Climate change — страница 4

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over longer periods also occur). They also include volcanic eruptions, injecting huge clouds of sulfur-containing gases, which tend to cool the Earth's surface and atmosphere over a few years. On the human-induced side, the balance can be changed by emissions from land-use changes and industrial practices that add or remove "heat-trapping" or "greenhouse" gases, thus changing atmospheric absorption of radiation. Greenhouse gases of policy significance include carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); the chlorofluorocarbons (CFCs) and their substitutes, including hydrofluorocarbons (HFCs); the long-lived fully fluorinated hydrocarbons, such as perfluorocarbons (PFCs); and ozone (O3). Although most of these gases occur naturally (the exceptions

are the CFCs, their substitutes, and the long-lived PFCs), the concentrations of all of these gases are changing as a result of human activities. For example, the atmospheric concentration of carbon dioxide has risen about 30 percent since the 1700s--an increase responsible for more than half of the enhancement of the trapping of the infrared radiation due to human activities. In addition to their steady rise, many of these greenhouse gases have long atmospheric residence times (several decades to centuries), which means that atmospheric levels of these gases will return to preindustrial levels only if emissions are sharply reduced, and even then only after a long time. Internationally accepted science indicates that increasing concentrations of greenhouse gases will raise

atmospheric and oceanic temperatures and could alter associated weather and circulation patterns. In a report synthesizing its second assessment and focusing on the relevance of its scientific analyses to the ultimate objective of the Convention, the IPCC concluded: Human activities--including the burning of fossil fuels, land use, and agriculture--are changing the atmospheric composition. Taken together, they are projected to lead to changes in global and regional climate and climate-related parameters, such as temperature, precipitation, and soil moisture. Some human communities--particularly those with limited access to mitigating technologies--are becoming more vulnerable to natural hazards and can be expected to suffer significantly from the impacts of climate-related

changes, such as high-temperature events, floods, and droughts, potentially resulting in fires, pest outbreaks, ecosystem loss, and an overall reduction in the level of primary productivity. The IPCC also concluded that, given the current trends in emissions, global concentrations of greenhouse gases are likely to grow significantly through the next century and beyond, and the adverse impacts from these changes will become greater. The remainder of this report seeks to elucidate the programs, policies, and measures being taken in the United States to begin moving away from this trend of increasing emissions, and to help move the world away from the trend of globally increasing concentrations of greenhouse gases. Principal Conclusions of the IPCC's Second Assessment Report While

the basic facts about the science of climate have been understood and broadly accepted for years, new information is steadily emerging--and influencing the policy process. In 1995, the IPCC released its Second Assessment Report, which not only validated most of the IPCC's earlier findings, but because of the considerable new work that had been undertaken during the five years since its previous full-scale assessment, broke new ground. The report is divided into three sections: physical sciences related to climate impacts; adaptation and mitigation responses; and cross-cutting issues, including economics and social sciences. The Climate Science Human activities are changing the atmospheric concentrations and distributions of greenhouse gases and aerosols. Global average

temperatures have increased about 0.3-0.6°C (about 0.5-1.0°F) over the last century. The ability of climate models to simulate observed trends has improved--although there is still considerable regional uncertainty with regard to changes. The balance of evidence suggests there is a discernible human influence on global climate. Aerosol sulfates (a component of acid rain) offset some of the warming by greenhouse gases. The IPCC mid-range scenario projects an increase of 2.0°C (3.7°F) by 2100 (with a range of 1.0-3.5°C (about 1.8-6.3°F). The average global warming projected in the IPCC mid-range scenario is greater than any seen in the last ten thousand years. Sea level is projected to rise (due to thermal expansion of the oceans, and melting of glaciers and ice sheets) by