ScienceWeek

Science 26 October 2007:
Vol. 318. no. 5850, pp. 582 - 583
DOI: 10.1126/science.1149988

http://scienceweek.com/2007/071028e.htm

Atmosphere: A Reappraisal of Climate Sensitivity

Myles R. Allen

Over the past 30 years, the climate research community has made valiant efforts to answer the "climate sensitivity" question: What is the long-term equilibrium warming response to a doubling of atmospheric carbon dioxide? Earlier this year, the Intergovernmental Panel on Climate Change (1) concluded that this sensitivity is likely to be in the range of 2° to 4.5°C, with a 1-in-3 chance that it is outside that range. The lower bound of 2°C is slightly higher than the 1.6°C proposed in the 1970s (2); progress on the upper bound has been minimal.

On page 629 of this issue, Roe and Baker (3) explain why. The fundamental problem is that the properties of the climate system that we can observe now do not distinguish between a climate sensitivity, S, of 4°C and S > 6°C. In a sense, this should be obvious: Once the world has warmed by 4°C, conditions will be so different from anything we can observe today (and still more different from the last ice age) that it is inherently hard to say when the warming will stop. Roe and Baker formalize the problem by showing how a symmetric constraint on the strength of the feedback parameter f (which determines how much energy is radiated to space per degree of surface warming) gives a strongly asymmetric constraint on S. The reason is simple: As f approaches 1, S approaches infinity. Roe and Baker illustrate the point with the information provided by recent analyses of observed climate change, atmospheric feedbacks, and "perturbed physics" experiments in which uncertain parameters are varied in climate models.

It might be objected that some models that displayed high sensitivities in perturbed physics experiments also poorly reproduce the energy budget at the top of the atmosphere (4) and hence perform poorly in short-term climate forecasts (5). Likewise, the fact that direct studies of atmospheric feedbacks provide only a weak constraint on S does not mean that no stronger constraint is possible. But these objections miss Roe and Baker's main point: The fact that uncertainties in climate processes add up to give an approximately Gaussian uncertainty in f means that there are innumerable ways of generating a climate model with f close to unity and hence a very high S. Ruling all of these out requires us to find observable quantities that are consistently related to S in all physically plausible climate models, and to show that observations of these quantities are inconsistent with a high S. Despite much searching, such observations remain elusive.

BOOK SOURCES:

global warming
climate change
IPCC
atmospheric science