The Global Hydrological Cycle and Atmospheric Shortwave Absorption in Climate Models under CO₂ Forcing

• Published in: Journal of climate Vol. 22; no. 21; pp. 5667 - 5675
• Main Author: Takahashi, Ken
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 01-11-2009
• Subjects: Absorption; Atmosphere; Atmospheric absorption; Atmospheric models; Atmospherics; Budgets; Carbon dioxide; Climate; Climate change; Climate cycles; Climate models; Climate prediction; Earth, ocean, space; Energy; Energy conservation; Enthalpy; Equivalence; Exact sciences and technology; Experiments; External geophysics; Fluid dynamics; General circulation models; Global climate; Global warming; Greenhouse gases; Heat; Heat flux; Heat transfer; Heating; Hydrologic cycle; Hydrological cycle; Hydrology; Intercomparison; Latent heat; Latent heat flux; Meteorology; Modeling; Physics; Radiative transfer; Radiative transfer models; Sensible heat; Sensible heat flux; Sensible heat transfer; Standard deviation; Studies; Surface chemistry; Temperature; Water cycle; Water vapor; Water vapor content; Water vapor distribution; Water vapour; sensible heat; short period waves; Coupled model; Long wave; atmospheric precipitation; radiative transfer; Carbon dioxide; climate warming; Climate models; global; Forecast model; greenhouse gas; Dynamical climatology; Clear sky; hydrologic cycle; Energy conservation; water vapor; global change; water content; Forcing; heat transfer; latent heat; climate change
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/2009jcli2674.1

The spread among the predictions by climate models for the strengthening of the global hydrological cycle [i.e., the global mean surface latent heat flux (LH), or, equivalently, precipitation] at a given level of CO₂-induced global warming is of the same magnitude as the intermodel mean. By comparing several climate models from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) database under idealized CO₂ forcings, it is shown that differences in the increase in global atmospheric shortwave heating (SWabs) induced by clear-sky absorption, presumably by water vapor, partly explains this spread. The increases in SWabsand LH present similar spreads across models but are anticorrelated, so the sum SWabs+LH increases more robustly than either alone. This is consistent with a recently proposed theory (Takahashi) that predicts that this sum (or, equivalently, the net longwave divergenceminus the surface sensible heat flux) is constrained by energy conservation and robust longwave physics.
The intermodel scatter in SWabschanges is explained neither by differences in the radiative transfer models nor in intermodel differences in global water vapor content change, but perhaps by more subtle aspects of the changes in the water vapor distribution. Nevertheless, the fact that the radiative transfer models generally underestimate the increase in SWabsrelative to the corresponding line-by-line calculation for a given change in water vapor content suggests that the climate models might be overestimating the rate of increase in the global hydrological cycle with global warming.