Climate hypersensitivity to solar forcing?

We compare the equilibrium climate responses of a quasi-dynamical energy ba lance model to radiative forcing by equivalent changes in CO2, solar total irradiance (S-tot) and solar UV (S-UV) The response is largest in the S-UV case, in which the imposed UV radiative forcing is preferentially absorbed in the layer above 250 mb, in contrast to the weak response from global-col umnar radiative loading by increases in CO2 or S-tot. The hypersensitive re sponse of the climate system to solar UV forcing is caused by strongly coup led feedback involving vertical static stability, tropical thick cirrus ice clouds and stratospheric ozone. This mechanism offers a plausible explanat ion of the apparent hypersensitivity of climate to solar forcing, as sugges ted by analyses of recent climatic records. The model hypersensitivity stro ngly depends on climate parameters, especially cloud radiative properties, but is effective for arguably realistic values of these parameters. The pro posed solar forcing mechanism should be further confirmed using other model s (e.g., general circulation models) that may better capture radiative and dynamical couplings of the troposphere and stratosphere.