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  • Michael E. Mann (a1)

The science underlying global warming, climate change, and the connections between these phenomena are reviewed. Projected future climate changes under various plausible scenarios of future human behavior are explored, as are the potential impacts of projected climate changes on society, ecosystems, and our environment. The economic, security, and ethical considerations relevant to determining the threat posed by climate change are subsequently assessed. The article then discusses the various means available for climate change mitigation, focusing on the relative strengths and weaknesses of various societal alternatives including ‘geoengineering’ and transitioning to less carbon intensive energy sources. The article concludes with the author's views as to what steps might most profitably be taken to avert dangerous anthropogenic interference with Earth's climate, and the ramifications if such steps are not taken.

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1 See Hansen, James et al. , “Global Temperature Change,” Proceedings of the National Academy of Sciences 103 (2006): 14288–293.

2 See Hansen, James et al. , “Potential Climate Impact of the Mount Pinatubo Eruption,” Geophysical Research Letters 19 (1992): 215–18.

3 This term was first coined by Nobel Prize winning scientist Paul Crutzen. See, e.g., Crutzen, P. J. and Stoermer, E. F., “The ‘Anthropocene,’Global Change Newsletter 41 (2000): 1213.

4 This conclusion is drawn in the Fourth Assessment Report of the United Nations Intergovernmental Panel on Climate Change. See, e.g., chapter 6 of the Working Group I report by Jansen, E. J. et al. , “Paleoclimate,” in Solomon, S., et al. , eds., Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge and New York: Cambridge University Press, 2007).

5 The stated objective of the Kyoto Protocol, which was agreed upon at a summit in Kyoto as a followup to the United Nations Framework Convention on Climate Change (UNFCCC), was to achieve “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”

6 S. Solomon et al., “Summary for Policymakers,” in Climate Change 2007: The Physical Science Basis.

7 Nakicenovic, N. and Swart, R., Special Report of the Intergovernmental Panel on Climate Change on Emissions Scenarios (Cambridge: Cambridge University Press, 2000).

9 An example of fallacious support for near-term inaction based on a neglect of low-probability, high-cost outcomes can be found in popular contrarian writings such as Lomborg, Bjørn, Cool It (New York: Knopf, 2007).

10 This use of the phrase ‘procrastination penalty’ appears, for example, in McKibben, Bill, “Warning on Warming,” The New York Review of Books, March 15, 2007.

11 See section of chapter 10 of the Working Group I report: G. A. Meehl et al., “Global Climate Projections,” in Solomon et al., eds., Climate Change 2007: The Physical Science Basis.

12 It is this (flawed) scientific premise that provides the basis for the plot of the disaster movie The Day After Tomorrow, released by Twentieth Century Fox in 2004, written and directed by Roland Emmerich.

13 This estimate is taken from a recent peer-reviewed study published in Science (Rahmstorf, S., “A Semi-Empirical Approach to Projecting Future Sea-Level Rise,” Science 315 [2007]: 368–70), which was published too late for inclusion in the IPCC Fourth Assessment Report. This recent work suggests sea-level-rise estimates that are moderately higher than suggested in the IPCC report. While the IPCC report is widely recognized as the most comprehensive assessment of the peer-reviewed climate change research, one shortcoming of the report was that the contribution to sea level from melting continental ice sheets was ignored, simply because its contribution is uncertain. This decision insured that the magnitude of sea-level rise would be systematically underestimated in the IPCC report, and more recent work supports modestly higher estimates, as cited above.

14 Rignot, E., “Recent Antarctic Ice Mass Loss from Radar Interferometry and Regional Climate Modelling,” Nature Geoscience 1 (2008): 106–10.

15 Emanuel, K., “Increasing Destructiveness of Tropical Cyclones over the Past 30 Years,” Nature 436 (2005): 686–88.

16 Parry, M. L. et al. , “Summary for Policymakers,” in Climate Change 2007: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge and New York: Cambridge University Press, 2007).

17 Le Bohec, C. et al. , “King Penguin Population Threatened by Southern Ocean Warming,” Proceedings of the National Academy of Sciences (2008): 10.1073/pnas.0712031105.

18 See Pounds, J. Alan et al. , “Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Warming,” Nature 439 (2006): 161–67.

19 Hoegh-Guldberg, O. et al. , “Coral Reefs Under Rapid Climate Change and Ocean Acidification,” Science 318 (2007): 1737–42.

20 Asnis, D. S. et al. , “The West Nile Virus Encephalitis Outbreak in the United States (1999–2000),” Annals of the New York Academy of Sciences 951 (2001): 161–71.

21 Hjelle, B., and Glass, G. E., “Outbreak of Hantavirus Infection in the Four Corners Region of the United States in the Wake of the 1997–1998 El Niño–Southern Oscillation,” Journal of Infectious Diseases 181 (2000): 1569–73.

22 Linthicum, K. J. et al. , “Climate and Satellite Indicators to Forecast Rift Valley Fever Epidemics in Kenya,” Science 285 (1999): 397400.

23 Confalonieri, U. et al. , “Human Health,” in Parry, M. L., et al. , eds., Climate Change 2007: Impacts, Adaptation, and Vulnerability, 391431.

24 Jacobson, M. Z., “On the Causal Link between Carbon Dioxide and Air Pollution Mortality,” Geophysical Research Letters 35 (2008): L03809, doi:10.1029/2007GL031101.

25 See, e.g., Bhattacharya, S., “European Heatwave Caused 35,000 Deaths,” New Scientist, October 10, 2003.

26 Ibid., 11.

27 Easterling, W. E. et al. , “Food, Fibre and Forest Products,” in Parry, et al. , eds., Climate Change 2007: Impacts, Adaptation, and Vulnerability, 273313.

28 Kundzewicz, Z. W. et al. , “Freshwater Resources and Their Management,” in Parry, et al. , eds., Climate Change 2007: Impacts, Adaptation, and Vulnerability, 173210.

29 Wilbanks, T. J. et al. , “Industry, Settlement, and Society,” in Parry, et al. , eds., Climate Change 2007: Impacts, Adaptation, and Vulnerability, 357–90.

30 Westerling, A. L. et al. , “Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity,” Science 313 (2006): 940–43.

31 See, e.g., Nordhaus, W. D., “To Slow or Not to Slow: The Economics of the Greenhouse Effect,” The Economic Journal 101 (1991): 920–37; and Nordhaus, W. D., “Economic Approaches to Greenhouse Warming,” in Global Warming: Economic Policy Responses, ed. Dornbusch, R. and Poterba, J. M. (Cambridge, MA: MIT Press, 1991).

32 This concept was first described by Lloyd, W. F., Two Lectures on the Checks to Population (Oxford: Oxford University Press, 1833).

33 Most serious economists would not advocate such a choice for the social discount rate. Nonetheless, libertarian activists sometimes do. See, e.g., Taylor, J., “Nordhaus vs. Stern,”

34 See, e.g., Nordhaus, W. D., ed., Economics and Policy Issues in Climate Change (Washington, DC: Resources for the Future, 1998).

35 Nordhaus, W. D., “Critical Assumptions in the Stern Review on Climate Change,” Science 317 (2007): 201–2.

36 See, e.g., Lomborg, Cool It.

37 Stern, N., The Economics of Climate Change: The Stern Review (Cambridge: Cambridge University Press, 2007).

38 Schneider, S. H. et al. , “Impacts, Adaptation, and Vulnerability,” in Parry, et al. , eds., Climate Change 2007: Impacts, Adaptation, and Vulnerability, 779810.

39 See, e.g., David Biello's interview of economist Gary Yohe of Wesleyan University, in Biello, D., “Clash: Gary Yohe,” Scientific American, November 26, 2007.

40 Raffensberger, C. and Tickner, J., eds., Protecting Public Health and the Environment: Implementing the Precautionary Principle (Washington, DC: Island Press, 1999).

41 The European Union Carbon Emission Trading scheme covers slightly less than half of the EU's energy- and industry-related greenhouse gas emissions. Emission allowances or “permits” are apportioned to major emitters for a period of several years at a time. The scheme requires emitters to monitor and report their emissions, and to return to the government a number of permits that is equivalent to their emissions on an annual basis. Permits can be bought from other emitters or the government as needed, or sold when they are available in excess of what is required by the emitter, thus creating a tradable emissions market. Upon instituting this scheme in 2005, the price of carbon credits began near the low end of the range of SCC estimates cited in the text, but then rose to around US$100/ton, close to the mid-range of estimates, before falling in 2006 (because the credits were believed to have been too generous). These fluctuations all fall within the range of SCC estimates cited in the text.

42 Campbell, K. K. et al. , The Age of Consequences: The Foreign Policy and National Security Implications of Global Climate Change (Washington, DC: Center for Strategic and International Studies, 2007).

43 Myers, N. and Kent, J., Environmental Exodus: An Emergent Crisis in the Global Arena (Washington, DC: The Climate Institute, 1995).

44 Myers, N., “Environmental Refugees: Our Latest Understanding,” Philosophical Transactions of the Royal Society B 356 (2001): 16.116.5.

45 Press Release (National Association of Evangelicals): “Evangelical, Scientific Leaders Launch Effort to Protect Creation” (January 17, 2007). The coalition released an “Urgent Call to Action” statement signed by twenty-eight evangelical and scientific leaders, describing a joint effort to protect the environment against human-caused threats to Creation—including climate change, habitat destruction, pollution, species extinction, the spread of human infectious diseases, and other dangers to the well-being of societies.

46 Adger, W. N. et al. , “Adaptation Practices, Options, Constraints, and Capacity,” in Parry, et al. , eds., Climate Change 2007: Impacts, Adaptation, and Vulnerability, 717–43.

47 China recently overtook the United States as the world's single largest emitter of greenhouse gases. See, e.g., “China Overtakes U.S. in Greenhouse Gas Emissions,”International Herald Tribute, June 20, 2007.

48 See, e.g., “Doffing the Cap,”The Economist, June 14, 2007.

49 Claussen, Eileen and Greenwald, Judith, “Handling Climate Change” (op-ed), Miami Herald, July 12, 2007.

50 To make comparisons across sectors, it is necessary to define a consistent unit of measurement that takes into account the impact of emissions of different types of greenhouse gases with different warming impacts. The preferred unit is the “CO2 equivalent,” which expresses the combined impact of multiple greenhouse gases (i.e., carbon dioxide, methane, nitrous oxide, etc.) in terms of the impact of an equivalent amount of carbon dioxide (CO2). The CO2 equivalent is typically measured in Gigatons (billions of metric tons) CO2, abbreviated as “Gt CO2 eq.”

51 An excellent summary of the topic is provided by Keith, D. W., “Geoengineering,” Nature 409 (2001): 420.

52 Buesseler, K. O. et al. , “The Effects of Iron Fertilization on Carbon Sequestration in the Southern Ocean,” Science 304 (2004): 414–17.

53 Goff, F. and Lackner, K. S., “Carbon Dioxide Sequestering Using Ultramafic Rocks,” Environmental Geoscience 5 (1998): 89101.

54 Bentley, M., BBC (online), “Synthetic Trees Could Purify Air,” (February 21, 2003).

55 Angel, R., “Feasibility of Cooling the Earth with a Cloud of Small Spacecraft Near the Inner Lagrange Point (L1),” Proceedings of the National Academy of Sciences 103 (2006): 17184–189.

56 See, e.g., Crutzen, P. J., “Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma,” Climatic Change 77 (2006): 211–20; and Wigley, T. M. L., “A Combined Mitigation/Geoengineering Approach to Climate Stabilization,” Science 314 (2006): 452–54.

57 See Lomborg, Cool It.

58 Hansen et al., “Global Temperature Change.”

59 This example is highlighted by Diamond, Jared in Collapse: How Societies Choose to Fail or Succeed (New York: Viking, 2004).

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Social Philosophy and Policy
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