The new questions, he continues, are why, and how heavily does climate influence conflict compared with other influences, such as personal or national wealth, political stability within a region or country, or how governments respond to migration within or across their national borders.
Other researchers exploring the links between climate, social, economic, and political conditions and conflict say the new study overstates climate's effect on the risk of conflict.
John O'Loughlin, a geographer at the University of Colorado at Boulder who explores the interplay of environmental conditions and conflict, notes that in his research, as in the new study, climate conditions play a role. But his own work suggests that warming temperatures or extreme precipitation are poor predictors of risk compared with the social, economic, and political drivers for conflict.
Still, the new study "really is a heroic effort. They're the first people to sit down and say: Can we make sense of this?" he says, referring to the 60 studies the team included in its analysis.
The studies span some 10,000 years of human history, "conflicts" ranging in scales from incidents of domestic violence to all-out war, and shifts in temperatures and rainfall that range from events lasting a few hours to centuries.
The studies involved criminologists, geographers, psychologists, and archaeologists who "all had been working on related issues," Hsiang says. "We set about trying to understand what everyone was finding."
The studies varied in important ways, he adds. But once his team set the studies into "a single coherent framework, we saw that people were seeing the same thing around the world."
The common thread: Higher temperatures, or higher extremes in precipitation – either more of it or less of it – "are associated with higher levels of human conflict around the world, throughout time, in different types of societies," Hsiang says.