Climate change — страница 7

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in any given year. Overall, U.S. greenhouse gas emissions have increased annually by just over one percent. The trend of U.S. emissions--which decreased from 1990 to 1991, and then increased again in 1992--is a consequence of changes in total energy consumption resulting from the U.S. economic slowdown in the beginning of this decade and its subsequent recovery. Carbon dioxide accounts for the largest share of U.S. greenhouse gases--approximately 85 percent--although the carbon sinks in forested lands offset CO2 emissions by about 8 percent. During 1990-95, greenhouse gas emissions continued to rise in the United States, with CO2 increasing approximately 6 percent, methane approximately 4 percent, N2O nearly 10 percent, and HFCs approximately 7 percent. Fossil fuel combustion

accounts for 99 percent of total U.S. CO2 emissions. (Chapter 3 of this report explains the use of MMTCE in converting emissions of greenhouse gases to carbon equivalents.) Although methane emissions are lower than CO2 emissions, methane's footprint is large: in a 100-year time span it is considered to be twenty-one times more effective than CO2 at trapping heat in the atmosphere and is responsible for about 10 percent of the warming caused by U.S. emissions. In addition, in the last two centuries alone, methane concentrations in the atmosphere have more than doubled. Emissions of methane are largely generated by landfills, agriculture, oil and natural gas systems, and coal mining, with landfills comprising the single largest source of the gas. In 1995, methane emissions from

U.S. landfills were 63.5 MMTCE, equaling approximately 36 percent of total U.S. methane emissions. Agriculture supplied about 30 percent of U.S. methane emissions in that same year. Nitrous oxide is also emitted in much smaller amounts than carbon dioxide in the United States and is responsible for approximately 2.4 percent of the U.S. share of the greenhouse effect. However, like methane, it is a more powerful heat trap--310 times more powerful than carbon dioxide at trapping heat in the atmosphere over a 100-year period. The main anthropogenic activities producing nitrous oxide are agriculture, fossil fuel combustion, and the production of adipic and nitric acids. Figures from 1995 show the agricultural sector emitting 46 percent of the total (18.4 MMTCE), with fossil fuel

combustion generating 31 percent. Hydrofluorocarbons (HFCs) are among the compounds introduced to replace ozone-depleting substances, which are being phased out as a result of the Vienna Convention and its Montreal Protocol on Substances That Deplete the Ozone Layer, and the Clean Air Act Amendments of 1990. Because HFCs have significant potential to alter the Earth's radiative balance, they are included in this inventory. Many of the compounds of this nature are extremely stable and remain in the atmosphere for extended periods of time, which results in a significant atmospheric accumulation over time. U.S. emissions of these gases have risen nearly 60 percent as they are phased in as substitutes for gases that are no longer allowed under the Montreal Protocol--a rate of growth

that is not anticipated to continue. Currently, HFCs account for less than 2 percent of U.S. radiative forcing. Mitigating Climate Change In October 1993, in response to the threat of global climate change, President Clinton and Vice President Gore announced the Climate Change Action Plan (CCAP). The Plan was designed to reduce U.S. emissions of greenhouse gases, while guiding the U.S. economy toward environmentally sound economic growth into the next century. This report updates the programs in the CCAP (including an appendix providing one-page descriptions of each program), describes several additional initiatives developed to further reduce emission growth rates, and estimates future emissions based on the current set of practices and programs. CCAP programs represent an

effort to stimulate actions that are both profitable for individual private-sector participants as well as beneficial to the environment. Currently, more than forty programs are in effect, combining efforts of the government at the federal, state, and local levels with those of the private sector. The CCAP has five goals: preserving the environment, enhancing sustainable growth environmentally and economically, building partnerships, involving the public, and encouraging international emission reductions. Carbon dioxide emissions constitute the bulk of U.S. greenhouse gas emissions. CCAP recognizes that investing in energy efficiency is the most cost-effective way to reduce these emissions. The largest proportion of CCAP programs contains measures that reduce carbon dioxide