On the additivity of climate response to anthropogenic aerosols and CO2, and the enhancement of future global warming by carbonaceous aerosols

Abstract

Climate responses to aerosol forcing at present-day and doubled CO2-levels are studied based on equilibrium simulations with the CCM-Oslo atmospheric GCM coupled to a slab ocean. Aerosols interact on-line with meteorology through life-cycling of sulphate and black carbon (BC), and tables for aerosol optics and CCN activation. Anthropogenic aerosols counteract the warming by CO2 through a negative radiative forcing dominated by the indirect effect. Anthropogenic aerosols reduce precipitation by 4%, while CO2 doubling gives a 5% increase, mainly through enhanced convective activity, including a narrower ITCZ. Globally, the aerosol cooling is insensitive to CO2, and the effects of CO2 doubling are insensitive to aerosols. Hence, global climate responses to these sources of forcing are almost additive, although sulphate and BC burdens are slightly increased due to reduced stratiform precipitation over major anthropogenic source regions and a modified ITCZ. Regionally, positive cloud feedbacks give up to 5 K stronger aerosol cooling at present-day CO2 than after CO2 doubling. Aerosol emissions projected for year-2100 (SRES A2) strongly increase BC and change the sign of the direct effect. This results in a 0.3 K warming and 0.1% increase in precipitation compared to the year 2000, thus enhancing the global warming by greenhouse gases. © 2008 The Authors Journal compilation © 2008 Blackwell Munksgaard.