U . S . Senate Committee on Commerce , Science , and Transportation Hearing on ‘ The Case for Climate Change Action ’ October 1 , 2003

Projections of future climate change made using state-of-the-art climate models suggest that changes over the coming century will be much larger than experienced over the past 100 years. The case for taking action to mitigate these human-induced (or ‘anthropogenic’) changes rests on the credibility of these models. There is a vast scientific literature on the development and testing of these models, summarized in the recent ‘Third Assessment Report’ (henceforth ‘TAR’) produced under the auspices of Working Group 1 of the Intergovernmental Panel on Climate Change (IPCC, Houghton et al., 2001). There are two main methods of model testing – comparing model simulations of the present state of the climate system (such as the geographical patterns of temperature, rainand snowfall, sealevel pressure, etc.) against observations, and comparing model simulations of past changes in climate with observations.