7. According to IPCC (Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, eds. M. L. Parry, O. F. Canziani, J. P. Palutikof, P. J. van der Linden, and C. E. Hanson [Cambridge, UK: Cambridge University Press, 2007]): “There is very high confidence [about 8 out of 10 chance of being correct]…that recent warming is strongly affecting terrestrial biological systems” and “high confidence, based on substantial new evidence, that observed changes in marine and freshwater biological systems are associated with rising water temperatures, as well as related changes in ice cover, salinity, oxygen levels and circulation.”

8. According to IPCC (Climate Change 2007 WG1, Summary for Policymakers): “Most of the observed increase in global average temperatures since the mid-20th century is very likely [greater than 90 percent likelihood] due to the observed increase in anthropogenic greenhouse gas concentrations.”

9. D. Lüthi, M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, U. Siegenthaler, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T. F. Stocker, “High-resolution carbon dioxide concentration record 650,000-800,000 years before present” (Nature 453[7193]:379-382, 2008, doi:10.1038/nature06949).

10. IPCC (Climate Change 2007 WG1, Summary for Policymakers): “The primary source of the increased atmospheric concentration of carbon dioxide since the pre-industrial period results from fossil fuel use, with land-use change providing another significant but smaller contribution.” This statement is based in part on the fossil fuel emissions data in Figure 1.2; in part on estimates of the other sources as “sinks” of atmospheric carbon dioxide like those provided by the Global Carbon Project (Le Quéré, C. M. R. Raupach, J. G. Canadell, G. Marland, L. Bopp, P. Ciais, T. J. Conway, S. C. Doney, R. A. Feely, P. Foster, P. Friedlingstein, K. Gurney, R. A. Houghton, J. I. House, C. Huntingford, P. E. Levy, M. R. Lomas, J. Majkut, N. Metzl, J. P. Ometto, G. P. Peters, I. C. Prentice, J. T. Randerson, S. W. Running, J. L. Sarmiento, U. Schuster, S. Sitch, T. Takahashi, N. Viovy, G. R. van der Werf, and F. I. Woodward, “Trends in the sources and sinks of carbon dioxide” [Nature Geoscience 2, 2009, doi: 10.1038/ngeo689]), which indicate that deforestation and other land use changes currently contribute about 12% of total human-induced CO2 emissions; and in part on the chemical “fingerprints” of CO2 and other gases in the atmosphere, which can only be explained by the burning of coal, oil, and natural gas (R. F. Keeling, S. C. Piper, A. F. Bollenbacher and J. S. Walker, “Atmospheric CO2 records from sites in the SIO air sampling network,” in Trends: A Compendium of Data on Global Change (Oak Ridge, TN: Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, 2009).

11. Methane comes from fossil fuel and biomass burning, natural gas management, animal husbandry, rice cultivation, and waste management (S. Houweling, T. Rockmann, I. Aben, F. Keppler, M. Krol, J. F. Meirink, E. J. Dlugokencky, and C. Frankenberg, “Atmospheric constraints on global emissions of methane from plants” [Geophysical Research Letters 33:L15821, 2006, doi:10.1029/2006GL026162]). The atmospheric concentration of methane rose sharply through the late 1970s before leveling off at about two-and-a-half times its estimated pre-industrial concentration. Methane levels have risen slightly in each of the past few years (E. J. Dlugokencky, L. Bruhwiler, J. W. C. White, L. K. Emmons, P. C. Novelli, S. A. Montzka, K. A. Masarie, P. M. Lang, A. M. Crotwell, J. B. Miller, and L. V. Gatti, “Observational constraints on recent increases in the atmospheric CH4 burden” [Geophysical Research Letters 36:L18803, 2009]) but the reasons for the changes are not completely clear. Nitrous oxide concentrations are steadily increasing primarily as a result of agricultural activities (especially the application of chemical fertilizers), but also a byproduct of fossil fuel combustion and certain industrial process. Halogenated gases include chlorofluorocarbons (CFCs), hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride, all of which are produced primarily by industrial processes. Many of these compounds also contribute to the depletion of ozone in the stratosphere, which is a related but largely separate environmental problem from climate change (see, e.g., CCSP, Trends in Emissions of Ozone-Depleting Substances, Ozone Layer Recovery, and Implications for Ultraviolet Radiation Exposure, Synthesis and Assessment Product 2.4 by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, eds. A. R. Ravishankara, M. J. Kurylo, and C. A. Ennis [Asheville, NC: National Oceanic and Atmospheric Administration, 2008]). Water vapor is also an important greenhouse gas, but its concentration in the lower atmosphere is controlled by the rates of evaporation and precipitation, which are natural processes that are much more strongly influenced by changes in atmospheric temperature and circulation than by human activities directly.

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