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Figure 5.16. Same as in Figure 5.15 , except for the cloud-radiation feedback mechanism
(from Curry et al., 1996 , by permission of AMS).
5.11.4 Water Vapor Feedback
Water vapor feedback is fairly straightforward in principle. The amount of water
vapor that the atmosphere can carry increases with increasing temperature. Assuming
there is water available to evaporate, an initial warming will result in an increase
in humidity. However, as water vapor is a greenhouse gas, the increase in humid-
ity leads to further warming. Like albedo feedback, the strength of the water vapor
feedback is a key determiner of the planet's climate sensitivity - that is, the change
in global average surface air temperature in response to a climate forcing of a given
magnitude. As discussed, the Arctic is now experiencing an outsized warming com-
pared the Northern Hemisphere as a whole. Both radiosonde data and output from
atmospheric reanalyses point to positive trends in column-integrated water vapor
over the Arctic over the past several decades, largest in summer and early autumn,
and most pronounced in the northern North Atlantic, including the Greenland,
Norwegian and Barents seas, the Canadian Arctic Archipelago and (on the Pacific
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