Biting critique of the study was voiced in Science as well. In a background article that accompanied the report, the Oslo-based economist Halvard Buhaug was quoted as saying that Hsiang and his team ignored certain data. "More worrisome," he wrote, is the fact that they seem to have used data "that return the strongest effects." Buhaug is the co-author of an April study that contradicts Hsiang's conclusions -- and which was not considered by Hsiang in his project.
Jochem Marotzke, director of the Max Planck Institute for Meteorology in Hamburg, echoed the criticism. He said that Hsiang and his team shrugged off alternative explanations for the increase in violence and thus "maximized the apparent explanatory power of changes in the climate." In general, Marotzke said, "he is skeptical when it comes to the robustness of the results."
Their allegedly selective approach to existing data isn't the only accusation that has been levelled at Hsiang's team. "The second major error is that they confuse climate and weather," says Richard Tol, professor of economy at the University of Sussex in the UK. He notes that it does indeed appear that heat waves make people more aggressive, a conclusion reached by earlier studies. That, though, has more to do with specific weather events, he says, while climate change is measured in decades. Tol says that most of the studies considered by Hsiang and his team focused on specific weather events. "Their projections of the impact of future climate change strongly exaggerate the effect," he says.
Sociologist Nico Stehr, for his part, says that the "biggest mistake" made in the Hsiang study is that it "underestimates" human innovation "when it comes to dealing with weather or climate events."
Authors Bite Back
The authors have been strident in defending themselves. It is certainly possible, they allow in a statement, that future societies will be better able to deal with environmental changes. But, they write, "it seems like hubris to just assume that 'this time things are different.'"
The team also denies having confused weather with climate. Rather, they say, their analysis showed that short-term and long-term changes in temperature have similar consequences for the frequency of conflict. Accusations of distorted data selection are likewise false, they say, claiming that they followed strict criteria in choosing studies for analysis. As a result, some studies that confirm a relationship between global warming and violence were likewise not included in their results.
Some experts are not convinced by the Hsiang-team defense. The researchers "compared apples to roller blading," wrote William Briggs, a professor of statistics at Cornell University in the US, in a sarcastic blog entry. "Data from last Tuesday was said to be equal in veracity to that culled from 8000 BC," and together it "produced lots of sharp graphics and one big quantitative result that hot, rainy weather is bad for you." That, he jokes, "is exactly why everybody is moving from Michigan to South Carolina ... to get to where the action is and join a gang." His verdict on the Hsiang study? "Complete and utter nonsense."
'Good for their Careers'