Climate sensitivity

In Intergovernmental Panel on Climate Change (IPCC) reports, equilibrium climate sensitivity refers to the equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO₂ concentration. This value is estimated, by the IPCC Fourth Assessment Report 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 Report, which said it was "likely to be in the range of 1.5 to 4.5°C" [1]. More generally, equilibrium climate sensitivity refers to the equilibrium change in surface air temperature following a unit change in radiative forcing, expressed in units of °C/(W/m²). In practice, the evaluation of the equilibrium climate sensitivity from models requires very long simulations with coupled global climate models, or it may be deduced from observations.

Climate sensitivity is not the same as the expected climate change at, say 2100: the TAR reports this to be an increase of 1.4 to 5.8°C over 1990.

Essentials

CO₂ climate sensitivity has a component directly due to CO₂'s radiative forcing, 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."

Radiative forcing due to doubled CO₂

In the 1979 NAS report (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). In 2001 the IPCC adopted the revised value of 3.7 W/m² due to a "stratospheric temperature adjustment".[2]

Sample calculation using industrial-age data

Rahmstorf (2008) provides an informal example of how climate sensitivity might be estimated. Suppose that the sensitivity, i.e. the increase in temperature due to the feedbacks as well as to CO₂ forcing, is x °C. If empirically the world experiences a temperature change of ΔT due to a forcing of ΔF, then we can say that (ΔT °C)/(x °C) = (ΔF W/m²)/(3.7 W/m²), i.e. that x = ΔT * 3.7/ΔF. The global temperature increase since the late 19th century is 0.8 °C, and the radiative forcing due to CO₂ emitted since that time is 2.6 W/m². However, ΔF also contains contributions due to solar activity (+0.3 W/m²), aerosols (-1 W/m²), and ocean heat uptake (-0.6 W/m²), all of which brings the total forcing ΔF to 1.3 W/m², producing a value for x of 2.3 degrees. (All numbers are approximate and somewhat 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".

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 Jule Charney, who in 1979 had convened a committee to report to the National Academy of Sciences on anthropogenic global warming. Only two sets of models were available; one, due to Syukuro Manabe, exhibited a climate sensitivity of 2°C, the other, due to James 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 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."

Other estimates

Gregory et al. (2002) estimate 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 20^th century. Recent work by Annan and Hargreaves [3] combines independent observational and model based estimates to produce a mean of about 3°C, and only a 5% chance of exceeding 4.5°C. A general discussion of some recent work is given here.

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 2.0°C (ranging between 0.9°C and 2.9°C at 99% confidence) if there is no cosmic-ray climate connection, or a typical sensitivity of 1.3°C (between 0.9°C and 2.5°C at 99% confidence), if the cosmic-ray climate link is real. More on climate sensitivity and this work can be found here.

Andronova and Schlesinger (2001) found that it could lie between 1 and 10°C, with a 54 percent likelihood that the climate sensitivity lies outside the IPCC range [4]. 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.

Frame et al. (2005) and Allen et al. note that the size of the confidence limits are dependent on the nature of the prior assumptions made.

Related concepts

The Transient climate response (TCR) - a term first used in the TAR - is the temperature change at the time of CO₂ doubling in a run with CO₂ increasing at 1%/year.

The effective climate sensitivity is a related measure that circumvents this requirement. It is evaluated from model output for evolving non-equilibrium conditions. It is a measure of the strengths of the feedbacks at a particular time and may vary with forcing history and climate state. Details are discussed in Section 9.2.1 of Chapter 9 in the TAR [5].

A "long-term sensitivity" can be defined which includes the effects of slower feedbacks.[6]

References

• Andronova, N., and M. E. Schlesinger. 2001. Objective Estimation of the Probability Distribution for Climate Sensitivity. J. Geophys. Res. 106, D19, 22605. [7] data: [8]
• Annan, J.D., and J. C.Hargreaves, 2006. Using multiple observationally-based constraints to estimate climate sensitivity. Geophysical Research Letters 33, L06704, 2006 [9] (preprint)
• Forest, C.E., P.H. Stone, A.P. Sokolov, M.R. Allen, and M.D. Webster, 2002. Quantifying uncertainties in climate system properties with the use of recent observations. Science, 295. 24 (preprint)
• Frame, D.J., B.B.B. Booth, J.A. Kettleborough, D.A. Stainforth, J.M. Gregory, M. Collins, and M.R. Allen, 2005. Constraining climate forecasts: the role of prior assumptions. Geophysical Research Letters, 32, L09702, doi:10.1029/2004GL022241. [10]
• Gregory J.M., R.J. Stouffer, S.C.B. Raper, P.A. Stott, and N.A. Rayner, 2002. "An observationally based estimate of the climate sensitivity. Journal of Climate, 15, 3117. [11]
• Shaviv, N.J., 2005. On climate response to changes in the cosmic ray flux and radiative budget. J. Geophys. Res. 110, A08105. [12] (preprint)

External links

• Estimates of climate sensitivity, 1896-2006