Rising temperatures are especially provocative. A shift toward greater warmth of one standard deviation caused personal violence to increase by 2.5 percent and intergroup conflict by 24 percent. (One standard deviation varies from place to place; in an African country, for example, it could amount to a warming of 0.6 degrees Fahrenheit for a year.)

Why? The science so far doesn’t answer this question, though it’s easy enough to imagine how subsistence farmers could come into greater conflict with one another as their croplands become less productive. Or how, in the face of rising sea levels, coastal dwellers could come to blows over shrinking habitable land. The scientists who did the review point as well to many psychological and economic studies that show people simply behave more aggressively or violently when temperatures are higher.

Unfortunately, humans also fight, if nonviolently, over climate change itself — specifically, over how, and how quickly, to reduce our destructive emissions. This makes the necessary reductions difficult. The United Nations Framework Convention on Climate Change, for example, originally aimed to have an accord setting firm greenhouse-gas-reduction targets for all countries by 2009. It is now hoping to accomplish this task by 2015. Meanwhile, in the U.S., President Barack Obama has been forced to act outside Congress, through the Environmental Protection Agency, to limit carbon-dioxide pollution from coal-fired power plants. Republicans in the House have predictably introduced legislation to forbid any such action.