Background
The author writes that "an evolving consensus that the earth is becoming warmer has led to increased interest in the social consequences of climate change," and he says that "a second consensus has begun to emerge among policymakers and opinion leaders that global warming may well result in increased civil and even interstate warfare, as groups and nations compete for water, soil, or oil." Although he further notes that "while anecdote and some focused statistical research suggests that civil conflict may have worsened in response to recent climate change in developing regions, these claims have been severely criticized by other studies," citing Nordas and Gleditsch (2007), Buhaug (2010), and Buhaug et al. (2010). In addition, he states that "the few long-term macro statistical studies actually find that conflict increases in periods of climatic chill (Zhang et al., 2006, 2007; Tol and Wagner, 2010)." And he reports that "research on the modern era reveals that interstate conflict has declined in the second half of the 20th century, the very period during which global warming has begun to make itself felt (Goldstein, 2011; Hensel, 2002; Levy et al., 2001; Luard, 1986, 1988; Mueller, 2009; Pinker, 2011; Sarkees et al., 2003)."

What was done
Gartzke explores "the relationship between climate change, liberal processes fueled by industrialization (development, democracy, international institutions) and interstate conflict," based on information gleaned from the Correlates of War (COR) Militarized Interstate Dispute (MID) dataset (Gochman and Maoz, 1984; Ghosn et al., 2004) and annual average temperature data provided by NASA's Goddard Institute for Space Studies, as well as the United Kingdom's Meteorological Office Hadley Centre and the Climatic Research Unit of the University of East Anglia, while measures of regime type come from the Polity IV project described by Gurr et al. (1989) and Marshall and Jaggers (2002).

What was learned
"Surprisingly," in the words of the University of California, San Diego researcher, "analysis at the system level suggests that global warming is associated with a reduction in interstate conflict," and that "incorporating measures of development, democracy, cross-border trade, and international institutions reveals that systemic trends toward peace are actually best accounted for by the increase in average international income," which in turn is driven by "the processes that are widely seen by experts as responsible for global warming."

What it means
"Ironically," as Gartzke writes in the concluding sentence of his paper's abstract, "stagnating economic development in middle-income states caused by efforts to combat climate change could actually realize fears of climate-induced warfare." And thus he states in the concluding section of his paper that "we must add to the advantages of economic development that it appears to make countries more peaceful," and that we must therefore ask ourselves if environmental objectives should be "modified by the prospect that combating climate change could prolong the process of transition from warlike to peaceful polities."

References
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Buhaug, H., Gleditsch, N.P. and Theisen, O.M. 2010. Implications of climate change for armed conflict. In: Mearns, R. and Norton, A. (Eds.). The Social Dimensions of Climate Change: Equity and Vulnerability in a Warming World. World Bank, Washington, DC, USA, p. 75-101.

Ghosn, F., Palmer, G. and Bremer, S. 2004. The MID 3 data set, 1993-2001: Procedures, coding rules, and description. Conflict Management and Peace Science 21: 133-154.

Gochman, C.S. and Maoz, Z. 1984. Militarized interstate disputes, 1816-1976: Procedure, patterns, and insights. Journal of Conflict Resolution 28: 585-615.

Goldstein, J.S. 2011. Winning the War on War: The Decline of Armed Conflict Worldwide. Dutton, New York, New York, USA.

Gurr, T.R., Jaggers, K. and Moore, W.H. 1989. Polity II: Political Structures and Regime Change, 1800-1986. Codebook, University of Colorado.

Hensel, P. 2002. The more things change ...: Recognizing and responding to trends in armed conflict. Conflict Management and Peace Science 19: 27-53.

Levy, J.S., Walker, T.C. and Edwards, M.S. 2011. Continuity and change in the evolution of warfare. In: Maoz, Z. and Gat, A. (Eds.). War in a Changing World. University of Michigan Press, Ann Arbor, Michigan, USA, p. 15-48.

Luard, E. 1986. War in International Society: A Study in International Sociology. Tauris, London, United Kingdom.

Luard, E. 1988. Conflict and Peace in the Modern International System: A Study of the Principles of International Order. Macmillan, Houndmills.

Marshall, M. and Jaggers, K. 2002. Polity IV: Political regime characteristics and transitions, 1800-2002 (http://www.systemicpeace.org/polity/polity4.htm).

Mueller, J. 2009. War has almost ceased to exist: An assessment. Political Science Quarterly 124: 297-321.

Nordas, R. and Gleditsch, N.P. 2007. Climate change and conflict. Political Geography 26: 627-638.

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Sarkees, M.R., Wayman, F.W. and Singer, J.D. 2003. Inter-state, intra-state, and extra-state wars: A comprehensive look at their distribution over time, 1816-1997. International Studies Quarterly 47: 49-70.

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Zhang, D., Jim, C.Y., Lin, G., He, Y.-Q., Wang, J. and Lee, H. 2006. Climatic change, wars and dynastic cycles in China over the last millennium. Climatic Change 76: 459-477.

Zhang, D., Brecke, P., Lee, H., He, Y.-Q. and Zhang, J. 2007. Global climate change, war, and population decline in recent human history. Proceedings of the National Academy of Sciences USA 104: 19,214-19,219.