GREENHOUSE GASES AND GLOBAL WARMING POTENTIAL VALUES EXCERPT FROM THE INVENTORY OF U.S. GREENHOUSE EMISSIONS AND SINKS: 1990-2000 U.S. Greenhouse Gas Inventory Program Office of Atmospheric Programs U.S. Environmental Protection Agency April 2002 Greenhouse Gases and Global Warming Potential Values Original Reference All material taken from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 - 2000, U.S. Environmental Protection Agency, Office of Atmospheric Programs, EPA 430-R-02- 003, April 2002. <www.epa.gov/globalwarming/publications/emissions> How to Obtain Copies You may electronically download this document from the U.S. EPA’s Global Warming web page on at: www.epa.gov/globalwarming/publications/emissions For Further Information Contact Mr. Michael Gillenwater, Office of Air and Radiation, Office of Atmospheric Programs, Tel: (202)564-0492, or e-mail [email protected] Acknowledgments The preparation of this document was directed by Michael Gillenwater. The staff of the Climate and Atmospheric Policy Practice at ICF Consulting, especially Marian Martin Van Pelt and Katrin Peterson deserve recognition for their expertise and efforts in supporting the preparation of this document. Excerpt from Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2000 Page 2 Greenhouse Gases and Global Warming Potential Values Introduction The Inventory of U.S. Greenhouse Gas Emissions elements of the Earth’s climate system. Natural and Sinks presents estimates by the United States processes such as solar-irradiance variations, government of U.S. anthropogenic greenhouse variations in the Earth’s orbital parameters, and gas emissions and removals for the years 1990 volcanic activity can produce variations in through 2000. The estimates are presented on climate. The climate system can also be both a full molecular mass basis and on a Global influenced by changes in the concentration of Warming Potential (GWP) weighted basis in various gases in the atmosphere, which affect the order to show the relative contribution of each Earth’s absorption of radiation. gas to global average radiative forcing. 1,2 The Earth naturally absorbs and reflects The Intergovenmental Panel on Climate Change incoming solar radiation and emits longer (IPCC) has recently updated the specific global wavelength terrestrial (thermal) radiation back warming potentials for most greenhouse gases in into space. On average, the absorbed solar their Third Assessment Report (TAR, IPCC radiation is balanced by the outgoing terrestrial 2001). Although the GWPs have been updated, radiation emitted to space. A portion of this estimates of emissions presented in the U.S. terrestrial radiation, though, is itself absorbed by Inventory continue to use the GWPs from the gases in the atmosphere. The energy from this Second Assessment Report (SAR). The absorbed terrestrial radiation warms the Earth's guidelines under which the Inventory is surface and atmosphere, creating what is known developed, the Revised 1996 IPCC Guidelines as the “natural greenhouse effect.” Without the for National Greenhouse Gas Inventories natural heat-trapping properties of these (IPCC/UNEP/OECD/IEA 1997) and the United atmospheric gases, the average surface Nations Framework Convention on Climate temperature of the Earth would be about 33oC Change (UNFCCC) reporting guidelines for lower (IPCC 2001). national inventories3 were developed prior to the Under the UNFCCC, the definition of climate publication of the TAR. Therefore, to comply change is “a change of climate which is attributed with international reporting standards under the directly or indirectly to human activity that alters UNFCCC, official emission estimates are the composition of the global atmosphere and reported by the United States using SAR GWP which is in addition to natural climate variability values. This excerpt of the U.S. Inventory observed over comparable time periods.” Given addresses in detail the differences between that definition, in its Second Assessment Report emission estimates using these two sets of GWPs. of the science of climate change, the IPCC Overall, these revisions to GWP values do not concluded that: have a significant effect on U.S. emission trends. Human activities are changing the Additional discussion on emission trends for the atmospheric concentrations and distributions United States can be found in the complete of greenhouse gases and aerosols. These Inventory of U.S. Greenhouse Gas Emissions and changes can produce a radiative forcing by Sinks: 1990-2000. changing either the reflection or absorption of solar radiation, or the emission and What is Climate Change? absorption of terrestrial radiation (IPCC Climate change refers to long-term fluctuations 1996). in temperature, precipitation, wind, and other Building on that conclusion, the more recent IPCC Third Assessment Report asserts that “[c]oncentrations of atmospheric greenhouse 1 See the section below entitled Global Warming Potentials for an explanation of GWP values. gases and their radiative forcing have continued 2 See the section below entitled What is Climate Change? to increase as a result of human activities” (IPCC for an explanation of radiative forcing. 2001). 3 See FCCC/CP/1999/7 at <www.unfccc.de>. Excerpt from Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2000 Page 3 Greenhouse Gases and Global Warming Potential Values The IPCC went on to report that the global concentrations, distributions and life cycles of average surface temperature of the Earth has these gases (IPCC 1996). increased by between 0.6 ± 0.2°C over the 20th Naturally occurring greenhouse gases include century (IPCC 2001). This value is about 0.15°C water vapor, carbon dioxide (CO2), methane larger than that estimated by the Second (CH4), nitrous oxide (N2O), and ozone (O3). Assessment Report, which reported for the period Several classes of halogenated substances that up to 1994, “owing to the relatively high contain fluorine, chlorine, or bromine are also temperatures of the additional years (1995 to greenhouse gases, but they are, for the most part, 2000) and improved methods of processing the solely a product of industrial activities. data” (IPCC 2001). Chlorofluorocarbons (CFCs) and While the Second Assessment Report concluded, hydrochlorofluorocarbons (HCFCs) are “the balance of evidence suggests that there is a halocarbons that contain chlorine, while discernible human influence on global climate,” halocarbons that contain bromine are referred to the Third Assessment Report states the influence as bromofluorocarbons (i.e., halons). Because of human activities on climate in even starker CFCs, HCFCs, and halons are stratospheric terms. It concludes that, “[I]n light of new ozone depleting substances, they are covered evidence and taking into account the remaining under the Montreal Protocol on Substances that uncertainties, most of the observed warming over Deplete the Ozone Layer. The UNFCCC defers the last 50 years is likely to have been due to the to this earlier international treaty; consequently increase in greenhouse gas concentrations” these gases are not included in national (IPCC 2001). greenhouse gas inventories. Some other fluorine containing halogenated substances— hydrofluorocarbons (HFCs), perfluorocarbons Greenhouse Gases (PFCs), and sulfur hexafluoride (SF6)—do not Although the Earth’s atmosphere consists mainly deplete stratospheric ozone but are potent of oxygen and nitrogen, neither plays a greenhouse gases. These latter substances are significant role in enhancing the greenhouse addressed by the UNFCCC and accounted for in effect because both are essentially transparent to national greenhouse gas inventories. terrestrial radiation. The greenhouse effect is primarily a function of the concentration of water There are also several gases that, although they vapor, carbon dioxide, and other trace gases in do not have a commonly agreed upon direct the atmosphere that absorb the terrestrial radiative forcing effect, do influence the global radiation leaving the surface of the Earth (IPCC radiation budget. These tropospheric gases— referred to as ambient air pollutants—include 1996). Changes in the atmospheric carbon monoxide (CO), nitrogen dioxide (NO2), concentrations of these greenhouse gases can sulfur dioxide (SO2), and tropospheric (ground alter the balance of energy transfers between the level) ozone (O3). Tropospheric ozone is formed atmosphere, space, land, and the oceans. A by two precursor pollutants, volatile organic gauge of these changes is called radiative forcing, compounds (VOCs) and nitrogen oxides (NOx) in which is a simple measure of changes in the the presence of ultraviolet light (sunlight). energy available to the Earth-atmosphere system Aerosols—extremely small particles or liquid (IPCC 1996). Holding everything else constant, droplets—often composed of sulfur compounds, increases in greenhouse gas concentrations in the carbonaceous combustion products, crustal atmosphere will produce positive radiative forcing materials and other human induced pollutants— (i.e., a net increase in the absorption of can affect the absorptive characteristics of the energy by the Earth). atmosphere. However, the level of scientific Climate change can be driven by changes in the understanding of aerosols is still very low (IPCC atmospheric concentrations of a number of 2001). radiatively active gases and aerosols. We have Carbon dioxide, methane, and nitrous oxide are clear evidence that human activities have