Working Group I: The Scientific Basis

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5.1 Introduction

Aerosols have a direct radiative forcing because they scatter and absorb solar and infrared radiation in the atmosphere. Aerosols also alter the formation and precipitation efficiency of liquid- water, ice and mixed-phase clouds, thereby causing an indirect radiative forcing associated with these changes in cloud properties.

The quantification of aerosol radiative forcing is more complex than the quantification of radiative forcing by greenhouse gases because aerosol mass and particle number concentrations are highly variable in space and time. This variability is largely due to the much shorter atmospheric lifetime of aerosols compared with the important greenhouse gases. Spatially and temporally resolved information on the atmospheric burden and radiative properties of aerosols is needed to estimate radiative forcing. Important parameters are size distribution, change in size with relative humidity, complex refractive index, and solubility of aerosol particles. Estimating radiative forcing also requires an ability to distinguish natural and anthropogenic aerosols.

The quantification of indirect radiative forcing by aerosols is especially difficult. In addition to the variability in aerosol concentrations, some quite complicated aerosol influences on cloud processes must be accurately modelled. The warm (liquid- water) cloud indirect forcing may be divided into two components. The first indirect forcing is associated with the change in droplet concentration caused by increases in aerosol cloud condensation nuclei. The second indirect forcing is associated with the change in precipitation efficiency that results from a change in droplet number concentration. Quantification of the latter forcing necessitates understanding of a change in cloud liquid-water content and cloud amount. In addition to warm clouds, ice clouds may also be affected by aerosols.

5.1.1 Advances since the Second Assessment Report

Considerable progress in understanding the effects of aerosols on radiative balances in the atmosphere has been made since the IPCC WGI Second Assessment Report (IPCC, 1996) (hereafter SAR). A variety of field studies have taken place, providing both process-level understanding and a descriptive understanding of the aerosols in different regions. In addition, a variety of aerosol networks and satellite analyses have provided observations of regional differences in aerosol characteristics. Improved instrumentation is available for measurements of the chemical composition of single particles.

Models of aerosols have significantly improved since the SAR. Because global scale observations are not available for many aerosol properties, models are essential for interpolating and extrapolating available data to the global scale. Although there is a high degree of uncertainty associated with their use, models are presently the only tools with which to study past or future aerosol distributions and properties.

The very simplest models represent the global atmosphere as a single box in steady state for which the burden can be derived if estimates of sources and lifetimes are available. This approach was used in early assessments of the climatic effect of aerosols (e.g., Charlson et al., 1987, 1992; Penner et al., 1992; Andreae, 1995) since the information and modelling tools to provide a spatially- and temporally-resolved analysis were not available at the time. At the time of the SAR, three-dimensional models were only available for sulphate aerosols and soot. Since then, three-dimensional aerosol models have been developed for carbonaceous aerosols from biomass burning and from fossil fuels (Liousse et al., 1996; Cooke and Wilson, 1996; Cooke et al., 1999), dust aerosols (Tegen and Fung, 1994; Tegen et al., 1996), sea salt aerosol (Gong et al., 1998) and nitrate and ammonia in aerosols (Adams et al., 1999, 2001; Penner et al., 1999a). In this report, the focus is on a temporally and spatially resolved analysis of the atmospheric concentrations of aerosols and their radiative properties.

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