Pursuant to the United Nations Framework Convention on Climate Change, the United Stateshas published "national inventories of anthropogenic emissions by sources and removals by sinksof all greenhouse gases not controlled by the Montreal Protocol, using comparable methodologies... agreed upon by the Conference of the Parties." (11) (See Table 1 .) Table 1. U.S. Greenhouse Gas Emissions (MMTCE),1990-2001 Source: Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2001 (April 2003), EPA 430R03004. (Datafor 1991-1994 provided by EPA.) Following international convention, EPA presents the data in teragrams ofCO 2 equivalent; CRS has converted thefigures to million metric tons of carbon equivalent (MMTCE), a metric that is more familiar to most U.S.policymakers. a Land-use changes and forestry sinks that sequester carbon; included in net emissions total only. The Environmental Protection Agency (EPA) publishes the official emissions data annually. (12) The United States also from timeto time reports on emissions andexplains its climate change programs in the Climate Action Report (CAR) to theUnited Nations; the third CAR was published in 2002. (13) The U.S. baselines for the UNFCCC and the Kyoto Protocol are shown in Table 2 . For the UNFCCC commitment, the baseline is 1990 emissions, or 1,675MMTCE; if the United States had acceded to the Kyoto targets, the baseline wouldhave been 1,676 MMTCE, a negligible difference. By definition, sinks are not included in calculating the baselines. Table 2. U.S. Baseline Year Greenhouse Gas Emissions Source: EPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 - 2001) April 2003, EPA 430R03004. The emissions baselines shown in Table 2 are not immutable. Each annual report includes updated estimates based on methodological and data revisions,although such changes are usually small. Revisions are discussed at some length ineach report. The criteria for calculating emissions agreed upon by the Conference ofParties hinge on both current technical knowledge and policy judgments. Newtechnical information can change factors, for example concerning calculation ofgreenhouse gas equivalents; and policy judgments can be adjusted, for exampleconcerning the time frame for calculating effects. In addition, a few technical issuesremain unresolved, for example in assigning emissions from fuels burned ininternational travel. However, any changes tend to be modest, seldom affecting totalsmuch more than plus or minus 1%, except for sequestration figures, which have beensubject to larger changes. (14) Besides actual quantities of emissions, an alternative measure of a nation's contribution to global warming is "greenhouse gas intensity of the economy" -- thatis, emissions per unit of gross domestic product (GDP). In effect, this measurefocuses on the efficiency of the economy in terms of greenhouse gas emissions: themore efficient, the fewer emissions per dollar of economic output and thus the lowerthe "greenhouse gas intensity." For the United States, greenhouse gas intensity hasbeen declining since at least the 1980s; for the 1990s, the decline in intensity wasabout 10%, based on net emissions (15) (see Table 3 ). Projecting greenhouse gas emissions involves modeling the nation's economicgrowth and activity, with special attention to variables affecting fossil fuelcombustion. The modeling also depends on assumptions about energy policydirections. If reducing emissions becomes a goal, then projections become subject to the outcome of unresolved issues in how the emissions reductions goals might bemet. For example, the major source of CO 2 emissions, fossil fuel combustion, is influenced by overall economic activity and growth as well as by energy policydecisions such as development of non-carbon based substitutes, the rate of adoptionof energy efficient technologies, and the retirement rate of nuclear facilities, amongothers. These policy factors are difficult to predict in the absence of a concreteclimate change policy. (16) The climate change planproposed by President Bush inFebruary 2002 provides some new policy directions, but many elements depend oncongressional action (e.g., for funding) or voluntary private sector initiatives, makingprojections of their impact problematic. Table 3. U.S. Greenhouse Gas Intensity (1990-2001) Source: Table 1; Economic Report of the President , February 2003, Table B-2; CRScalculations. The third U.S. Climate Action Report ( CAR 2002 ) projects greenhouse gas emissions at 5-year intervals through 2020. For this report, the projections arefollowed only to 2010 (see Figures 1 and 2 ), because of the difficulties in projectinginto the more distant future. Also, 2010 provides a basis for evaluating a relationshipto the Kyoto Protocol targets. Sources: Historical data (through 2001): EPA, Inventory of U.S. Greenhouse GasEmissions and Sinks: 1990 - 2001 , April 2003, EPA 430R03004, pp.ES-2 - ES-4. Projections (to 2010): U.S. Department of State, Climate Action Report 2002 , May 2002. [Data converted to MMTCE by CRS.] Sources: Historical data (through 2001): EPA, Inventory of U.S. Greenhouse GasEmissions and Sinks: 1990 - 2001 , April 2003, EPA 430R03004, pp.ES-2 - ES-4. Projections (to 2010): U.S. Department of State, Climate Action Report 2002 , May 2002. [Data converted to MMTCE by CRS.] The CAR 2002 estimate for aggregate gross greenhouse emissions (17) in 2010 is 2,213MMTCE (see Figure 3 ). The President's 2002 initiative to reduce greenhouse gas intensityproposes a series of policy initiatives that it estimates "will achieve 100 million tons ofreduced emissions in 2012." (18) Extrapolatingbetween the CAR 2002 projections from 2010to 2015 (a 1.9% annual growth rate), the 2012 projected emission level would be 2,298MMTCE. The President's initiative suggesting a decline in emissions of 100 million tonsin 2012 would reduce this to 2,198 MMTCE, representing a reduction of about 4.4% from"business as usual" gross greenhouse emissions in that year. In addition, largely separatefrom federal activities, a number of state and local governmental initiatives, as well as avariety of private sector activities, are underway to address greenhouse gas emissions. CAR 2002 only makes point estimates, but some sense of the implications of varying assumptions that affect the estimates can be gleaned from examining an alternative sourceof CO 2 emissions data, the Energy Information Agency's (EIA's) Annual Energy Outlook series. (19) (Because of minor differences in datacalculation and presentation, EIA's annualemissions figures differ slightly from EPA's.) The EIA report's projections of CO 2 emissions include sensitivity analyses to various changes in assumptions, and since CO 2 from fuel combustion accounts for about 80% of U.S.greenhouse gas emissions, the analysis is a reasonable test of the projections. Theassumptions EIA examines include economic growth, technological innovation, oil prices,electricity demand, and others. The first two, economic growth and technologicalinnovation, have the greatest effect on variance in projections of CO 2 emissions (see Table4 ). For 2010, compared to EIA's "reference case" (which is equivalent to a "business asusual" case), low economic growth would reduce projected emissions by about 2%, (20) whilehigh economic growth would increase projected emissions by about 3%. Compared to thereference case that assumes anticipated technological developments, static technology wouldresult in emissions rising about 2%, while a "high-technology" case is projected to reduceemissions about 2%. The point reference case -- CAR's point estimate -- effectivelyassumes the several variances affecting emissions cancel out. But if all the variancesincreasing emissions prove true and cumulative, then projected emissions for 2010 could be5% or more higher than the point estimate; conversely, if all the variances decreasingemissions prove true and cumulative, emissions could be 5% or more lower. Sources: Historical data (through 2001): EPA, Inventory of U.S. Greenhouse GasEmissions and Sinks: 1990 - 2001 , April 2003, EPA 430R03004, pp.ES-2 - ES-4. Projections (to 2010): U.S. Department of State, Climate Action Report 2002 , May 2002. [Data converted to MMTCE by CRS.] Upperand lower bound equal + 5% and - 5%, as discussed in text. President's initiative is from documents on theAdministration plan at http://www.whitehouse.gov/news/releases/2002/02/climatechange.html . Table 4. Impact of Economic Assumptions on Projections of CO2Emissions Source: EIA, Annual Energy Outlook 2003 (January 2003) DOE/EIA-0383(2003), pp. 174,218. Some studies suggest that even greater variance in projections is possible -- for example, that new energy-efficient technologies could be deployed more quickly thangenerally assumed if appropriate policies were instituted. A November 2000 DOE study,commonly called the "New 5-Lab Study," shows that energy efficiency gains in thetransportation, industry, commercial, and residential sectors could reduce emissions from the"business as usual" scenario. (21) The "business asusual" scenario in this study is very similarto EIA's reference case, though it projects somewhat smaller emissions in 2010 (1,769MMTCE from fossil fuel combustion, compared to EIA's most recent projection of 1,800).