What accounts for the collapse of history's great civilizations? Retired anthropologist and author Brian Fagan blames it on the weather, and in particular on the pernicious effects of El Niño, a periodic warming of the Pacific Ocean that brings a hotter, drier climate to tropical countries worldwide. Fagan and others have suggested that El Niño had a hand in the biblical droughts in Egypt, the disappearance of the Mayan Empire, and even the French Revolution.
A newly released study in this week's Nature suggests that there may be some science underlying this speculation and that El Niño's effects on the wealth and stability of nations has continued into recent times. Combining cutting-edge climate science with data on civil conflicts during the second half of the 20th century, the researchers estimate that more than a fifth of violent flare-ups worldwide may have been triggered by El Niño. And if climate experts are correct in predicting that in the next century El Niño-like conditions will become more commonplace due to climate change, the study's findings point to a hotter, drier, and more violent planet in the years ahead.
The primary mechanism by which warmer, drier weather potentially causes civil conflict is none too subtle—heat and drought reduce the food supply, leaving hungry, rebellious populations in their wake. The unruly hordes take up arms against the government or fight amongst themselves for scarce resources.
El Niño promises not just one lean season but up to 18 months of heat and drought, which can sometimes be predicted well in advance. Indigenous societies have for centuries had ways of divining El Niño's arrival. Andean potato farmers, for example, learned to predict El Niño's onset from the brightness of stars in the Pleiades constellation. Why bother tilling the soil if you know there's nothing but drought on the horizon? Better to prepare for war instead, especially if you expect your neighbors will be taking up arms as well.
The new Nature study isn't the first to consider the link between global climactic changes and war. But most large-scale shifts in weather take place over centuries or millennia, amid technology revolutions and social upheaval that make it impossible to discern any measurable effect due to climate. Saying that the world is less violent now than it was 500 years ago—or more violent than it was during the last ice age—isn't a very useful observation, even if we had comparable data on violence going back that far. El Niño causes the tropics to randomly flip back and forth between weather extremes every few years, which allows the researchers to compare the level of civil conflict within a given country under very different climactic conditions but just a few years apart.
The authors (who include eminent oceanographer Mark Cane, the first person to successfully forecast El Niño through numerical modeling) classify El Niño months based on equatorial Pacific sea surface temperatures, with extended periods of warming as the indicator of El Niño's arrival. Unusually cool surface temperatures indicate the onset of La Niña, El Niño's kinder, wetter mirror image. Using data on armed conflicts during 1950-2004 compiled by Swedish and Norwegian researchers, they compare the likelihood that conflicts flare up during El Niño versus La Niña years. (The researchers use a standard measure of conflict onset, defined as a new civil dispute that breaks out between the government and an organized adversary, resulting in at least 25 battle-related deaths in that year.)
In regions affected by El Niño fluctuations the authors find a doubling of conflict risk during El Niño as compared to La Niña. El Niño is a relatively frequent event, occurring every four to seven years, and it affects fully half of the world's population (including most of Latin American, South and Southeast Asia, and all of Sub-Saharan Africa). Given this high frequency and wide-ranging impact, the paper's findings suggest that El Niño has had a hand in triggering as much as 21 percent of recent conflicts worldwide. In a "placebo" set of countries relatively removed from El Niño's influence—including most of North Africa, the Middle East, and northern parts of Asia—conflict risk is the same during both El Niño and La Niña periods, bolstering the claim that weather is indeed behind their results. The authors also found that El Niño's impact on conflict took place relatively soon after its arrival, and well before food shortages would have really set in. While they don't speculate on the explanation for these patterns in the data, they are broadly consistent with the critical role that expectations of future hardship play in causing people to switch from agriculture to waging war.
These findings are of grave concern, if you believe—as many climate scientists do—that climate change will produce global weather patterns that are more El Niño-like, with just as much year-to-year climate variability as exists today, but with warmer and drier conditions overall. While it would be premature to draw any conclusions, given the randomness in year-to-year appearances of El Niño and the extreme complexity of global climate systems, it does hint at one more reason to worry about a warming planet.
But linking the onset of violence to a relatively predictable phenomenon like El Niño might help us head off impending conflict before it starts. Scientists can with some certainty spot an El Niño half a year away, which means aid organizations should have more than enough time to deliver El Niño-dependent relief that can diminish the conflict-inducing effects of drought and famine before bad weather even arrives. By uncovering the link between El Niño and violence, this study may serve as a very early first step in shielding the developing world from the ill effects of its hotter, drier future.