Peru’s civil war started in an El Niño year (Image: Enrique Castro-Menoivil/Reuters/Corbis)

THE Peruvian highlands were hit hard by El Niño in 1982, and crops were destroyed. The same year, guerrilla attacks by the Shining Path movement erupted into a civil war that would last 20 years. Random coincidence? Possibly not.

The first study to link global climate patterns to the onset of civil conflict places El Niño on a par with factors like poverty and social exclusion.

Solomon Hsiang, a researcher in international affairs at Princeton University, and colleagues looked at data on conflicts between 1950 and 2004 that killed more than 25 people in a year. They compared El Niño years, which happen roughly every five years, with La Niña years. El Niño tends to bring hotter, drier conditions – and La Niña cooler ones – to tropical countries, but both have less of an influence on temperate countries.


The analysis included 175 countries and 234 conflicts, over half of which caused more than 1000 deaths. It found that the risk of conflict in tropical countries rose from 3 per cent during La Niña years to 6 per cent during El Niño years. The effect was absent from countries only weakly affected by these climate cycles (Nature, DOI: 10.1038/nature10311).

“I was surprised by the strength of the effect,” says Halvard Buhaug of the Peace Research Institute Oslo in Norway. “Doubling of risk is a large increase, about on a par with poverty and ethno-political exclusion.”

Doubling the risk of conflict is a large increase, about on a par with poverty and ethno-political exclusion

Buhaug has been sceptical of similar studies, and though he finds the statistics convincing, he says he is puzzled, as the study offers no explanation for how El Niño might exert an influence over stressed human societies.

Hsiang’s team found that El Niño appeared to have an immediate effect – in their analysis, conflicts erupted within months of the onset of El Niño events – but the correlation was independent of local weather events like drought, which can bring famine and increased tension.

Hsiang cannot yet explain what is causing the link. One possibility is that international markets spread climate signals around the world. For instance, widespread drought in an El Niño year could cause global food prices to rise.

But as Andrew Solow of the Woods Hole Oceanographic Institution in Massachusetts points out, “People do not start wars simply because they are hot.” And until we know what it is about El Niño that increases the likelihood of conflict, it will be impossible to say whether this means we should expect more unrest due to climate change.