(Click through to see other half of the map.) This figure shows which countries have a climate that is strongly linked to El Nino, according to the study. Countries coloured red are considered 'teleconnected', meaning that a majority of the population in red countries lives in locations that become much warmer during El Nino years. Countries coloured blue are considered 'weakly-affected', meaning that either (1) temperatures in these countries do not rise substantially during El Nino years and/or (2) regions where temperatures rise during El Nino are not heavily populated.
Credit: Hsiang et al. Nature
This figure shows which countries have a climate that is strongly linked to El Nino, according to the study. Grey countries are not included in the study because they have no data.
Credit: Hsiang et al. Nature
SHROPSHIRE: Cyclical changes in the Earth's climate have been linked to patterns of modern-day human conflict.
A new study has shown that the probability of a civil war breaking out is doubled when the tropical Pacific Ocean is in a warm El Niño state.
"It has long been hypothesised that civil violence responds to changes in the climate," said co-author Kyle Meng from Columbia University in New York of the paper published in Nature today. "We show that civil wars across the tropics respond strongly to the El Niño-Southern Oscillation (ENSO) cycle, one of the most important types of year-to-year climate change."
Change-making climates
Several studies over the past decade have suggested that climate variability can dictate wars and topple civilisations. Drought is a particular harbinger of doom, having been linked to civil war in Africa, fighting within Chinese dynasties, and the downfall of the Maya and Angkor civilisations in Mesoamerica and Cambodia respectively.
However, this new study is the first time that climate variations have been linked to modern-day social changes. The culprit in this case is ENSO, the seesaw between warm (El Niño) and cool (La Niña) ocean temperatures in the eastern tropical Pacific Ocean.
An El Niño typically occurs once every two to seven years and involves a warming of up to 4°C off the west coast of South America. Conversely, a La Niña is associated with a corresponding drop in ocean temperatures.
Unsurprisingly, these large temperature fluctuations can lead to extreme weather events. For instance, an El Niño often generates flooding in Peru and Ecuador, but brings drought to Australia and Indonesia. Impacts as far afield as North America, Africa and Asia are also possible via the propagation of atmospheric waves.
Heat of the battle
The interdisciplinary team of researchers at Columbia University's Earth Institute looked for a correlation between ENSO and the onset of civil war for the period 1950-2004.
A relationship between tropical Pacific Ocean temperatures and civil conflict was discovered in the countries that are strongly affected by ENSO. In these regions, the probability of a new civil war doubles from 3% during a La Niña to 6% during an El Niño.
On the other hand, countries that lie outside ENSO's sphere of influence have only a 2% chance of civil war outbreak, regardless of the state of the tropical Pacific. The authors propose that ENSO may have played a role in one-fifth of all civil wars since 1950.
Societal impacts
So how might an El Niño precipitate a civil war? Two suggested factors are weather extremes and disease outbreaks, both of which could have negative impacts on agriculture, economy and aggression levels. But Meng is keen to point out that ENSO alone does not start wars. "Four-fifths of the observed conflicts are occurring independently of ENSO," he said. "This means that other determinants of violence, such as political institutions or economic development, could also play key roles."
The researchers also found that low-income countries are more sensitive to ENSO than richer countries. "If you have social inequality and ... underlying tensions, it seems possible that climate can deliver the knockout punch," said lead author Solomon Hsiang, now at Princeton University in the U.S. "When crops fail, people may take up a gun simply to make a living."
At the other end of the spectrum, Australia has never experienced a civil war despite being significantly affected by ENSO (the recent flooding in Queensland is one example of La Niña's influence on Australian weather).
Correlation without a cause?
"The most remarkable thing about this study is not that it reports a statistical connection between ENSO and conflict risk in El Niño-sensitive countries," said Halvard Buhaug, a political scientist at Norway's Peace Research Institute Oslo. "Rather, it is the magnitude of this effect - a doubling of the conflict risk - that stands out."
However, Buhaug doubts whether the correlation indicates a true, real-time connection: "I am not convinced that what we observe is a causal relationship. A careful study of a set of selected cases where El Niño overlaps with the onset of civil war might provide some answers. I do not dismiss Hsiang et al.'s results out of hand - the study is solid and intriguing - but it takes more than a correlation to claim a causal relationship. I am confident that this is not the last we will see of research on global climate systems and conflict."
A final straw maybe?
It has long been hypothesized that civil disturbance breaks out when one additional factor turns an extreme situation into an intolerable one. Drought on top of poverty would fulfil that role without being causal, one would think.