Aerosol-cloud-climate interactions in the climate model CAM-Oslo

A new aerosol module is integrated on-line in the atmospheric GCM CAM-Oslo coupled to a slab ocean for equilibrium climate response studies. The response to an anthropogenic change in aerosols since pre-industrial times is compared with that of a future 63% increased CO2 level. The aerosol module calculates concentrations of sea-salt, mineral dust, sulphate, black carbon (BC) and particulate organic matter (POM). Look-up tables, constructed from first principles, are used to obtain optical parameters and cloud droplet numbers (CDNC) for any given aerosol composition. Anthropogenic aerosols thus produce a global near-surface cooling of 1.94 K and a 5.5% precipitation decrease, including amplifications by positive cloud feedbacks. In comparison, the CO2 increase gives a warming of 1.98 K and a 3.8% precipitation increase, causing slightly reduced sulphate, BC, POM and sea-salt burdens. A minor increase in mineral dust is ascribed to reduced subtropical precipitation downwind of Sahara over the Atlantic Ocean. The modelled indirect effects are probably overestimated, mainly due to neglected natural aerosol components and the diagnostic scheme for CDNC. Adding 15 cm(-3) to CDNC everywhere reduces the indirect forcing from -2.34 to -1.36 Wm(-2), whilst solving a prognostic equation for CDNC reduces it from -2.34 to -1.44 Wm(-2).