Climate Science

How Wet Will It Get?

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Science  20 Jan 2012:
Vol. 335, Issue 6066, pp. 264
DOI: 10.1126/science.335.6066.264-d

Theory and models agree that global mean precipitation should increase as global warming continues, but they differ on the extent of that increase, and even different models vary by a factor of 3 in their predictions of how much more rain will fall. Why do models differ so dramatically? Pendergrass and Hartmann examine how black carbon forcing influences the amount of global mean precipitation calculated by models used in the A1b scenario (assuming rapid economic growth and a balance of fossil fuel and non-fossil fuel energy sources) of the IPCC'S AR4, and find that black carbon has a significant effect on clear-sky atmospheric shortwave absorption, which drives precipitation changes, whereas the radiative forcing supplied by CO2 does not substantially affect the model spread. Therefore, they conclude, black carbon forcing differences between models can explain a substantial portion, but not all, of the intermodel spread in global mean precipitation in the A1b scenario of AR4. These results may help reconcile disagreements between models, leaving the differences between models and theory still to be bridged.

Geophys. Res. Lett. 39, L01703 (2012).

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