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Table 1-1 summarizes the recent trends in U.S. greenhouse gas emissions from 1990 to 1995. The three most important anthropogenic greenhouse gases are carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Hydrofluorocarbons (HFCs) are also inventoried. Consistent with the requirements in the Climate Convention only to address emissions of gases not controlled by the Montreal Protocol on Substances That Deplete the Ozone Layer, chlorofluorocarbon (CFC) emissions are not inventoried, nor are mitigation measures for these compounds described.

Table 1-1 Recent Trends in U.S. Greenhouse Gas Emissions: 1990-1995 (MMTs of Carbon Equivalent)
Gases and Sources	Emissions--Direct and Indirect Effects
	1990	1991	1992	1993	1994	1995
Carbon Dioxide (CO2)	1,228	1,213	1,235	1,268	1,291	1,305
Fossil Fuel Combustion	1,336	1,320	1,340	1,370	1,391	1,403
Industrial Processes and Other	17	16	17	18	19	19
Total	1,353	1,336	1,357	1,388	1,410	1,422
Forests (sink)*	(125)	(123)	(122)	(120)	(119)	(117)
Methane (CH4)	170	172	173	171	176	177
Landfills	56	58	58	60	62	64
Agriculture	50	51	52	52	54	55
Coal Mining	24	23	22	20	21	20
Oil and Natural Gas Systems	33	33	34	33	33	33
Other	6	7	7	6	6	6
Nitrous Oxide (N2O)	36	37	37	38	39	40
Agriculture	17	17	17	18	18	18
Fossil Fuel Consumption	11	11	12	12	12	12
Industrial Processes	8	8	8	8	9	9
HFCs	12	12	13	14	17	21
PFCs	5	5	5	5	7	8
SF6	7	7	8	8	8	8
U.S. Emissions	1,583	1,570	1,592	1,624	1,657	1,676
Net U.S. Emissions	1,458	1,447	1,470	1,504	1,538	1,559
Note: The totals presented in the summary tables in this chapter may not equal the sum of the individual source categories due to rounding. * These estimates for the conterminous United States for 1990-91 and 1993-95 are interpolated from forest inventories in 1987 and 1992 and from projections through 2040. The calculation method reflects long-term averages, rather than specific events 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 emissions while simultaneously enhancing domestic productivity and competitiveness. Other programs seek to reduce carbon dioxide emissions by investing in renewable-energy and other low-carbon, energy-supply technologies, which will also provide longer-term benefits, such as increased efficiency and related cost-savings and pollution prevention. A smaller number of programs are targeted at methane, nitrous oxide, and other greenhouse gases (Table 1-2).

A review and update of the CCAP was initiated in 1995, involving a federal government interagency review process and a public hearing and comment period. Revisions to the CCAP (and to the calculation of the effects of its measures) were initiated in light of comments received during this process and are reflected in this document. In addition, as called for under FCCC reporting guidelines, the projections of the effects of measures taken are extended to the year 2020, with the understanding that uncertainties become greater in more distant years.

One of the principal products of the review was an assessment of the effectiveness of the CCAP programs, which were rated to be successful at reducing emissions. Currently, more than 5,000 organizations are participating in programs around the United States. The pollution-prevention benefits of these innovative programs are beginning to multiply rapidly in response to the groundwork laid and the partnerships made. In all, the programs are expected to achieve a large portion of the reductions projected in the CCAP. In fact, it is estimated that these programs will result in energy cost savings of $10 billion annually in 2000.

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