est for trading schemes or offset projects, the report focuses on the national emission totals that include these activities. Only public domain data (not classified or commercial data) are considered because confidence in a treaty relies on open data for transparency and scientific scrutiny.

Greenhouse Gases

This report considers the anthropogenic greenhouse gases required by the committee charge—CO2, CH4, N2O, CFCs, HFCs, and PFCs—and SF6, but not the optional soot or precursors of tropospheric ozone. The greenhouse gases required by the committee charge, along with SF6, are currently covered by international agreements (CFCs under the Montreal Protocol and the others under the UNFCCC) and were the targets of negotiations at the 2009 United Nations Climate Change Conference (COP 15) in Copenhagen. Thus, there is an immediate practical need to verify emissions of the gases included in this report, which does not extend to the greenhouse agents that were omitted. The short-lived greenhouse agents (soot and other aerosols, aerosol precursors, and precursors to tropospheric ozone) are not covered by international agreements, although many countries have a highly developed capability to monitor them to support air pollution regulations. A comparable capability for the greenhouse gases discussed in this report does not exist.

The focus of international agreements on CO2, CH4, N2O, CFCs, HFCs, PFCs, and SF6 is likely to continue for three reasons. First, these gases are collectively more important greenhouse agents than soot, sulfur compounds, and precursors of tropospheric ozone (Figure 1.1). Commonly cited mitigation targets, such as a maximum of 2C of warming or a maximum concentration of 450 parts per million (ppm) CO2 equivalent, cannot be achieved without large reductions in emissions of CO2, CH4, N2O, CFCs, HFCs, PFCs, and SF6. Second, the gases included in the report are long-lived in the atmosphere (decades to millennia or more), whereas the omitted gases and soot are short-lived (less than a year). Longevity in the atmosphere means that delayed mitigation is costly—CO2 emissions today will add to global climate change for centuries. In contrast, short-lived greenhouse agents

FIGURE 1.1 The relative importance of emissions of anthropogenic greenhouse gases and soot (black carbon) and other aerosols. The bars show the 20- (lower panel) and 100-year (upper panel) radiative forcing of emissions in 2000. SOURCE: Figure 2.22 from IPCC (2007a), Cambridge University Press.

FIGURE 1.1 The relative importance of emissions of anthropogenic greenhouse gases and soot (black carbon) and other aerosols. The bars show the 20- (lower panel) and 100-year (upper panel) radiative forcing of emissions in 2000. SOURCE: Figure 2.22 from IPCC (2007a), Cambridge University Press.

do not entail the same penalties for delay. Because they are removed from the atmosphere in less than a year, today’s emissions will have a smaller impact on global warming in coming decades when the problem becomes most acute. Third, the net radiative forcing from the emission of short-lived gases and aerosols depends greatly on the location and timing of emissions. The time required for air to mix globally is on the order of 2 weeks in the east-west direction and 1 year in the north-south direction across the equator, which is less than the lifetime of short-lived greenhouse agents. For this and other reasons, the greenhouse impact of the ozone precursor NOx (nitrogen oxide) can vary by a factor of 10, depending on whether it is emitted in northern Europe or in the tropics (Wild et al., 2001; see also Table 2.15 of Forster et al., 2007). This makes



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