  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.
