Historians have speculated for years that global environmental changes caused some ancient wars to erupt, or even societies to collapse. Such connections may still exist—because new research finds that the risk of civil war in tropical countries increases during hot, dry El Nino years as opposed to cooler La Nina periods. The study is in the journal Nature. [Solomon M. Hsiang, Kyle C. Meng and Mark A. Cane, Civil conflicts are associated with the global climate]

Researchers used a database of global conflicts over the last six decades, which included clashes resulting in at least 25 deaths in battle. They compared those data to the three- to seven-year cycling of El Nino. And they found that the risk of conflict doubled during El Nino years—but only in tropical countries most affected by the climate cycle. El Nino may have played a role in spurring a fifth of the 234 conflicts studied.

This study can’t determine a cause. But the researchers say hot El Nino conditions can diminish harvests, causing food shortages and sparking conflicts. And since El Nino can be predicted up to two years in advance, governments and NGOs might be better able to plan for the possibility of a civil war.

—Christopher Intagliata

[The above text is a transcript of this podcast.]

[Scientific American is part of the Nature Publishing Group.]