The global hydrological cycle and atmospheric shortwave absorption in climate models under CO₂ forcing.

Ken Takahashi, 2008

Submitted to Journal of Climate: June 10, 2008

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This paper provides further support that the change in the global mean evaporation and precipitation is constrained by the changes in the radiative fluxes. In particular, it suggests that the relevant longwave physics are robust among models and that the differences between them regarding the hydrological cycle changes are associated with shortwave absorption in the atmosphere. The results give some support to the idealized model I proposed earlier.

Abstract

The spread among the predictions by climate models for the rate of strengthening of the global hydrological cycle (i.e. the global mean surface latent heat flux, LH) associated with CO₂-induced global warming is of the same magnitude as the inter-model mean. It is shown that this spread is partly explained by a lack of agreement in the predicted rate of increase in global shortwave atmospheric absorption (SW_abs), related mainly to the absorption by water vapor in the clear-sky atmosphere. The rates of increase in SW_abs and in LH present similar spreads among models but, since they are anticorrelated, the increase in the sum (SW_abs+LH) presents less spread than for each flux separately, as would be expected from robust longwave physics and energy conservation. Since SW_abs is not strongly coupled to the processes controlling the hydrological cycle, this result suggests that the changes in LH for each model could be determined as a residual from the energy budget given the changes in longwave fluxes and in SW_abs which would imply that LH changes are not directly dependent on the details of the hydrological cycle physics in the models.

The inter-model differences in the rate of change in SW_abs can not be solely accounted for by differences in the radiative transfer schemes or in the modeled changes in the global water vapor content, which suggests that subtler aspects of the change in the water vapor distribution might be important for the change in SW_abs However, the results from the RTMIP project suggest that climate models generally underestimate the change in the clear-sky SW_abs by water vapor relative to detailed line-by-line calculations. In the light of the results presented here, this would suggest that the climate models might be overestimating the rate of increase in the global hydrological cycle with global warming.

June, 2008.