Does El Nino—and Climate Change—Really Cause Civil Wars?

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For several years now, a few academics have been fighting a civil war over the possible effects of climate and global warming on, well, civil war. In 2009 Marshall Burke, an economist at the University of California-Berkeley, co-authored a paper arguing that higher temperatures increased the risk of civil conflict—and that the warming predicted by 2030 could cause a 54% increase in armed civil conflict in Africa, leading to 393,000 additional battle deaths. Then the following year Halvard Buhaug, a senior researcher at the Centre for the Study of Civil War in Oslo, published a study arguing that Burke’s work was essentially poppycock. Buhaug combed through the same data Burke used, finding little evidence that temperature had much impact on the intensity of civil conflict—and noting that the past 10 to 15 years, when temperatures were unusually high, also saw a drop in African warfare. If warming had a discernible effect on civil wars, it was far less important than geopolitical factors, population growth and economic changes.

Now a new shot has been fired in this climate civil war—albeit from a different side. In a study published in the August 24 Nature, researchers from Princeton University and Columbia University’s Earth Institute looked at the impact of the El Nino climate cycle on civil conflict over the past half-century. (Quick background: El Nino is one half of the El Nino-Southern Oscillation cycle, or ENSO, which involves the periodic warming and cooling of the tropical Pacific Ocean. The El Nino phase—which occurs every three to seven years—tends to bring unusually high temperatures and dry weather to tropical countries.) The Nature researchers found that the arrival of an El Nino phase doubled the risk of civil conflict across 90 affected tropical countries, and may help account for a fifth of worldwide conflicts over the past half-century. “This is the first major evidence that the global climate is a major factor in organized violence around the world,” says Solomon Hsiang, an expert in sustainable development and the study’s lead author.

Scientists and writers have suspected that strong climatic or environmental change can trigger conflict, and even cause societies to fall. In his great book Collapse, Jared Diamond argued that unusually long and punishing droughts—in addition to population pressures—may have helped virtually wipe out the Anasazi Indians in the American Southwest. Other researchers have used proxy measures for climate change—like tree rings or titanium sediments in the ocean—to match extreme climatic events with the collapse of ancient civilizations, including the Maya, Angkor in Cambodia and Mesopotamia’s Akkadian empire. More recently, the author Mike Davis made a controversial case that persistent, El Nino-influenced famines in the late 19th century—along with colonial mismanagement—effectively crippled what we’d eventually come to refer to as the developing world.

But Hsiang and his colleagues say they’ve managed to nail down a more persuasive case with current data, arguing that climate changes can still have a strong effect on conflict even in the modern era, when we’re more insulated from the effects of weather. The researchers tracked El Nino from 1950 to 2004 and correlated it with onsets of civil conflicts that killed more than 25 people in a given year. (Altogether their data set included 175 countries and 234 conflicts.) For tropical nations affected by the El Nino cycle, they found that the chance of civil war breaking out during an El Nino was 6%, double the chance during La Nina years. (La Nina is the flip side of the El Nino cycle.) Overall, they calculated that El Nino may play a role in nearly 30% of the civil conflicts in countries strongly affected by the cycle. “We’re showing that global climate influences local conflict,” says Hsiang.

It’s less clear how El Nino might help trigger civil conflict, though. Tropical countries tend to see high temperatures and drought during El Nino years, which can lead to famine and damage economies. That in turn might lead to increased income inequality and social unrest—especially if governments botch the response to higher food prices. In essence, El Nino could be what defense types call a “threat multiplier,” helping to push broken societies over the edge into civil conflict. It’s also possible that higher temperatures just make people a little more willing to engage in violence, which anyone who’s lived through a New York City summer would know. Studies have even shown that in baseball games, when a pitcher’s teammates have been hit by a pitch earlier in the game, the chance that pitcher will retaliate by hitting opposing batters rises sharply during hotter days. “It’s hard to disentangle which factor is more important than the other,” says Kyle Ming Meng, a study co-author. “But this hotter and drier weather has clear adverse effects on society.”

Now the caveats. It’s important to understand that the Nature study does not say that El Nino alone causes civil wars. Rather, the authors argue that adverse weather changes are one factor among many—including economics, politics, and social factors that can be tougher to translate into raw data. Nor does this study–unlike Burke’s earlier research—make clear claims on how manmade climate change might impact civil wars. El Nino is a natural part of the climate, and they’ll continue to occur in the future no matter how manmade carbon emissions affect global temperature. (Although some studies have indicated that climate change might make the El Nino weather-state more common, which might be bad news for tropical countries.) And El Nino is hardly a definitive factor—rich Australia, which is heavily impacted by El Nino, has never been close to a civil war. El Nino seems to have a much bigger effect on the chance of civil conflict in the poorest countries—but is that due to the weather, or to poverty? “We’re in no way claiming that climate variation is the sole cause of conflict,” says Mark Cane, a climate scientist at Columbia and a co-author of the paper. “But what we can say is that things external to human social systems like climate can help cause conflict.”

Still, while the Nature study is elegantly prepared, there’s a slight “duh” element here. It’s hardly surprising that droughts and famines can help push poor, unstable countries over the edge into civil conflict. But is the causation really that strong? Buhaug, already skeptical of the connection between climate change and conflict, is doubtful:

The study fails to improve on our understanding of the causes of armed conflicts, as it makes no attempt to explain the reported association between ENSO cycles and conflict risk. Correlation without explanation can only lead to speculation.

When it comes to conflicts, climate effects take a backseat—at least for now—to manmade factors over which we have more control, like population growth and economic development. Take the devastating famine underway in Somalia right now. While the Horn of Africa is experiencing the worst drought in decades, the horrific death toll from the famine has more to do with civil conflict within the country than it does with the lack of rain. (As Hsieng points out,  forecasters had predicted a major drought for East Africa this year, yet aid groups failed to respond in time.) The Nature researchers are right that even today, we’re still more vulnerable to the weather and to the climate than we might think—but that’s one more reason to build the richer, resilient societies that can weather that risk.

Bryan Walsh is a senior writer at TIME. Find him on Twitter at @bryanrwalsh. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME