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Several human and natural activities can change the balance between the energy absorbed by the Earth and that emitted in the form of long-wave infrared radiation. On the natural side, these include changes in solar radiation (the sun's energy varies by small amounts--approximately 0.1 percent over an eleven-year cycle--and variations 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 about 50 centimeters (20 inches) by 2100, with a range of 15-95 centimeters (about 6-38 inches). Even after a stabilization of greenhouse gas concentrations, temperatures would continue to increase for several decades, and sea level would continue to rise for centuries. Vulnerability, Likely Impacts, and Possible Responses Climate change is likely to have wide-ranging and mostly adverse effects on human health. Direct and indirect effects can be expected to lead to increased mortality. Coastal infrastructure is likely to be extremely vulnerable. A 50-centimeter (20-inch) rise in sea level would place approximately 120 million people at risk. Natural and managed ecosystems are also at risk: forests, agricultural areas, and aquatic and marine life are all susceptible. However, adaptation and mitigation options are numerous. Significant reductions in net greenhouse gas emissions are technically possible and can be economically feasible, using an extensive array of technologies and policy measures that accelerate technology development, diffusion, and transfer. Socioeconomic Issues Early mitigation may increase flexibility in moving toward a stabilization of atmospheric concentrations of greenhouse gases. Economic risks of rapid abatement must be balanced against risks of delay. Significant "no regrets" opportunities are available in most countries. Next steps must recognize equity considerations. Costs of stabilization of emissions at 1990 levels in OECD countries could range considerably (from a gain of $60 billion to a loss of about $240 billion) over the next several decades.

National Circumstances

In responding to the threat of global climate change, U.S. policymakers must consider the special circumstances created by a unique blend of challenges and opportunities. The National Circumstances chapter of this report attempts to explain the particular situation in the United States--including its climate, natural resources, population trends, economy, energy mix, and political system--as a backdrop for understanding the U.S. perspective on global climate change.

The United States is unusual in that it encompasses a wide variety of climate conditions within its borders, from subtropical to tundra. This diversity complicates the discussion of impacts of global climate change within the United States because those impacts would vary widely. This diversity also adds to U.S. emission levels, as heating and cooling demands drive up emissions. Recent record levels of precipitation--both in snowfall and rain--consistent with what could be expected under a changed climate, have raised the awareness of climate impacts at the local and regional levels, and may make it somewhat easier to predict the effects of increased precipitation.

The United States also is uncommonly rich in land resources, both in extent and diversity. U.S. land area totals about 931 million hectares (2.3 billion acres), including grassland pasture and range, forest, and cropland. Forested land has been increasing, while grasslands and croplands are slowly declining and being converted to other uses. The decline in wetlands has slowed significantly as a result of the "no net loss" policy being implemented.

With just over 265 million people, the United States is the third most populous country in the world, although population density varies widely throughout the country, and is generally very low. Although population increase is moderate from a global perspective, it is high relative to the average for all industrialized countries. Moreover, the number of households is growing rapidly. These and other factors drive U.S. emissions to higher per capita rates than those in most other countries with higher population densities, smaller land areas, or more concentrated distribution of resources to population centers.

The U.S. market economy is based on property rights and a reliance on the efficiency of the market as a means of allocating resources. The government plays a key role in addressing market failures and promoting social welfare, including through the imposition of regulations on pollutants and the protection of property rights, but is cautious in its interventions. Thus, the infrastructure exists to limit emissions of greenhouse gases--although the strong political and economic preference is to undertake such controls through flexible and cost-effective programs, including voluntary programs and market instruments, where appropriate.

U.S. economic growth averaged 3 percent annually from 1960 to 1993, and employment nearly tripled as the overall labor force participation rate rose to 66 percent. The service sector--which includes communications, utilities, finance, insurance, and real estate--has grown rapidly, and now accounts for more than 36 percent of the economy. The increasing role of trade in the U.S. economy heightens concerns about the competitiveness effects of climate policies.

During the 1980s, the U.S. budget deficit grew rapidly, as did the ratio of debt to gross domestic product, and a political consensus emerged on the goal of a balanced budget. The result is a tighter federal budget with many competing priorities.

The United States is the world's largest energy producer and consumer. Abundant resources of all fossil fuels have contributed to low prices and specialization in relatively energy-intensive activities. Energy consumption has nearly doubled since 1960, and would have grown far more, because of growth in the economy, population, and transportation needs, had it not been for impressive reductions in U.S. energy intensity. Industrial energy intensity has declined most markedly, due to structural shifts and efficiency improvements. In the residential and commercial sectors, efficiency improvements largely offset the growth in the number and size of both residential and commercial buildings. Likewise, in the transportation sector, efficiency moderated the rise in total fuel consumption from 1973 to 1995 to only 26 percent, despite dramatic increases in both the number of vehicles and the distances they are driven. Fossil fuel prices below levels assumed in the 1993 Climate Change Action Plan, however, have contributed to the unexpectedly large growth in U.S. emissions.

While unique national circumstances point to the reasons for the current levels (and increases) in U.S. emissions, they also suggest the potential for emission reductions. Successful government and private-sector programs are beginning to exploit some of the inefficiencies in the manufacturing sector. The development of new, climate-friendly technologies is a rapidly growing industry, with significant long-term potential for domestic and international emission reductions.

Greenhouse Gas Inventory

Inventorying the national emissions of greenhouse gases is a task shared by several departments within the executive branch of the federal government, including the Environmental Protection Agency, the Department of Energy and the Department of Agriculture. The Greenhouse Gas Inventory chapter summarizes the most current information on U.S. greenhouse gas emission trends--and represents the 1997 submission from the United States in fulfillment of its annual inventory reporting obligation. The estimates presented in this chapter were compiled using methods consistent with those recommended by the IPCC Guidelines for National Greenhouse Gas Inventories; therefore, the U.S. emissions inventory should be comparable to those submitted by others under the FCCC.

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