7.1.3 Atmospheric Chemistry and Climate

Interactions between climate and atmospheric oxidants, including ozone, provide important coupling mechanisms in the Earth system. The concentration of tropospheric ozone has increased substantially since the pre-industrial era, especially in polluted areas of the world, and has contributed to radiative warming. Emissions of chemical ozone precursors (carbon monoxide, CH₄, non-methane hydrocarbons, nitrogen oxides) have increased as a result of larger use of fossil fuel, more frequent biomass burning and more intense agricultural practices. The atmospheric concentration of pre-industrial tropospheric ozone is not accurately known, so that the resulting radiative forcing cannot be accurately determined, and must be estimated from models. The decrease in concentration of stratospheric ozone in the 1980s and 1990s due to manufactured halocarbons (which produced a slight cooling) has slowed down since the late 1990s. Model projections suggest a slow steady increase over the next century, but continued recovery could be affected by future climate change. Recent changes in the growth rate of atmospheric CH₄ and in its apparent lifetime are not well understood, but indications are that there have been changes in source strengths. Nitrous oxide continues to increase in the atmosphere, primarily as a result of agricultural activities. Changes in atmospheric chemical composition that could result from climate changes are even less well quantified. Photochemical production of the hydroxyl radical (OH), which efficiently destroys many atmospheric compounds, occurs in the presence of ozone and water vapour, and should be enhanced in an atmosphere with increased water vapour, as projected under future global warming. Other chemistry-related processes affected by climate change include the frequency of lightning flashes in thunderstorms (which produce nitrogen oxides), scavenging mechanisms that remove soluble species from the atmosphere, the intensity and frequency of convective transport events, the natural emissions of chemical compounds (e.g., biogenic hydrocarbons by the vegetation, nitrous and nitric oxide by soils, DMS from the ocean) and the surface deposition on molecules on the vegetation and soils. Changes in the circulation and specifically the more frequent occurrence of stagnant air events in urban or industrial areas could enhance the intensity of air pollution events. The importance of these effects is not yet well quantified.

7.1.4 Aerosol Particles and Climate

Atmospheric aerosol particles modify Earth’s radiation budget by absorbing and scattering incoming solar radiation. Even though some particle types may have a warming effect, most aerosol particles, such as sulphate (SO₄) aerosol particles, tend to cool the Earth surface by scattering some of the incoming solar radiation back to space. In addition, by acting as cloud condensation nuclei, aerosol particles affect radiative properties of clouds and their lifetimes, which contribute to additional surface cooling. A significant natural source of sulphate is DMS, an organic compound whose production by phytoplankton and release to the atmosphere depends on climatic factors. In many areas of the Earth, large amounts of SO₄ particles are produced as a result of human activities (e.g., coal burning). With an elevated atmospheric aerosol load, principally in the Northern Hemisphere (NH), it is likely that the temperature increase during the last century has been smaller than the increase that would have resulted from radiative forcing by greenhouse gases alone. Other indirect effects of aerosols on climate include the evaporation of cloud particles through absorption of solar radiation by soot, which in this case provides a positive warming effect. Aerosols (i.e., dust) also deliver nitrogen (N), phosphorus and iron to the Earth’s surface; these nutrients could increase uptake of CO₂ by marine and terrestrial ecosystems.