Historical evolution of radiative forcing of climate

We have compiled the evolution of the radiative forcing for several mechanisms based on our radiative transfer models using a variety of information sources to establish time histories. The anthropogenic forcing mechanisms considered are well-mixed greenhouse gases, ozone, and tropospheric aerosols (direct and indirect effect). The natural forcing mechanisms taken into account are the radiative effects of solar irradiance variation and particles of volcanic origin. In general there has been an increase in the radiative forcing during the 20th century. The exception is a decline in the radiative forcing in the 1945-1970 period. We have found that the evolution of anthropogenic particle emissions in the same period may have been a major cause of this decline in the forcing. We have discussed uncertainties in the various forcings and their evolution. The uncertainties are large for many forcing mechanisms, especially the impact of anthropogenic aerosols. In particular the indirect effect of aerosols on clouds is difficult to quantify. Several evolutions of their effect may have bean possible, strongly influencing the evolution of the total anthropogenic radiative forcing. (C) 2001 Elsevier Science Ltd. All rights reserved.