2 00 6 Can we believe in high climate sensitivity ?

The climate response to anthropogenic forcing has long been one of the dominant uncertainties in predicting future climate change (Houghton et al., 2001). Many observationally-based estimates of climate sensitivity (S) have been presented in recent years, with most of them assigning significant probability to extremely high sensitivity, such as P (S > 6C) > 5%. However, closer examination reveals that these estimates are based on a number of implausible implicit assumptions. We explain why these estimates cannot be considered credible and therefore have no place in the decision-making process. In fact, when basic probability theory is applied and reasonable assumptions are made, much greater confidence in a moderate value for S (≃ 2.5C) is easily justified, with S very unlikely to be as high as 4.5C.