Estimated Climate Sensitivity Constrained by Temperature Reconstructions Over the Past Seven Centuries
The magnitude and impact of future global climate change depends on the sensitivity of the climate system to changes in greenhouse gas concentrations. The commonly accepted range for the equilibrium global mean temperature change in response to a doubling of the atmospheric carbon dioxide concentration, termed climate sensitivity, is 1.5° – 4.5°C. A number of observational studies, however, find a substantial probability of significantly higher sensitivities, yielding upper limits on climate sensitivity of 7.7 °C to above 9°C. In the April 20 issue of Nature, DOE-sponsored researchers Hegerl et al. demonstrate that observational estimates of climate sensitivity can be tightened if reconstructions of Northern Hemisphere temperature over the past several centuries are considered. Using large-ensemble energy balance modeling to simulate the temperature response to past solar, volcanic and greenhouse gas forcing, they determine which climate sensitivities yield simulations that are in agreement with proxy reconstructions. After accounting for the uncertainty in reconstructions and estimates of past external forcing, they find an independent estimate of climate sensitivity that is very similar to those from instrumental data. If the latter are combined with the result from all proxy reconstructions, then the 5% -95 % range of climate sensitivity shrinks to 1.5 ° 6.2 °C, thus substantially reducing the probability of very high climate sensitivity.
Gabriele C. Hegerl, Thomas J. Crowley, William T. Hyde & David J. Frame: Climate sensitivity constrained by temperature reconstructions over the past seven centuries. Nature, 2006 DOI:10.1038/nature04679