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). The study compares "moderate" and "advanced" scenarios "that are defined by policies thatare consistent with increasing levels of public commitment and political resolve to solvingthe nation's energy-related challenges." Policies examined include "fiscal incentives,voluntary programs, regulations, and research and development." (22) Under the "moderate scenario," energy efficiency is improved through such policies as expanded labeling, new efficiency standards, tax credits, and cost-shared R&D; renewableenergy grows more rapidly than in the "business as usual" scenario, and a higher proportionof nuclear power is retained. Under the "advanced scenario," which has more aggressivedemand- and supply-side policies and a doubling of R&D, a federally sponsored carbontrading system is announced in 2002 and implemented in 2005 with a clearing equilibriumprice of $50 per ton of carbon. (23) The results ofthis analysis are shown in Table 5 . Table 5. Impact of Technology/Efficiency Assumptions on Projectionsof CO2 Emissions Source: DOE, Interlaboratory Working Group, Scenarios for a Clean Energy Future ,ORNL/CON-476 and LBNL-44029 (Oak Ridge, TN: Oak Ridge National Laboratory;Berkeley, CA: Lawrence Berkeley National Laboratory, 2000), Table 1.8, p. 1.18. This "New 5-Lab Study" thus suggests that if specified policies were adopted, emissions could be considerably lower than even EIA's high-technology scenario indicates, by as muchas 17% compared to EIA's high-technology reduction in emissions of about 2%. EPA andthe Department of Energy (DOE) have underway a number of programs to foster thedevelopment and deployment of energy-efficient technologies. (24) The President's greenhouse gas intensity reduction initiative is a new variable affecting projections. It would have the effect of reducing anticipated emissions below "business asusual" levels in the future (see Figure 3 ); however, the initiative does not reflect the levelof aggressiveness assumed by the "New 5-Lab Study" for policy interventions to achieve its"advanced scenario" for rapid penetration of energy-efficient technologies. Under the UNFCCC, the United States committed to the voluntary goal of holdinggreenhouse gas emissions at the end of the 1990s to their 1990 levels. If the United Stateshad met this goal, its greenhouse gas emissions for 2000 would have been 1,675 MMTCE. However, U.S. emissions in 2000 were 1,921 MMTCE (not counting sinks). These figuresindicate that in 2000, the nation was exceeding its UNFCCC greenhouse gas emissionscommitment by 246 MMTCE, or nearly 15%. If the United States had acceded to the Kyoto Protocol, its greenhouse gases emissions target for the period 2008-2012 would have been 5 times 93% of the 1,676 MMTCEbaseline, or 1,559 MMTCE on average per year for the period. This hypothetical goal wouldimply reductions equal to the difference between the goal and what would be "business asusual emissions" for the period 2008-2012. Based on the CAR projection that emissions willbe 2,213 MMTCE in 2010, the average annual reduction that would be necessary for theUnited States to meet the Kyoto target of 1,559 MMTCE per year for 2008-2012 would be654 MMTCE per year, or about 30% below the estimated level of "business as usual"emissions. Higher than base case economic growth or lower penetration of energy-efficienttechnologies would mean that emissions would be even higher (and reductions necessary tomeet a goal like Kyoto greater). Slower economic growth, or faster penetration ofenergy-efficient technologies as suggested by the 5-Lab Study, would decrease emissions(and hence reductions to meet a goal). The President's greenhouse gas initiative has the goal of reducing, through voluntary activities, the intensity of net greenhouse gas emissions per unit of economic activity by 18%over the next 10 years; this compares to a projected "business as usual" decline in intensityof 14% for the period -- compared to a decline during the 1990s of about 10% (see Table3 ). According to the White House announcement, this goal means that the current (2002)intensity of 183 metric tons of carbon emissions per million dollars of GDP would fall to 151MMTCE per million dollars of GDP in 2012 (see Figure 4 ). (25) At the anticipated increasedrate of intensity decline, total emissions would decline 100 MMTCE below "business asusual" emissions (although the absolute amount of emissions would continue to rise). It istoo early to assess progress toward the Administration's goal of diminishing greenhouse gasintensity. Source: Global Climate Change Policy Book http://yosemite.epa.gov/oar/globalwarming.nsf/UniqueKeyLookup/SHSU5BNMAJ/$File/bush_gccp_021402.pdf These projected emissions levels (and any implied reductions) are gross estimates and do not take sinks into account (except for the intensity projection). As previously noted, thebaseline could be revised, at least slightly. More important, such projections depend onassumptions about economic trends as well as about policy actions at the local, domestic, andinternational levels. However, whatever the assumptions, the trend in total emissionsexperienced over the past decade and projected for the next decade is clearly upward, whilethe UNFCCC goal was for stabilization and the Kyoto Protocol calls for emissions levelsof developed nations to decline. If one is concerned about assessing the implications of possible reduction requirementsin the future, two further factors must be considered. One is sequestration, which removesCO 2 from the atmosphere, thereby reducing gross emissions. The second is a series ofproposed trading mechanisms that could allow a country to take credit for reductions itsponsors in other countries. The United States was a strong supporter of including both thesevariables in the Kyoto Protocol. Sequestration could directly diminish a country's reductionrequirement; trading does not change a reduction requirement, but it could affect costs andwho would actually achieve the reductions. Carbon Sequestration. Atmospheric greenhouse gas levels are affected not only by emissions, but also by carbon sinks --processes that remove and sequester carbon from the atmosphere. Activities that affectsequestration include farming and forestry practices. For example, a positive net growth oftrees removes carbon from the atmosphere; clearing forests typically releases carbon. Table1 , "U.S. Greenhouse Gas Emissions, 1990 -2001," includes figures for carbon sequestrationfrom land-use activities and forestry, which are the difference between "total emissions" and"net emissions." The UNFCCC states that signatory nations shall commit to "promote sustainable management, and promote and cooperate in the conservation and enhancement, asappropriate, of sinks and reservoirs of all greenhouse gases ... , including biomass, forestsand oceans as well as other terrestrial, coastal and marine ecosystems" (Article 4(1)(d)). The Kyoto Protocol also would provide that sinks can be taken into account in calculating a nation's emissions and its reduction obligation. "The net changes ingreenhouse gas emissions from sources and removals by sinks resulting from directhuman-induced land-use change and forestry activities, limited to afforestation, reforestation,and deforestation since 1990, measured as verifiable changes in stocks ... shall be used tomeet" the 2008-2012 commitments (Article 3(3)). In general, then, a net increase inhuman-induced carbon sequestration from forestry practices between 1990 and 2008-2012would be subtracted from emissions during the period, thereby reducing the amount of actualemissions that will have to be curtailed. Conversely, net negative sequestration from forestrypractices would be added to the emissions that will have to be reduced. Just how this calculation would be done is not prescribed in the Protocol, and disagreements on how much carbon sequestration could be counted toward a nation'sreduction obligations were debated through several subsequent conferences. In July 2001,the Sixth Conference of Parties in Bonn (COP6) agreed to limits on sequestration activitiesthat could be credited against the Protocol's reduction requirements. Although the UnitedStates chose not to participate in these proceedings, the Conference stated in a footnote (26) thatunder the methodology agreed upon, the United States could take credit for net increases ofsequestration of up to 28 million metric tons per year. Emissions Trading. Emissions trading, strongly supported by the United States in the Kyoto negotiations, derives from the principleof economic efficiency -- that reductions, if necessary, should be achieved at the lowestcost. Trading mechanisms thus are designed to allow low-cost reductions to substitute forhigher-cost ones. The idea is that a country could achieve its reduction goal not only byreducing its domestic emissions, but also by reducing emissions elsewhere. Trading does notactually reduce a nation's reduction requirement, but it does allow it to contract for and tocount reductions elsewhere that are cheaper to achieve than domestic ones. The Kyoto Protocol provides for emissions trading mechanisms (27) that can be used to"supplement" domestic reductions; this offers the possibility that actual domestic greenhousegas reductions achieved by a party to the Kyoto Protocol will be less than the party's actualcommitment. Some portion of the reduction requirement could be shifted elsewhere. TheClinton Administration argued that emission trading would be critical to U.S. compliancewith Kyoto; (28) a Clinton Administration economicanalysis suggested that U.S. compliancecosts would drop from $193 per ton with no international emissions trading to $23 per tonwith global trading. (29) COP6 agreed that therewould be no quantitative limit on the amountof credit a country could receive from trading, but that domestic action must constitute asignificant part of a nation's reduction efforts. (30) With no quantitative limit on trading, anyestimate of actual domestic reduction required to comply with the Kyoto Protocol, or of thecosts involved, remains problematic -- and is moot as long as the United States declines toparticipate in the Kyoto process. The precise numerical projections of greenhouse gas emissions (or of proposedreductions) should be viewed as indicative (see Figure 3 ). They are less accurate than theyappear, given the potential for revisions in data and the uncertainties of projections. But inassessing the status of U.S. greenhouse gas emissions, the trendline for aggregate greenhousegas emissions is clear: for the United States, the overall trend is up. None of the reviewedscenarios using assumptions that diminish emissions -- low economic growth, putting offretirement of nuclear facilities, accelerated fostering of energy-efficient technologies, thePresident's voluntary program to reduce greenhouse gas intensity -- reverses the upwardtrend in aggregate greenhouse gas emissions by 2010. (31) Historical data show that the United States failed to meet its voluntary commitment under the UNFCCC for returning aggregate emissions at the end of the 1990s decade to the1990 level. Any goal to reduce emissions to or below 1990 levels would require thecontinuing upward trend to turn down. Even with the potential for sequestration andemissions trading to reduce domestic reduction efforts, a goal to reverse greenhouse gasemissions trends would represent an extraordinary technical and political challenge for U.S.energy and environmental policy. 

Summary: This report reviews U.S. emissions of greenhouse gases in the contexts both of domestic policy and of international obligations and proposals. On October 15, 1992, the United States ratified theUnited Nations Framework Convention on Climate Change (UNFCCC), which entered into forceon March 21, 1994. This committed the United States to "national policies" to limit "itsanthropogenic emissions of greenhouse gases," with a voluntary goal of returning "emissions ofcarbon dioxide [CO 2 ] and other greenhouse gases [methane (CH 4 ), nitrous oxide(N 2 O),hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF 6 )]" at the "endof the decade" to "their 1990 levels." Subsequently, in the 1997 Kyoto Protocol to the UNFCCC, the United States participated in negotiations that ended with agreement on further reductions that could become legally binding. TheUnited States signed the Kyoto Protocol in 1998, but President Clinton did not send it to the Senatefor advice and consent. President Bush has said that he rejects the Protocol, and former U.S.Environmental Protection Agency Administrator Christine Todd Whitman told reporters that theAdministration would not be pursuing the UNFCCC commitment either. Instead, President Bushhas proposed to shift the nation's climate change program from a goal of reducing emissions per seto a goal of reducing energy intensity -- the amount of greenhouse gases emitted per unit ofeconomic productivity. Under the proposal, the intensity, which has been declining for a numberof years, would decline 18% between 2002 and 2012, as opposed to a 14% projected "business asusual" decline. Meanwhile, the UNFCCC "end of the decade" deadline has passed and U.S. greenhouse gas emissions continue on an upward trend, though with dips in 1991 and in 2001, attributed mostly toeconomic slowdowns. Based on historical data, 2001 emissions were about 13% in excess of theUNFCCC goal. Overall, from 1990 to 2001, U.S. greenhouse gas emissions (weighted by globalwarming potential) have increased an average of about 1.1% per year. Projections suggest that U.S.emissions will continue to rise for at least the next decade. Reversing the upward trend ingreenhouse gas emissions would represent an extraordinary technical and political challenge to U.S.energy and environmental policy. This report will be updated as necessary.