Interpretation of high projections for global-mean warming

T. M. L. Wigley

National Center for Atmospheric Research

S. C. B. Raper

Climatic Research Unit, University of East Anglia, Norwich, NR47TJ, U.K., and Alfred-Wegener-Institut for Polar and Marine Research, D-27515 Bremerhaven, Germany


The Intergovernmental Panel on Climate Change (IPCC) has recently released its Third Assessment Report in which new projections are given for global-mean warming in the absence of policies to limit climate change. These projections are based on a new set of emissions scenarios, and incorporate recent advances in the science of climate change. The full warming range over 1990-2100, 1.4-5.8oC, is substantially higher than the range given previously in the IPCC Second Assessment Report, viz. 0.8-3.5oC. Here we interpret the new warming range in probabilistic terms accounting for uncertainties in emissions, the climate sensitivity and the carbon cycle. We show that the probabilities of warming values at both the high and low ends of the range are very low, and that the most likely warming in the absence of climate mitigation policies is in a more restricted range, 1.7-5.0oC with 90% confidence.
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