Cost-effective reductions of non-CO 2 greenhouse gases

Abstract

To date, most of the focus on greenhouse gas emission reductions has been on energy-related CO 2 emissions. This is understandable since CO 2 emissions currently account for about 82 percent of the total US greenhouse gas emissions weighted by 100-year global warming potentials (EPA, www.epa.gov/globalwarming/publications/emissions, 2001a). 1 1 As noted in EPA, 2001a, the concept of global warming potential (GWP) has been developed to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas over a given period of time. Carbon dioxide is the reference gas, consistent with guidelines developed by the Intergovernmental Panel on Climate Change (IPCC). US estimates of greenhouse gas emissions are typically presented in units of million metric tons of carbon equivalents (MMTCE). Carbon comprises 12/44ths of carbon dioxide by weight. In order to convert emissions reported in teragrams (Tg) of greenhouse gas to MMTCE, the following equation was used: MMTCE=(Tg of gas)×(GWP)×(12/44). For example, a source emitting 1Tg of methane (GWP=21) would be emitting 5.73 MMTCE, as follows: (1Tg)×(21)×(12/44)=5.73 MMTCE. While any time period may be selected, the 100-year GWPs recommended by the IPCC, and employed by the United States for policy making and reporting purposes, were used in this report ( Intergovernmental Panel on Climate Change 1999). The effect of selecting alternative time periods depends on the atmospheric lifetime of the gas in relation to the atmospheric lifetime of the reference gas (carbon dioxide). Thus, since methane's lifetime is about 12 years, its 20-year GWP is 56 and its 500-year GWP is 6.5. In contrast, the lifetime of sulfur hexafluoride is 3200 years and thus its GWP increases from 16,300 over 20 years, 23,900 over 100 years, and 34,900 over 500 years. However, a number of analyses suggest that the non-CO 2 greenhouse gases included in the Kyoto Protocol—methane, nitrous oxide, and the high-GWP (global warming potential) gases (HFCs, PFCs, and SF 6)—can make a significant contribution to cost-effective emission reductions for the US and other countries. Our current estimate for the US is a reduction in non-CO 2 emissions of 105 million metric tons of carbon equivalent (MMTCE) at $50/ton carbon equivalent in 2010. This paper provides a perspective on the current and projected emissions of greenhouse gas; outlines the potential methods for achieving emissions reductions for various sources; and summarizes several recent studies on the cost of reductions for the US and other countries. Although the paper does not specifically address the potential for reductions of these gases in individual countries outside the US and the European Union, its findings are generally applicable to many countries.