A comparison of large scale changes in surface humidity over land in observations andCMIP3 general circulation models

Abstract

Observed changes in the HadCRUH global land surface specific humidity and CRUTEM3surface temperature from 1973 to 1999 are compared to CMIP3 archive climate modelsimulations with 20th Century forcings. Observed humidity increases are proportionatelylargest in the Northern Hemisphere, especially in winter. At the largest spatio-temporal scalesmoistening is close to the Clausius–Clapeyron scaling of the saturated specific humidity (∼7% K − 1). At smaller scales in water-limited regions, changes in specific humidity arestrongly inversely correlated with total changes in temperature. Conversely, in someregions increases are faster than implied by the Clausius–Clapeyron relation.The range of climate model specific humidity seasonal climatology and varianceencompasses the observations. The models also reproduce the magnitude of observedinterannual variance over all large regions. Observed and modelled trends andtemperature–humidity relationships are comparable except for the extratropicalSouthern Hemisphere where observations exhibit no trend but models exhibitmoistening. This may arise from: long-term biases remaining in the observations; therelative paucity of observational coverage; or common model errors. The overalldegree of consistency of anthropogenically forced models with the observationsis further evidence for anthropogenic influence on the climate of the late 20thcentury.