Further comment on “A perspective on global warming, dimming, and brightening”



In a recent Eos Forum article, Stanhill [2007] suggested that there is an unexplained inconsistency between “global dimming” and global warming. This contention is mistaken.

Stanhill's argument is based on select local observations that show reductions in the surface shortwave downwelling radiation (SWD) from the 1950s to the 1980s of about 20 watts per square meter (W/m2) [Stanhill and Cohen, 2005]. Stanhill errs in equating these surface flux changes (the “dimming”) with the radiative forcing from anthropogenic greenhouse gases (currently about 2.6 W/m2). Radiative forcing is defined at the tropopause (not the surface) specifically to provide a good prediction for eventual model responses. Changes in aerosols and clouds result in large changes in SWD, but they do not translate into similarly large changes in tropopause forcing since the aerosols and clouds absorb energy in addition to reflect it.