Global climate changes and rising tropospheric ozone levels may exacerbate urban air pollution problems. Projections based on some SRES scenarios show increases in tropospheric ozone of more than 40 ppb over most of the Northern Hemisphere mid-latitudes. Such increases would approximately double the baseline levels of ozone entering many metropolitan regions, substantially degrading air quality. Climate change would affect the meteorological conditions (regional temperature, cloud cover, and surface wind) that influence photochemistry, and the occurrence of major pollution episodes. While warmer temperatures would generally contribute to more urban ozone, the change in frequency and intensity of pollution episodes has not been evaluated. Adverse health effects attributable to urban air quality would be exacerbated by increases in heat waves that would accompany anthropogenic climate change.

The sulfate aerosols formed from sulfur emissions from the burning of fossil fuels lead to both acid deposition and a cooling of the climate system. Acid deposition has adverse impacts on both terrestrial and aquatic ecosystems and causes damage to human health and many materials. Some of these impacts could be exacerbated by climate change (e.g., through increase in humidity and temperature). Actions to reduce sulfur emissions have been taken in many countries, and declines in sulfate deposition have been observed in some regions in recent years (see Table 8-3). In the SRES scenarios, this situation has led to projections of future sulfate aerosol abundances that are lower than those in the SAR. This has led, in turn, to less negative projections for the radiative forcing by aerosols, hence less of a cooling effect to offset the greenhouse gas-induced warming.

Stratospheric Ozone Depletion and Climate Change