Uncertainties in projections of future climate change have not lessened substantially in past decades. Both models and observations yield broad probability distributions for long-term increases in global mean temperature expected from the doubling of atmospheric carbon dioxide, 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, and we derive a simple analytic form for the shape that fits recent published distributions very well. 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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Recent studies have shown that global radiative and hydrological fluxes are strongly linked to microphysical processes in clouds. The sensitivity of predictions of climate variations to assumptions about the microphysical processes has led to new approaches to atmospheric measurements and to heightened interest and progress in understanding the physical chemistry, radiative properties, and kinetics of small solid and liquid aqueous particles.