentration (ΔTx2). This value is estimated, by the IPCC Fourth Assessment Report (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.[1] This is a slight change from the IPCC Third Assessment Report (TAR), which said it was "likely to be in the range of 1.5 to 4.5 °C".[2] More recent work continues to support a best-guess value around 3 °C.[3]

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 CO2 doubling in a transient simulation with CO2 increasing at 1% per year.[4] 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".[5] [6]

A less commonly used concept, the long-term sensitivity, can be defined which includes the effects of slower feedbacks.[7]

Although climate sensitivity is usually used in the context of radiative forcing by CO2, it is thought of as a general property of the climate system: the change in surface air temperature (ΔTs) following a unit change in radiative forcing (RF) and expressed in units of °C/(W/m2). For this to be so, the measure must be independent of the nature of the forcing (e.g. from greenhouse gases or solar variation); 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.

$\Delta T_s = \lambda \cdot RF$

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 TAR forecasts this to be an increase of 1.4 to 5.8 °C over 1990.

## Essentials

CO2 climate sensitivity has a component directly due to radiative forcing by CO2 (or any other change in Earth's radiative balance), and a further contribution arising from feedbacks, positive and negative. "Without any feedbacks, a doubling of CO2 (which amounts to a forcing of 3.7 W/m2) 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 feedback, the cloud feedback, and the lapse rate feedback";[8] 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[9] (p. 7), the radiative forcing due to doubled CO2 is estimated to be 4 W/m2, as calculated (for example) in Ramanathan et al. (1979).[10] In 2001 the IPCC adopted the revised value of 3.7 W/m2, the difference attributed to a "stratospheric temperature adjustment".[11] More recently an intercomparison of radiative transfer codes (Collins et al., 2006)[12] showed substantial discrepancies among climate models and between climate models and more exact radiation codes in the forcing attributed to doubled CO2 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 CO2 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)[8] 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 CO2 (taken as 3.7 W/m2), as x °C. If Earth were to experience an equilibrium temperature change of ΔT (°C) due to a sustained forcing of ΔF (W/m2), then one might say that x/(ΔT) = (3.7 W/m2)/(ΔF), i.e. that x = ΔT * (3.7 W/m2)/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 CO2 and other long-lived greenhouse gases (mainly methane, nitrous oxide, and chlorofluorocarbons) emitted since that time is about 2.6 W/m2. 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/m2), aerosols (-1 W/m2), ozone (0.3 W/m2) and other lesser influences, bringing the total forcing over the industrial period to 1.6 W/m2 according to best estimate of the IPCC AR4, 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/m2)/(ΔF-H). Taking planetary heat uptake rate as the rate of ocean heat uptake, estimated by the IPCC AR4 as 0.2 W/m2, 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 interglacial (no ice age) periods. The change in temperature, revealed in ice core samples, is 5 °C, while the change in solar forcing is 7.1 W/m2. The computed climate sensitivity is therefore 5/7.1 = 0.7 K(W/m2)−1. We can use this empirically derived climate sensitivity to predict the temperature rise from a forcing of 4 W/m2, arising from a doubling of the atmospheric CO2 from pre-industrial levels. The result is a predicted temperature increase of 3 °C."[13] Based on analysis of uncertainties in total forcing, in Antarctic cooling, and in the ratio of global to Antarctic cooling of the last glacial maximum 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.[14]

## History of the concept

### 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 Sciences and chaired by Jule Charney. Only two sets of models were available; one, due to Syukuro Manabe, 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..."[15]

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."[9]

### Subsequent developments

In 2008 climatologist Stefan Rahmstorf 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."[8]

### Other estimates

Idso, (1998)[16] reprinted in[17] 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.[18] 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)[19] 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)[20] 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)[21] 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 link is real. (Note Shaviv quotes a radiative forcing equivalent of 3.8Wm−2. [ΔTx2=3.8 Wm−2 λ].)

Frame, et al. (2005)[22]] 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)[23] 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)[24] 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)[25] determined climate sensitivity within a major part of the Phanerozoic. 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.[26]

## References

1. ^ IPCC (2007). "2.3 Climate sensitivity and feedbacks". In Pachauri, R.K and Reisinger, A. (Eds.). Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Geneva, Switzerland: Intergovernmental Panel on Climate Change. Retrieved 2010-07-03
2. ^ Prentice, I.C. et al (2001). "Technical Summary: F.3 Projections of Future Changes in Temperature". In Houghton J.T. et al (Eds.). Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Geneva, Switzerland: Intergovernmental Panel on Climate Change. Retrieved 2010-07-03
3. ^ Knutti, Reto; Hegerl, Gabriele C. (2008-10-26). "The equilibrium sensitivity of the Earth's temperature to radiation changes". Nature Geoscience 1 (11): 735–743. doi:10.1038/ngeo337. Retrieved 2010-07-03.
4. ^ Randall, D.A., et al (2007). "8.6.2 Interpreting the Range of Climate Sensitivity Estimates Among General Circulation Models, In: Climate Models and Their Evaluation.". In Solomon, S., D. et al (Eds.). Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. Retrieved 2010-07-03
5. ^ Prentice, I.C. et al (2001). "9.2.1 Climate Forcing and Climate Response, in chapter 9. Projections of Future Climate Change". In Houghton J.T. et al (Eds.). Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.. ISBN 9780521807678. Retrieved 2010-07-03, 2011-10-05(url)
6. ^ Solomon, S., D. et al (Eds.) (2007). "Glossary A-D, Climate sensitivity". Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.. Retrieved 2011-10-05
7. ^ "Target CO2". RealClimate. April 2008.
8. ^ a b c Rahmstorf, Stefan (2008). "Anthropogenic Climate Change: Revisiting the Facts". In Zedillo, E. (PDF). Global Warming: Looking Beyond Kyoto. Brookings Institution Press. pp. 34–53.
9. ^ a b Ad Hoc Study Group on Carbon Dioxide and Climate (1979). "Carbon Dioxide and Climate: A Scientific Assessment" (PDF). National Academy of Sciences.
10. ^ V. Ramanathan, M.S. Lian, and R.D. Cess (1979) (PDF). Increased Atmospheric CO2: Zonal and Seasonal Estimates of the Effect on Radiative Energy Balance and Surface Temperature. Journal of Geophysical Research.
11. ^ O. Boucher, et al (2001). "6.3.1 Carbon Dioxide in: Chapter 6 Radiative Forcing of Climate Change". In Houghton J.T. et al (Eds).. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. Retrieved 2010-07-03
12. ^ Collins, W. D., et al. (2006). "Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4)". J. Geophys. Res. 111 (D14317): D14317. Bibcode 2006JGRD..11114317C. doi:10.1029/2005JD006713. Retrieved 2010-07-03.
13. ^ John Farley (2008). "The Scientific Case for Modern Anthropogenic Global Warming". Monthly Review.
14. ^ Ganopolski, A., and T. Schneider von Deimling (2008). "Comment on "Aerosol radiative forcing and climate sensitivity deduced from the Last Glacial Maximum to Holocene transition" by Petr Chylek and Ulrike Lohmann". Geophys. Res. Lett. 35 (L23703): L23703. Bibcode 2008GeoRL..3523703G. doi:10.1029/2008GL033888. Retrieved 2010-07-03.
15. ^ Richard A. Kerr (13 August 2004). "Three Degrees of Consensus". Science 305 (5686): 932–4. doi:10.1126/science.305.5686.932. PMID 15310873.
16. ^ Idso, Sherwood (1998). "CO2-induced global warming: A skeptic’s view of potential climate change". Climate Research 10: 69–82. doi:10.3354/cr010069.
17. ^ Gerhard, L.C.; Harrison, W.E.; Hanson, W.E. (2001). "Geological perspectives of global climate change". AAPG Special Publication SG47: 317–336.
18. ^ Andronova, N.; Schlesinger, M. E. (2001). "Objective Estimation of the Probability Distribution for Climate Sensitivity". J. Geophys. Res. 106 (D19): 22605. Bibcode 2001JGR...10622605A. doi:10.1029/2000JD000259.  as PDF data
19. ^ Forest, C.E.; Stone, P.H.; Sokolov, A.P.; Allen, M.R.; Webster, M.D. (2002). "Quantifying uncertainties in climate system properties with the use of recent observations". Science 295 (5552): 113–7. doi:10.1126/science.1064419. PMID 11778044.  as PDF
20. ^ Gregory, J.M.; Stouffer, R.J.; Raper, S.C.B.; Stott, P.A.; Rayner, N.A. (2002). "An observationally based estimate of the climate sensitivity". Journal of Climate 15 (22): 3117. doi:10.1175/1520-0442(2002)015<3117:AOBEOT>2.0.CO;2.  as PDF
21. ^ Shaviv, N.J. (2005). "On climate response to changes in the cosmic ray flux and radiative budget". J. Geophys. Res. 110: A08105. Bibcode 2005JGRA..11008105S. doi:10.1029/2004JA010866.  (preprint)
22. ^ Frame, D.J.; Booth, B.B.B.; Kettleborough, J.A.; Stainforth, D.A.; Gregory, J.M.; Collins, M.; Allen, M.R. (2005). "Constraining climate forecasts: the role of prior assumptions". Geophysical Research Letters 32 (9): L09702. Bibcode 2005GeoRL..3209702F. doi:10.1029/2004GL022241.
23. ^ Annan, J.D.; Hargreaves, J. C. (2006). "Using multiple observationally-based constraints to estimate climate sensitivity". Geophysical Research Letters 33 (6): L06704. Bibcode 2006GeoRL..3306704A. doi:10.1029/2005GL025259.  as PDF
24. ^ Forster, Piers M. de F.; Gregory, Jonathan M. (2006). "The Climate Sensitivity and Its Components Diagnosed from Earth Radiation Budget Data". Journal of Climate 19 (1): 39–52. doi:10.1175/JCLI3611.1.
25. ^ Royer, Dana L.; Berner, Robert A.; Park, Jeffrey (29 March 2007). "Climate sensitivity constrained by CO2 concentrations over the past 420 million years". Nature 446 (7135): 530–2. doi:10.1038/nature05699. PMID 17392784.
26. ^ Sceptics as Jan Veizer have pointed out that while data for the whole Phanerozoic are available Royer et al. left out the time span younger than 420 Ma with an ice age and extremely high carbon dioxide content during the Hirnantian.

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