Everyone has seen the graph presented by the Intergovernmental Panel for Climate Change (IPCC) illustrating radiative forcing caused by various human-induced perturbations. This diagram illustrates the globally averaged watts per square meter of forcing produced since the pre-industrial era in response to changing atmospheric concentrations of carbon dioxide and other "well mixed" greenhouse gases, stratospheric and tropospheric ozone, and aerosols.

This diagram is somewhat misleading because it does not differentiate between the forcings that are global (e.g., CO₂) and those that clearly have a regional nature (e.g., aerosols). Furthermore, the total forcing (derived by adding the best estimates of IPCC) is not significantly larger than the uncertainty associated with the estimated total forcing. If the geographical inhomogeneity in the radiative forcing is taken into account, the picture becomes even more complicated and uncertain. An important task for the scientific community, therefore, is to define a research agenda that rapidly leads to a clearer picture and, hence, a clearer message for decision-makers.

One major uncertainty is associated with the direct and indirect radiative effects of aerosols. In early climate models this type of forcing was ignored, but more recent investigations have attempted to account for the effects of sulfate particles and other aerosols (dust, soot, etc.). Even this approach remains an oversimplification because in many cases, aerosols are multi-component particles with a chemical composition that can vary in space and time.

Recent field campaigns (e.g., ACE-1, ACE-2, TARFOX and others) have shown the important role of aerosols for the climate system. Local and even regional effects much larger (20 Wm^–2) than the globally averaged forcing (a few Wm^–2) have been reported, which could modify our current understanding of human-induced climate change. These particles also contribute dramatically to regional pollution, which has become a major issue in many areas of Asia and Africa, especially during the dry season.

Much work involving the atmospheric chemistry and climate communities is needed before fundamental issues related to the microphysics and chemistry of aerosols are fully resolved. It is therefore important that funding agencies recognize the urgency of the problem. Field campaigns such as the IGAC-sponsored ACE-Asia project (which is about to start) will provide new information regarding aerosol formation and fate in eastern Asia, a region where aerosol sources are exceptionally high. The solution to the aerosol/climate question requires a comprehensive approach including laboratory studies, coordinated field campaigns, space observations and modeling.

The present issue of the IGAC Newsletter provides a synthetic view of some important issues related to cloud chemistry and the indirect effects of aerosols.