Abstract
This chapter provides a synopsis of aerosol observations, source inventories, and the theoretical understanding required to enable an assessment of radiative forcing from aerosols and its uncertainty. The chemical and physical properties of aerosols are needed to estimate and predict direct and indirect climate forcing. Aerosols are liquid or solid particles suspended in the air. They have a direct radiative forcing because they scatter and absorb solar and infrared radiation in the atmosphere. Aerosols also alter warm, ice and mixed-phase cloud formation processes by increasing droplet number concentrations and ice particle concentrations. They decrease the precipitation efficiency of warm clouds and thereby cause an indirect radiative forcing associated with these changes in cloud properties. Aerosols have most likely made a significant negative contribution to the overall radiative forcing. An important characteristic of aerosols is that they have short atmospheric lifetimes and therefore cannot be considered simply as a long-term offset to the warming influence of greenhouse gases.