In the first study of its kind, researchers have linked a natural global climate cycle to periodic（周期的） increases in warfare. The arrival of El Niño, which every three to seven years boosts temperatures and cuts rainfall, doubles the risk of civil wars across 90 affected tropical countries, and may help account for a fifth of worldwide conflicts during the past half-century, say the authors. The paper, written by an interdisciplinary（各学科间的） team at Columbia University's Earth Institute, appears in the current issue of the leading scientific journal Nature.

In recent years, historians and climatologists have built evidence that past societies suffered and fell due in connection with heat or droughts that damaged agriculture and shook governments. This is the first study to make the case for such destabilization in the present day, using statistics to link global weather observations and well-documented outbreaks of violence. The study does not blame specific wars on El Niño, nor does it directly address the issue of long-term climate change. However, it raises potent questions, as many scientists think natural weather cycles will become more extreme with warming climate, and some suggest ongoing chaos in places like Somalia are already being stoked by warming climate.

"The most important thing is that this looks at modern times, and it's done on a global scale," said Solomon M. Hsiang, the study's lead author, a graduate of the Earth Institute's Ph.D. in sustainable development. "We can speculate that a long-ago Egyptian dynasty was overthrown during a drought. That's a specific time and place, that may be very different from today, so people might say, 'OK, we're immune to that now.' This study shows a systematic pattern of global climate affecting conflict, and shows it right now."

The cycle known as the El Niño-Southern Oscillation, or ENSO, is a periodic warming and cooling of the tropical Pacific Ocean. This affects weather patterns across much of Africa, the Mideast, India, southeast Asia, Australia, and the Americas, where half the world's people live. During the cool, or La Niña, phase, rain may be relatively plentiful in tropical areas; during the warmer El Niño, land temperatures rise, and rainfall declines in most affected places. Interacting with other factors including wind and temperature cycles over the other oceans, El Niño can vary dramatically in power and length. At its most intense, it brings scorching（灼热的，激烈的） heat and multi-year droughts. (In higher latitudes, effects weaken, disappear or reverse; La Niña conditions earlier this year helped dry the U.S. Southwest and parts of east Africa.)

The scientists tracked ENSO from 1950 to 2004 and correlated it with onsets of civil conflicts that killed more than 25 people in a given year. The data included 175 countries and 234 conflicts, over half of which each caused more than 1,000 battle-related deaths. For nations whose weather is controlled by ENSO, they found that during La Niña, the chance of civil war breaking out was about 3 percent; during El Niño, the chance doubled, to 6 percent. Countries not affected by the cycle remained at 2 percent no matter what. Overall, the team calculated that El Niño may have played a role in 21 percent of civil wars worldwide -- and nearly 30 percent in those countries affected by El Niño.

Coauthor Mark Cane, a climate scientist at Columbia's Lamont-Doherty Earth Observatory, said that the study does not show that weather alone starts wars. "No one should take this to say that climate is our fate. Rather, this is compelling（引人注目的） evidence that it has a measurable influence on how much people fight overall," he said. "It is not the only factor--you have to consider politics, economics, all kinds of other things." Cane, a climate modeler, was among the first to elucidate（阐明） the mechanisms of El Niño, showing in the 1980s that its larger swings can be predicted -- knowledge now used by organizations around the world to plan agriculture and relief services.