CR 23:1-9 (2002)  -  doi:10.3354/cr023001

ABSTRACT: Temperature projections for the 21st century made in the Third Assessment Report (TAR) of the United Nations Intergovernmental Panel on Climate Change (IPCC) indicate a rise of 1.4 to 5.8°C for 1990-2100. However, several independent lines of evidence suggest that the projections at the upper end of this range are not well supported. Since the publication of the TAR, several findings have appeared in the scientific literature that challenge many of the assumptions that generated the TAR temperature range. Incorporating new findings on the radiative forcing of black carbon (BC) aerosols, the magnitude of the climate sensitivity, and the strength of the climate/carbon cycle feedbacks into a simple upwelling diffusion/energy balance model similar to the one that was used in the TAR, we find that the range of projected warming for the 1990-2100 period is reduced to 1.1-2.8°C. When we adjust the TAR emissions scenarios to include an atmospheric CO₂ pathway that is based upon observed CO₂ increases during the past 25 yr, we find a warming range of 1.5-2.6°C prior to the adjustments for the new findings. Factoring in these findings along with the adjusted CO₂ pathway reduces the range to 1.0-1.6°C. And thirdly, a simple empirical adjustment to the average of a large family of models, based upon observed changes in temperature, yields a warming range of 1.3-3.0°C, with a central value of 1.9°C. The constancy of these somewhat independent results encourages us to conclude that 21st century warming will be modest and near the low end of the IPCC TAR projections.

KEY WORDS: Temperature projections · Climate change · Global warming · Climate models · Impact assessment