Climate Change 2001:
Synthesis Report
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Figure 8-1: Climate is controlled by geochemical processes and cycles resulting from the interplay among the environment's components involved, as affected by human action. The scheme shows some of these issues. For simplicity, the single double-ended arrows between issues represent some of the linkages involved. For example, biological and ecological processes play an important role in modulating the Earth's climate at both regional and global scale by controlling the amounts of water vapor and other greenhouse gases that enter into or are depleted from the atmosphere. Changes in climate affect the boundaries, composition, and functioning of ecological systems, such as forests, and changes in the structure and functioning of forests affect the Earth's climate system through changes in the biogeochemical cycles, particularly cycles of carbon, nitrogen, and water. There are other linkages such as the connection between air quality and forestry, directly or through acid precipitation, which for simplicity are not shown here.


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.

WGI TAR Sections 4.4.4 & 4.5-6, & WGII TAR Sections & 9.6
  Acid Deposition and 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.

WGI TAR Sections, 5.5.3, 6.7, & 6.15, WGII TAR Sections 5.6, 5.7.3, &, & SRES Section 3.6.4

Stratospheric Ozone Depletion and Climate Change

8.10 Depletion of the stratospheric ozone layer leads to an increased penetration of UV-B radiation and to a cooling of the climate system. Ozone depletion has allowed for increased penetration of UV-B radiation, with harmful effects on human and animal health, plants, etc. During the last 2 decades, the observed losses of stratospheric ozone have decreased the downward infrared emissions to the troposphere from the (now colder) lower stratosphere. Stratospheric ozone depletion has also altered tropospheric ozone concentrations, and, by allowing more ultraviolet sunlight into the troposphere, it has led to more rapid photochemical destruction of CH4 thereby reducing its radiative forcing. These effects lead also to a cooling of the climate system.

WGI TAR Sections 4.2.2 & 6.4
8.11 Many of the halocarbons that cause depletion of the ozone layer are also important greenhouse gases. Chlorofluorocarbons, for example, add a notable fraction to the total positive radiative forcing since the pre-industrial era. The negative radiative forcing from the associated stratospheric ozone depletion (noted above) reduces this by about half.
The Montreal Protocol will eventually eliminate both of these radiative-forcing contributions. However, one class of substitutes for the now-banned chlorofluorocarbons is hydrofluorocarbons, which are among the greenhouse gases listed under the Kyoto Protocol. This overlap can give rise to a potential conflict beween the goals of the two Protocols.

WGI TAR Sections 4.2.2 & 6.3.3
8.12 Climate change will alter the temperature and wind patterns of the stratosphere, possibly enhancing chlorofluorocarbon depletion of stratospheric ozone over the next 50 years. Increases in greenhouse gases lead in general to a colder stratosphere, which alters stratospheric chemistry. Some studies predict that current rates of climate change will result in significant increases in the depletion of the Arctic stratospheric ozone layer over the next decade before chlorofluorocarbon concentrations have declined substantially. Although many climate/ozone-layer feedbacks have been identified, no quantitative consensus is reached in this assessment.
WGI TAR Sections 4.5, 6.4, &

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