Tuesday, 11 January 2005: 2:15 PM
Global climate sensitivity
Historically, the global climate sensitivity, defined as the equilibrium globally averaged surface air temperature change for a doubling of atmospheric CO2, has been listed as 1.5C to 4.5C, with an equal probability of the actual sensitivity being anywhere in that range. In the IPCC Third Assessment Report (TAR), a new quantity was introduced called the transient climate response (TCR) which was defined as the globally averaged surface air temperature change in a transient 1% per year CO2 increase experiment in a global coupled climate model at the time of CO2 doubling. The range of TCR in the TAR was 1.1C to 3.1C. The reduced range of TCR compared to the canonical equilibrium range was ascribed to the compensatory effects of ocean heat uptake and climate sensitivity in the coupled climate models. However, climate system response to radiative forcing related to climate sensitivity still presents uncertainties regarding quantification of future climate change, and will be addressed again in the upcoming IPCC Fourth Assessment Report (AR4). Consequently, an IPCC workshop was convened in Paris in July 2004, to address this issue. The new generation of climate models appears to show a narrowing of the equilibrium sensitivity range due to improved understanding of physical processes that translates into better parameterizations in the models, in particular related to clouds. But all the new model results are not yet in hand, and higher sensitivities cannot necessarily be ruled out yet. Several other techniques have been explored recently to provide more information regarding climate sensitivity. Two groups have run large numbers of perturbed parameter ensembles in an attempt to ascribe probabilities to possible climate sensitivity values from models. Additionally, paleoclimate evidence and analyses of observed data, in particular related to volcanic eruptions, have contributed to a better understanding of what the actual climate sensitivity may be. The combination of all these lines of evidence points to a more probable value of climate sensitivity near the middle of the previously listed range.