Greenhouse Gas Emission Trends

Despite an international agreement signed by the United States and 153 other nations in 1992 to stabilize atmospheric GHG concentrations "at a level that would prevent dangerous anthropogenic interference with the climate system,"1 GHG emissions have continued to rise. Energy-related carbon dioxide (CO2) emissions constitutes roughly

BOX 1.2

Non-CO2 Greenhouse Gases and Aerosols

International negotiations and domestic policy debates have focused largely on reducing CO2 emissions from fossil fuels, both because these emissions account for a large fraction of total GHG emissions and because they can be estimated fairly accurately based on fuel-use data." This report follows suit by focusing primarily on energy-related CO2 emissions. It is important to recognize, however, that there are other important sources of CO2 (such as tropical deforestation), and there are other compounds in the atmosphere that affect the earth's radiative balance and thus play a role in climate change.

This includes long-lived GHGs such as methane, nitrous oxide, and fluorinated compounds (which arise from a variety of human activities including agriculture and industrial activities). It also includes shorter-lived gases that are precursors to tropospheric ozone (which directly affects human health, in addition to influencing climate), and a variety of aerosols that can exert either warming or cooling effects, depending on their chemical and physical properties. Some of these other compounds are explicitly included in climate policy negotiations and emissions reductions plans, but in general it is much more difficult to measure and verify reductions in emissions of many of these substances than for CO2.b See NRC, Advancing the Science of Climate Change and Limiting the Magnitude of Climate Change for more extensive discussion of these other gases and aerosols.

a It should be noted, however, that there is as yet no sufficiently accurate way to verify countries' self-reported estimates using independent data. A recent study (NRC, Verifying Greenhouse Gas Emissions: Methods to Support International Climate Agreements,Washington,D.C.:National Academies Press,2010) recommended a set of strategic investments that would improve self-reporting and provide a verification capability within 5 years. b NRC, Verifying GHG Emissions.

83 percent of total U.S. GHG emissions2 and thus will be the primary focus of the discussion in this report (non-CO2 GHGs are discussed briefly in Box 1.2). Figure 1.1 shows recent and projected energy-related CO2 emissions for the United States. The increase in emissions over the past few decades occurred despite the fact that the "intensity" of America's CO2 emissions (the amount of emissions created per unit of economic output, often presented as emissions per dollar of GDP) decreased by almost 30 percent.3 Thus, the general tendency for industrialized nations to become more efficient and less carbon intensive has slowed but not prevented the growth of domestic CO2 emissions.

The upward trend in U.S. emissions has been punctuated by brief declines, usually during economic downturns. By far the most significant of these downturns was the roughly 6 percent decrease in energy-related CO2 emissions in 2009, related to the economic recession. 4 However, the U.S. Energy Information Administration's latest

AMERICA'S CLIMATE CHOICES

FIGURE 1.1 Energy-related U.S. CO2 emissions for 1980-2009 (estimated) and 2010-2035 (projected). Given in billion metric tons CO2. The long-term upward trend in emissions has been punctuated by declines during economic downturns, most notably around 2009. SOURCE: Adapted from Energy Information Administration (EIA), International Energy Outlook, Report # DOE/EIA-0484(2010) (Washington, D.C.: U.S. Department of Energy, 2010, available at http://www.eia.doe.gov/oiaf/ieo/, accessed March 4, 2011).

FIGURE 1.1 Energy-related U.S. CO2 emissions for 1980-2009 (estimated) and 2010-2035 (projected). Given in billion metric tons CO2. The long-term upward trend in emissions has been punctuated by declines during economic downturns, most notably around 2009. SOURCE: Adapted from Energy Information Administration (EIA), International Energy Outlook, Report # DOE/EIA-0484(2010) (Washington, D.C.: U.S. Department of Energy, 2010, available at http://www.eia.doe.gov/oiaf/ieo/, accessed March 4, 2011).

projections are that emissions return to an upward trend in 2010, and that under a "business-as-usual" scenario through 2035 (that is, assuming no major actions to reduce domestic GHG emissions or additional major economic downturns), CO2 emissions will grow by an average of roughly 0.2 percent per year.5 This is a slower growth rate than that of the past three decades, but it does indicate that emissions will exceed pre-recession levels by the year 2028, and by the year 2035 they will be about 8.5 percent higher than pre-recession levels.

Recent studies have highlighted the commitments to further climate change that are implied by construction of new, long-lived infrastructure (e.g., electricity production facilities, highways). Once constructed, the emissions from these facilities can be locked in for as much as 50 years or more. This is an especially serious concern in regards to rapidly developing countries, where huge investments in new energy generation and energy use systems are being made. 6 As a result, global GHG emissions are projected to increase steeply. The U.S. Energy Information Administration esti mates that by 2035, global emissions will be more than 40 percent larger than in 2007 in the absence of aggressive policies to reduce emissions, with most of the increase expected to occur in developing economies. 7

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