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.a 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

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