Introduction

In Chapter 4, we examined evidence suggesting that human production of aerosol particles has local and regional impacts on clouds, precipitation, and atmospheric temperature. In this chapter we examine the evidence indicating potential impacts of anthropogenic aerosol on global climate.

Estimating the effects of aerosol on climate is particularly challenging. We noted in Chapter 8 that the radiative response to aerosol particles varies with size and chemical composition of the particles relative to the wavelength of the incident radiation. Moreover, because most aerosol particles are heterogeneous in structure, some components of the particles are very absorbing while others are reflecting. Table 2-2 from National Research Council (2005) (reproduced as Table 8.3 in Chapter 8) summarizes the major recognized climate forcings of aerosols. As a consequence only gross estimates of the radiative properties can be made. Aerosol particles also have limited lifetimes in the atmosphere. Particles greater than a few micrometers may survive for only a few days, while particles on the order of 0.1μm and less may reside in the lower troposphere for several weeks. This results in pronounced regional and hemispheric variations in aerosol concentrations. Unfortunately, there have been few systematic long-term observations of aerosols, their size spectra, and chemical composition. It is, therefore, usually necessary to resort to a variety of less-than-direct measurements, regional field campaigns, and global models to infer changes in aerosol concentrations and their radiative effects.