"There’s a surprisingly strong link between climate change and violence." That's the headline of a recent article by journalist Chris Mooney in The Washington Post. As fossil-fuel emissions push temperatures higher, we can "expect more wars, civil unrest, and strife, and also more violent crime in general," Mooney says. But the evidence for this alarming claim is surprisingly weak, not strong.*

Mooney's evidence is a meta-analysis by economists Marshall Burke, Solomon Hsiang and Edward Miguel of 55 studies of linkages between climate and violence. The violence, Burke et al. state, includes "both interpersonal conflict--such as domestic violence, road rage, assault, murder, and rape--and intergroup conflict -- including riots, ethnic violence, land invasions, gang violence, civil war and other forms of political instability, such as coups."

The researchers assert that each one-degree-Celsius rise in temperature increases "the frequency of contemporaneous interpersonal conflict by 2.4% and of intergroup conflict by 11.3%." Burke and his colleagues have advanced this thesis before, notably in Climatic Change and Science.

Here are some problems with the thesis:

Problem One: In spite of the recent surge in violence in the Middle East, war-related casualties have fallen over the last half-century, as temperatures have risen, as illustrated by the two charts in this column. According to a 2014 report from the Human Security Report Project at Simon Fraser University, between 1950 and 2007 the annual combat-related mortality rate fell from 240 per million people to less than 10. Rates of homicide unrelated to war "are declining in every region of the developing world except Latin and Central America," according to the Simon Fraser report.

Problem Two: In his Washington Post article, Mooney quotes Romeo and Juliet—"these hot days, is the mad blood stirring"—to support his speculation that people get cranky and hence more likely to behave violently when they're hot. Others argue, rather more plausibly, that global warming might provoke conflict by causing shortages of water, food and other necessities. As I have pointed out previously—see for example this 2012 column--anthropological research finds a weak linkage between resource scarcity and war.

Problem Three: Today, many people making decisions that lead to large-scale violence—politicians, generals, warlords, drug kingpins and so on—work indoors in climate-controlled conditions, insulated from shifts in atmospheric temperature.

Problem Four: One chapter of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, published this year, concludes on the basis of the work of Burke et al. that "the effect of climate change on conflict and insecurity has the potential to become a key risk." But another chapter, written by different authors, examines a broader range of research and concludes that “collectively the research does not conclude that there is a strong positive relationship between warming and armed conflict.”

Problem Five: The research of Burke et al. has been critiqued by 26 researchers led by Halvard Buhaug of the Peace Research Institute Oslo (PRIO) in Norway. In a new analysis in Climatic Change, Buhaug and his colleagues note that the research of Burke et al. "suffers from shortcomings with respect to sample selection and analytical coherence. A modified assessment that addresses some of these problems suggests that scientific research on climate and conflict to date has produced mixed and inconclusive results." Buhaug's group probes in detail one subset of the studies analyzed by Burke et al., involving conflicts between "organized non-state actors and state military forces." Buhaug's group found "no evidence of a convergence of findings on climate variability and civil conflict. Recent studies disagree not only on the magnitude of the impact of climate variability but also on the direction of the effect."

Problem Six: Green scientists, activists and journalists want politicians and the public to take global warming seriously and take steps to counteract it. But when they promote dubious claims, Burke, Mooney and others undermine their credibility and hence their cause. Their efforts may lead to other harmful consequences. A 2014 report of the U.S. Defense Department contends that climate change is a "threat multiplier" that "will intensify the challenges of global instability, hunger, poverty, and conflict."

Predictions of warming-induced war are more likely to result in higher military budgets than lower fossil-fuel emissions.

*Postscript: See my followup post: "Hawkish U.S. Policies Pose Bigger Threat to Peace Than climate Change."