Human activities release greenhouse gases into the atmosphere. The atmospheric concentrations of carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and tropospheric ozone have all increased over the past century. These rising levels of greenhouse gases are expected to cause climate change. By absorbing infrared radiation, these gases change the natural flow of energy through the climate system. The climate must somehow adjust to this "thickening blanket" of greenhouse gases to maintain the balance between energy arriving from the sun and energy escaping back into space. This relatively simple picture is complicated by increased amounts of sulfate aerosol from human activities that modulate incoming solar radiation and tend to cause a cooling effect on climate, at least on regional and hemispheric scales.

Global mean surface air temperatures have increased by 0.3-0.6�C since the late 19th century, and recent years have been among the warmest on record. Any human-induced effect on climate, however, is superimposed on natural climate variability resulting from climate fluctuations (e.g., El Ni�o) and external causes such as solar variability and volcanic eruptions. More sophisticated approaches are now being applied to the detection and attribution of the causes of change in climate by looking, for example, for spatial patterns expected from climate-forcing change by greenhouse gases and aerosols. To date, the balance of the evidence suggests that there is a discernible human influence on the global climate.

Model projections of future climate, based on the present understanding of climate processes and using emission scenarios (IS92) based on a range of economic and technological assumptions, estimate a rise in global mean temperature of 1-3.5�C (best estimate 2�C) between 1990 and 2100. In all cases, the average rate of warming would probably be greater than any in the past 10,000 years, though actual annual-to-decadal changes would include considerable natural variability. A general warming is expected to lead to an increase in the occurrence of extremely hot days and a decrease in the occurrence of extremely cold days. Regional temperature changes could differ substantially from the global mean value, and there are many uncertainties about the scale and impacts of climate change, particularly at the regional level. The mean sea level is expected to rise 15-95 cm (best estimate 50 cm) by 2100, with some flooding of low-lying areas. Forests, deserts, rangelands, and other unmanaged ecosystems would face new climatic stresses, partly as a result of changes in the hydrological cycle; many could decline or fragment, with some individual species of flora or fauna becoming extinct. Because of the delaying effect of the oceans, surface temperatures do not respond immediately to greenhouse gas emissions, so climate change would continue for many decades even if atmospheric concentrations were stabilized.

Achieving stabilized atmospheric concentrations of greenhouse gases would demand a major effort. For CO2 alone, freezing global emissions at their current rates would result in a doubling of its atmospheric concentrations from pre-industrial levels soon after 2100. Eventually, emissions would have to decrease well below current levels for concentrations to stabilize at doubled CO2 levels, and they would have to continue to fall thereafter to maintain a constant CO2 concentration. The radiative forcing of greenhouse gas levels (including methane, nitrous oxide, and others, but not aerosols) could equal that caused by a doubling of pre-industrial CO2 concentrations by 2030 and a trebling or more by 2100.

The international community is tackling this challenge through the United Nations Framework Convention on Climate Change (UNFCCC). Adopted in 1992, the Convention seeks to stabilize atmospheric concentrations of greenhouse gases at safe levels. More than 170 countries have become Parties to the Convention. Developed countries have agreed to take voluntary measures aimed at returning their emissions to 1990 levels by the year 2000, with further legally binding emissions cuts after the year 2000 proposed at Kyoto in late 1997. Developed countries have also agreed to promote financial and technological transfers to developing countries to help them address climate change.

Source: IPCC, 1996a.

Although ozone can be measured throughout much of the atmosphere, most of it is found in the stratosphere in a layer centered about 20 km above the Earth's surface. Stratospheric ozone is beneficial to life on Earth because it blocks much of the dangerous ultraviolet light (UV-B) radiated by the sun. If unnaturally high levels of UV-B radiation reach the Earth's surface, many forms of life can be harmed. For instance, UV-B can cause skin cancers in humans and may reduce crop yields.

Natural ozone amounts in the stratosphere result from a balance of production and loss processes involving chemistry, meteorology, and solar radiation. Since the 1960s, however, increases in atmospheric concentrations of human-generated chlorine- and bromine-containing compounds (principally chlorofluorocarbons and halons) have caused additional ozone loss. This trend has resulted in declines in stratospheric ozone amounts at middle and high latitudes in both hemispheres. The most dramatic manifestation is the Antarctic ozone hole, where more than half of the ozone is destroyed in a 6-week period each spring. In recent winters, similar features-but with half the ozone loss (20-30%)-have been observed over the Arctic, and at northern mid-latitudes a long-term decline of 5-10% has occurred over the past 20-30 years. Annual amounts of biologically active UV-B radiation have increased by about 10% over mid-latitudes since 1979. No significant loss of ozone or increase in UV-B radiation has been found in the tropics.

Concern that chlorofluorocarbons might destroy ozone was first raised in the 1970s. Following the general realization that these human-generated chemicals posed a real threat to the ozone layer, the Vienna Convention for the Protection of the Ozone Layer was adopted in 1985. Shortly afterwards, the Antarctic ozone hole was discovered, leading to renewed pressure to control ozone-depleting substances. In 1987, the Montreal Protocol on Substances that Deplete the Ozone Layer was agreed; it has since been ratified by more than 160 countries. Initially, the Montreal Protocol imposed clear limits on the future production of chlorofluorocarbons and halons only; as scientific evidence about ozone depletion has mounted, however, the Protocol has been modified to include other chemicals.

As a direct result of these controls, there have been marked reductions in emissions of these substances into the atmosphere; assuming full compliance with the Protocol, these reductions will result in reduced atmospheric amounts of chlorine and bromine. However, the magnitude of the ozone loss at any given time depends on a number of factors. Although the amount of chlorine and bromine clearly is important, other influences include the temperature of the ozone layer, the atmosphere's chemical composition, and long-term changes in atmospheric circulation related to climate change. How all of these factors evolve over coming decades will determine future ozone amounts as chlorine and bromine are reduced.

Source: WMO, 1999.