|Countries where the majority of the population lives in areas that become much warmer in El Niño years (red) are more likely to experience wars than those where temperatures are less affacted (blue).|
Tropical conflicts double during El Niño years.
Natural climate cycles seem to have a striking influence on war and peace around the equator. Tropical countries face double the risk of armed conflict and civil war breaking out during warm, dry El Niño years than during the cooler La Niña phase of the El Niño/Southern Oscillation (ENSO), according to an analysis published today in Nature1.
The study throws light on the hotly contested issue of whether climate change has any notable effect on violence and societal stability, particularly in poor countries. The authors of several popular books have previously proposed a link, but there are disagreements within the scientific literature over whether a robust climate signal can be detected in conflict statistics.
Previous studies have focused on the question of how anthropogenic climate change might increase conflict risk. A 2009 study by economist Marshall Burke at the University of California, Berkeley, and his co-workers found that the probability of armed conflict in sub-Saharan Africa was about 50% higher than normal in some unusually warm years since 1981. But critics point to statistical problems — for instance when linking possibly random local temperature and rainfall variations with outbreaks of civil war — that may have resulted in a false appearance of causality.
To overcome this problem, Solomon Hsiang, an economist currently at Princeton University in New Jersey, and his colleagues opted to look at how historical changes in the global, rather than local, climate affect conflict risk1.
The team designed a 'quasi-experiment' for which they divided the world into regions strongly affected by the ENSO — the tropical parts of South America, Africa and the Asia–Pacific region, including parts of Australia — and regions only weakly affected by it. They then searched for a link between climate and armed conflicts that arose in the first group between 1950 and 2004.
A very clear signal appeared in the data. The team found that the risk of annual civil conflict doubles, from 3% to 6%, in countries of the ENSO-affected, or 'teleconnected', group during El Niño years relative to La Niña years. In many cases, conflicts that might have broken out anyway may have occurred earlier owing to the effects of El Niño, Hsiang suggests.
Civil conflicts have been by far the most common form of organized political violence in recent decades, Hsiang says. Globally, one-fifth of the 240 or so civil conflicts since 1950 could be linked to the 4–7-year climate cycle originating in the southern Pacific, the study concludes. The results were unaffected by any modification to the statistical set-up of the analysis — such as excluding particularly crisis-prone African countries — which the team performed to confirm the robustness of their findings.
"A doubling of risk is a very strong effect," says Halvard Buhaug, a conflict researcher with the Peace Research Institute Oslo, who was not involved in the study.
Buhaug, who has previously criticized claims such as Burke's, says he feels "surprised and a bit puzzled" by the results. He grants that the study is "very competently executed" but adds that the issue is nonetheless far from being settled. "I don't dismiss that a correlation exists, but it is a correlation we so far don't understand," he says. "I remain sceptical about any potential causal connection."
A more detailed analysis of the 'narratives' of historical conflicts that have occurred during El Niño years is needed to establish whether any factors that may have caused these conflicts — such as harvest failures that led to food shortages — can be traced to El Niño events, he says.
The authors of the study are aware of its limitation and of the difficulties involved in establishing a causal link between climate and conflict. But, says Hsiang, case studies are ongoing at Columbia University in New York and elsewhere on how El Niño events might link to local outbreaks of violence.
"Different hypotheses have been proposed as to how one phenomenon causes the other, and we aren't sure yet what the correct narrative is," he says. "It could be that agricultural income in El Niño years drops to levels that can trigger violence. Furthermore, psychologists think that aggressive behaviour gets generally more widespread during exceptionally warm conditions."
El Niño events, he adds, are by no means the sole factor leading to conflict. But although these natural climate cycles do seem to play a part in the peacefulness of nations, the authors warn against rushing to the conclusion that anthropogenic greenhouse warming will lead to more armed conflict and political instability.
Climate models give ambiguous projections as to how ENSO will change in a warming world. "El Niño is different in structure than anthropogenic climate change," says Hsiang. "It would therefore be hard to map our results onto future changes."
But Burke, who was not involved in the study, says that the work could be useful for at-risk countries. "The fact that ENSO is itself somewhat predictable makes their findings policy-relevant," he says. "If we think an El Niño is coming, then governments in teleconnected regions could put in place measures and safety nets to try to reduce the risk of conflict in that year."
El Niño appearances tied to civil conflicts in tropical countries
Political conflicts are extremely complex, and we almost never understand all the factors that are involved in their timing, the course they take, and their eventual outcome. In this week’s Nature, a paper suggests a new variable to consider: the climate. According to researchers from Columbia and Princeton, there is good evidence that global climate variations can play a role in the onset of civil conflicts.
The El Niño-Southern Oscillation cycle, known as ENSO, is a climatic pattern that repeats about every 5 years. Variations in water temperatures and air pressure in the Pacific Ocean cause El Niño years to be warm and dry, while La Niña years are cooler and wetter. These oscillations are felt most strongly in tropical countries, while mid-latitude areas are much less affected.
The authors of the study hypothesized that these cycles might play a role in the onset of civil conflicts. Although some research has been done on the correlation between climate and conflict, methodological difficulties and inconclusive results have caused lots of confusion. Climate studies are difficult because there is no Earth we can control to experiment with; we are stuck examining global patterns with few controls. However, in this study, ENSO provides a convenient experimental setup: volatile El Niño years provide a “treatment” group, while calmer La Niña years serve as a control.
175 countries were included in the dataset, including 93 tropical countries that are highly prone to the effects of El Niño and 82 other countries that are weakly affected by ENSO. Countries were classified as experiencing "conflict onset" in a given year if more than 25 people had died as a direct result of a new political dispute between two groups. Conflicts from 1950 to 2004 were included. Then the researchers calculated the annual conflict risk (or ACR) for both the tropical countries and the weakly affected countries. The ACR is the probability that a randomly selected country from the group experienced conflict onset in a particular year.
For the countries that aren’t highly affected by ENSO, such as Greece, Afghanistan, and Sweden, the ACR was about 2 percent in both El Niño and La Niña years, indicating that these climate cycles are unlikely to affect civil conflict in these countries. However, the ACR for tropical countries such as Australia, Sudan, and Trinidad doubled during El Niño years, increasing from 3 percent to 6 percent. From their analyses, the researchers concluded that the ENSO cycle may have affected 21 percent of civil conflicts since 1950.
The results are remarkably robust; the researchers repeated the analysis with various types of statistical models and with different ENSO indices, and their conclusions remained the same. Furthermore, the results held up even when high-conflict countries were excluded from the dataset, and when other variables, such as a country's age structure, income growth, and agricultural reliance, were included in the analyses.
Two additional findings were particularly intriguing. Many of the conflicts affected by ENSO are particularly deadly recurring conflicts. By changing the requirement for the length of "peaceful periods" between conflicts, the researchers found that the relationship between ENSO and large conflicts decreased. Additionally, low-income countries were the hardest hit by El Niño years, indicating that poorer countries are particularly sensitive to ENSO patterns.
Although the relationship between ENSO and civil conflicts is quite clear, the reasons behind this correlation are still not understood. Warm, dry El Niño years might decrease agricultural output, stressing a country’s resources and increasing food prices. ENSO patterns affect the frequencies of natural disasters, such as hurricanes and cyclones, which put countries at risk for upheaval. Extreme conditions can also cause psychological stress and alter human behavior. Finally, ENSO is a widespread phenomenon, and may cause suboptimal conditions on a large scale.
In order to use the results of this study to predict or alleviate conflicts, we need to better understand the mechanisms driving the relationship between climate patterns and political conflict.
First proof that climate is a trigger for conflict: study
Climate shift has at times been fingered as a culprit in triggering conflict, fuelling for instance the 1789 French Revolution by wrecking harvests and driving hungry peasants to the city.
Evidence to back the theory has often been contested as sketchy or anecdotal - but the case has been boosted by the first scientific study to declare an unmistakeable link between climate fluctuations and violence.
It says tropical countries affected by the notorious El Nino weather event are twice as likely to be hit by internal unrest compared to the phenomenon’s cooler, wetter counterpart, La Nina.
The civil war and famine gripping the Horn of Africa is a typical example of what happens when a climate swing causes drought and overstresses an already fragile society, say its authors.
The inquiry, appearing in Thursday’s issue of the journal Nature, focusses on a naturally occurring pattern of climate change.
But its authors say there is a disturbing lesson about violence driven by man-made warming, which is expected to bite deep in coming decades.
"What it does show and show beyond any doubt is that even in this modern world, climate variations have an impact on the propensity of people to fight," said Mark Cane, a climate scientist at the Lamont-Doherty Earth Observatory at Columbia University in New York.
"It’s difficult to see why that won’t carry over to a world that’s disrupted by global warming."
Formally known as the El Nino-Southern Oscillation (ENSO), the cycle occurs every two to seven years and last from nine months to two years, often inflicting massive losses on agriculture, forestry and fishing.
It starts when warm water builds on the western side of the tropical Pacific and shifts across the ocean.
This part of the cycle, El Nino, can cause dramatic changes in rainfall patterns and temperatures, unleashing scorching heat or drying winds in much of Africa, South and Southeast Asia and Australia.
When the cycle goes into reverse, a phase called La Nina, the water in the eastern Pacific cools, often bringing heavy rain to those regions.
The study looked at ENSOs from 1950 to 2004 and overlaid this data with civil conflicts - violence that had taken place within national borders, as opposed to cross-border wars - that had killed more than 25 people in a given year.
The data included 175 countries and 234 conflicts, more than half of which caused more than 1,000 battle-related deaths.
In countries whose weather cycles are determined by ENSO, the risk of civil conflict occurring during La Nina was about three percent; during El Nino, this doubled to six percent, the paper says.
Countries not affected by ENSO remained at two percent regardless.
Overall, according to the study, El Nino may have played a role in 21 percent of civil wars worldwide, and nearly 30 percent in those countries that are specifically affected by El Nino.
Lead author Solomon Hsiang of Columbia’s Earth Institute said El Nino was an invisible factor - but not the only one - in driving intra-border conflict.
By causing crop losses, hurricane damage or helping to spread epidemics of water-borne disease, it amplified hunger, loss, unemployment and inequality, which in turn fuelled resentment and division.
Other factors that could affect risk and the outcome are the country’s population growth and prosperity and whether its government is able to manage El Nino events properly.
"Even though we control for all of these factors simultaneously, we still find that there’s a large and pervasive El Nino effect on civil conflicts," Hsiang said in a teleconference.
Although the current crisis in the Horn of Africa occurred beyond the parameters of the study, it was a "perfect example" of the hidden destruction of an El Nino.
"Forecasters two years ago predicted that there would be a famine in Somalia this year, but donors in the international aid community did not take that forecast seriously," said Hsiang.
"We hope our study can provide the international community and governments and aid organisations with additional information that might in the future help avert humanitarian crises that are associated with conflict."