Effects of surface temperature and clouds on the CO2 forcing

Abstract

CO2 forcing is defined as the initial change in heating rate, with no feedbacks included, that is the direct response to an increase in the CO2 concentration in the atmosphere. We have conducted a study of the effects of surface temperature and clouds on the CO2 forcing, based on use of the Colorado State University general circulation model. We report results from a pair of perpetual July simulations in which the sea surface temperatures differ by 4 K. The precipitable water is about 1.5 times larger in the warm run. The increased water vapor concentration amplifies the radiative effects of CO2, leading to greater CO2 forcing in the warm run. In the colder run the globally averaged reduction in upward longwave radiation due to a doubling of CO2 is 4.3 W m−2 at the level of maximum forcing, or the “CO2 tropopause.” Above and below this level the CO2 forcing decreases, resulting in a net tropospheric warming of 3.3×10−2 K day−1, and a net stratospheric cooling. In the warm run the CO2 forcing at the CO2 tropopause is 4.6 W m−2, and is associated with a tropospheric warming of 4.×10−2 K day−1. The clear‐sky CO2 forcing at the CO2 tropopause is 5.0 W m−2 in the cold run, and 5.2 W m−2 in the warm run. By blocking infrared radiation that would otherwise be blocked by CO2, the clouds reduce the CO2 forcing of the surface‐troposphere system by 0.66 W m−2 in the cold run, and by 0.59 W m−2 in the warm run. Our results for the CO2 forcing are model‐dependent, of course. Every GCM‐based study of CO2‐induced climate change produces a CO2 forcing, however, and the warming scenarios generated depend very directly on this forcing. A thorough investigation of the CO2 forcings produced by GCMs is thus a rather basic prerequisite for understanding the climate change predictions produced by the models.