Nonlinear aspects of the climate response to greenhouse gas and aerosol forcing

In a series of equilibrium experiments the climate response to present-day radiative forcings of anthropogenic greenhouse gases and aerosol particles is calculated. The study was performed with a model system consisting of the ECHAM4 atmospheric general circulation model coupled to a slab ocean and thermodynamic sea ice model. The model includes transport of the relevant chemical constituents, a sulfur chemistry model that calculates sulfate production in the gas and aqueous phase, and an aerosol model that accounts for source and sink processes. The aerosol cycle, the hydrological cycle, and the atmospheric dynamics are fully interactive. The climate response to aerosol forcing is not just a mirror image of the response to greenhouse forcing. This applies to the temperature changes, which are regionally more uniform for greenhouse forcing than for aerosol forcing as is already well known, and, in particular, to the hydrological cycle: the global hydrological sensitivity (Deltaprecip/Deltatemp) to a 1-K surface temperature change is almost 3 times higher for aerosol forcing than for greenhouse forcing. When both forcings are combined, a global warming is simulated while evaporation and precipitation decrease by about 2% K(-1), resulting in a negative hydrological sensitivity. A strong dependency of the response to the type of forcing has also been found for the cloud water content and, consequently, for the change in cloud radiative forcing, which is substantially larger in the combined forcing experiment than in either of the individual forcing experiments. Consequently, the global warming for combined forcing is significantly smaller (0.57 K) than that obtained by adding the individual changes (0.85 K). Due to feedbacks between temperature changes and the hydrological cycle the simulated aerosol load, applying the same source strength, is considerably lower in a warmer climate (-17% K(-1) warming). A consequence of this aerosol-temperature feedback could be that a future increase in greenhouse gases may reduce the aerosol burden even if the source strength would not change.