What kind of information from the climate science community is the most useful for policy
makers, and which uncertainties matter most? Constraining climate sensitivity -
the long-term increase in global mean temperature expected from the doubling of atmospheric
carbon dioxide - has been one of the main benchmark goals of climate science.
However uncertainties in our estimates of climate sensitivity have not lessened substantially in
past decades. Both models and observations yield broad probability distributions for climate
sensitivity, with small but finite probabilities of very large increases. We show that the shape
of these probability distributions is an inevitable and general consequence of the nature of the
climate system. Further, we show that the breadth of the distribution and, in particular, the
probability of large temperature increases are relatively insensitive to decreases in
uncertainties associated with the underlying climate processes.
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