Cyclical climatic changes double the risk of civil wars, with analysis showing that 50 of 250 conflicts between 1950 and 2004 were triggered by the El Nino cycle, according to scientists.
Researchers connected the climate phenomenon known as El Nino, which brings hot and dry conditions to tropical nations and cuts food production, to outbreaks of violence in countries from southern Sudan to Indonesia and Peru.
Solomon Hsiang, who led the research at Columbia University, New York, said: "We can speculate that a long-ago Egyptian dynasty was overthrown during a drought. This study shows a systematic pattern of global climate affecting conflict right now. We are still dependent on climate to a very large extent."
Hsiang said that pre-emptive action could prevent bloodshed because El Nino events could be predicted up to two years ahead. "We hope our study may help reduce humanitarian suffering."
Global warming caused by humans, with the continual ramping up of temperature and extreme weather, differs from the natural El Nino cycle, the scientists are careful to note.
Mark Cane, a member of the team, said global warming would have greater climatic impacts than El Nino, making it "hard to imagine" it would not provoke conflicts.
The scientists are beginning work to discover the factors involved in the climate-conflict link. Food is likely to be key as crop yields and incomes from agriculture are known to fall heavily in El Nino years. "When crops fail, people may take up a gun to make a living," said Hsiang.
Other factors could include rises in unemployment and natural disasters, such as hurricanes. "Also, previous work has shown that when people get warm and uncomfortable, they are more prone to fight," said Cane.
The research, published in Nature, uses a statistical approach to show that the risk of a conflict doubles from 3% to 6% in El Nino cycles (which occur every three to seven years) in affected nations. Unaffected nations showed no such pattern.
The analysis shows that a fifth of the 250 civil conflicts between 1950 and 2004 were precipitated by hotter, drier weather. Differing levels of poverty, democracy and population did not alter the strength of the climate-conflict link, nor did the impact of the end of colonial rule in many countries by 1975.
However, bad weather does appear to tip less developed countries into chaos more easily, said Hsiang, pointing to the example of southern Sudan, where intense warfare broke out in the El Nino year of 1963.
After a flare-up in another El Nino year, 1976, a severe El Nino, in 1983, saw the start of more than 20 years of fighting, which left 2 million people dead and culminated only this year when South Sudan was formed as a separate nation.
By contrast, Australia's climate is controlled by El Nino cycles, but has had no civil conflicts. "One hypothesis is the poorest countries lack the resources to cope with [the impacts] of El Nino," said Hsiang. "Another is that they could be physically more vulnerable to El Nino, prompting war and leading to poverty."
Marshall Burke, an economist at the University of California, Berkeley, said the research gave very convincing evidence of a connection.Andrew Solow, an environmental statistician, at the Woods Hole Oceanographic Institution, said: "Careful statistical analyses such as this one, which relate complex human behaviour to environmental factors, can be invaluable."
Yaneer Bar-Yam, president of the New England Complex Systems Institute in the US, said: "It is part of the progress we are making in understanding the drivers of human social behaviour."