Climate model calculations of the impact of aerosols from road transport and shipping

Abstract

Road transport and shipping are copious sources of aerosols, which exert a significant radiative forcing, compared to, for example, the CO2 emitted by these sectors. An advanced atmospheric general circulation model, coupled to a mixed-layer ocean, is used to calculate the climate response to the direct radiative forcing from such aerosols. The cases considered include imposed distributions of black carbon and sulphate aerosols from road transport, and sulphate aerosols from shipping; these are compared to the climate response due to CO2 increases. The difficulties in calculating the climate response due to small forcings are discussed, as the actual forcings have to be scaled by large amounts to enable a climate response to be easily detected. Despite the much greater geographical inhomogeneity in the sulphate forcing, the patterns of zonal and annual-mean surface temperature response (although opposite in sign) closely resembles that resulting from homogeneous changes in CO2. The surface temperature response to black carbon aerosols from road transport is shown to be notably non-linear in scaling applied, probably due to the semi-direct response of clouds to these aerosols. For the aerosol forcings considered here, the most widespread method of calculating radiative forcing significantly overestimates their effect, relative to CO2, compared to surface temperature changes calculated using the climate model.