Is climate change a matter of national security? In a warming world, sea-level rise, drought and soil degradation are putting basic human needs such as food and shelter at risk. In March, the U.S. Department of Defense called climate change a "threat multiplier," saying that competition for resources "will aggravate stressors abroad such as poverty, environmental degradation, political instability and social tensions—conditions that can enable terrorist activity and other forms of violence."

Connecting climate change to a global increase in violence is tricky, and attempts to make such a link receive a fair amount of criticism. A hotter planet doesn't automatically become a more conflict-ridden one. The 2000s, for instance, saw some of the highest global temperatures in recorded history—and some of the lowest rates of civil conflict since the 1970s.

But there are historical examples of civilizations that did not fare well when faced with drastic environmental change, and those examples may offer a window into the future—and even help prevent catastrophe. "We can never know with 100-percent certainty that the climate was the decisive factor [in a conflict]," says Solomon Hsiang, assistant professor of public policy at the University of California, Berkeley. "But there's a lot of cases where things look pretty conspicuous."