How Do Aerosols Affect Earth's Climate and Our Health?

he aim is to better understand this complex system thereby improving computer models that can be used to predict the sensitivity of climate to changes produced by natural phenomena and human activities. One of the most difficult challenges in unraveling the climate puzzle is the role of aerosols in regulating planetary temperature. People from a range of disciplines and backgrounds are working on the multiple dimensions of this scientific problem, including our scientists, engineers from private industry and students and faculty from university and precollege partners. Collectively, knowledge and skills are being gained in the areas of measurement of aerosols from the ground and space, scientific analysis of aerosol type, size, concentration and distribution, applications to public issues concerning health and classroom impact by involving students and teachers in aerosol research. Although aerosols represent only a tiny fraction of the mass of the atmosphere (less than one part per billion), they have the potential to cool climate by reflecting sunlight to space (a direct effect) and indirectly by altering cloud properties. Through processes that mix chemicals in the atmosphere and by providing surfaces on which chemical reactions can occur aerosols can even modify concentrations of greenhouse gases. It is broadly agreed in the scientific community that the average surface temperature of the Earth has increased in the past century by several tenths of a degree (IPCC, 1992; 1994), and it is generally acknowledged that the bulk of this temperature increase is probably the consequence of increasing atmospheric amounts of anthropogenic greenhouse gases. Interpretation of observed climate change is complicated not only by measurement uncertainties but also by the fact that climate can fluctuate naturally without any external forcing. However, if increasing amounts of greenhouse gases were the only forcing agent then global warming would have already emerged above the level of climate fluctuations. Unfortunately, the climate problem is not that simple. Other forcings exist that are sufficiently large that they compete with the forcing provided by greenhouse gases. Thus, unless all of the major climate forcings are measured, it will not be possible to interpret the significance of observed climate change nor to accurately predict future trends. To place the aerosol forcing in perspective, the present day forcing due to greenhouse gases is roughly +2.0 to +2.8 W/m2 while the present day global mean forcing (direct and indirect) due to man-made aerosols is estimated to be between –0.3 to –2.5 W/m2. Unlike greenhouse