IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis

7.5 Aerosol Particles and the Climate System

Aerosols are an integral part of the atmospheric hydrological cycle and the atmosphere’s radiation budget, with many possible feedback mechanisms that are not yet fully understood. This section assesses (1) the impact of meteorological (climatic) factors like wind, temperature and precipitation on the natural aerosol burden and (2) possible effects of aerosols on climate parameters and biogeochemistry. The most easily understood interaction between aerosols and climate is the direct effect (scattering and absorption of shortwave and thermal radiation), which is discussed in detail in Chapter 2. Interactions with the hydrological cycle, and additional impacts on the radiation budget, occur through the role of aerosols in cloud microphysical processes, as aerosol particles act as cloud condensation nuclei (CCN) and ice nuclei (IN). The suite of possible impacts of aerosols through the modification of cloud properties is called ‘indirect effects’. The forcing aspect of the indirect effect at the top of the atmosphere is discussed in Chapter 2, while the processes that involve feedbacks or interactions, like the ‘cloud lifetime effect’[6], the ‘semi-direct effect’ and aerosol impacts on the large-scale circulation, convection, the biosphere through nutrient supply and the carbon cycle, are discussed here.

7.5.1 Aerosol Emissions and Burdens Affected by Climatic Factors

Most natural aerosol sources are controlled by climatic parameters like wind, moisture and temperature. Hence, human-induced climate change is also expected to affect the natural aerosol burden. The sections below give a systematic overview of the major natural aerosol sources and their relations to climate parameters while anthropogenic aerosol emissions and combined aerosols are the subject of Chapter 2.

  1. ^  The processes involved are more complex than can be encompassed in a single expression. The term ‘cloud lifetime effect’ thus should be understood to mean that aerosols can change precipitation efficiency in addition to increasing cloud albedo.