Climate change may be linked to increased rates of conflict, recent studies suggest, adding to reasons why the environmental phenomenon is such a hot-button issue.
Scientists are now studying how extreme changes in climate could usher the rise or fall of societies, in a concept known as climatic determinism.
Researchers at Columbia University published a study in the journal Nature in August that found global climate patterns could cause civil unrest, as hot temperatures brought droughts and other agricultural crises.
The team, led by international affairs expert Solomon Hsiang, analysed conflicts that happened between 1950 and 2004, checking how many of them happened in El Nino years, where oceans are warmer, and La Nina years, where ocean temperatures drop significantly.
They found that El Nino - which happens roughly every three to seven years - doubled the risk of civil war across 90 tropical countries and could help account for one-fifth of global conflicts during the past half-century.
'Since 1950, one out of five civil conflicts have been influenced by El Nino,' Hsiang said. 'This represents the first major evidence that global climate is a major factor in organised patterns of violence.'
Their analysis covered 175 countries and 234 conflicts, over half of which caused more than 1,000 deaths. Their sample included conflicts with death tolls of more than 25 people within a given year.
The Columbia University team also ran a comparative simulation estimating the number of civil wars that could occur in a world hit by El Nino as opposed to a world unaffected by the phenomenon. They determined that the hotter, drier conditions helped stoke 48 civil wars, while the simulated El-Nino-free world had none.
'Even in this modern world, climate variability has an impact on the propensity of people to fight,' said climate scientist Mark Cane, part of the team. 'When people get warmer than comfortable, they get irritable and they are more prone to fight.'
This could spell bad news for today's world, where oceans are becoming warmer because of increasing concentrations of greenhouse gases such as carbon dioxide in the atmosphere.
'It is frankly difficult to see why that [conflict scenario] won't carry over to a world that is disrupted by global warming,' Cane said.
El Nino and La Nina affect tropical regions, but have less influence on temperate countries.
In a separate study, however, University of Hong Kong researchers found a causal relationship between colder climate and social crises based on data from pre-industrial Europe.
David Zhang, from the university's geography department, and his colleagues said earlier this month that cooler conditions from 1560 to 1660 resulted in shorter plant-growing seasons and reductions in arable land, pushing grain prices up by 300 per cent. During these times, wars become 'longer and bloodier'.
In the study, published in the latest Proceedings of the National Academy of Sciences, researchers analysed data on tree-ring widths - used as a gauge on past climate, as colder weather slows down tree growth - from 1500 to 1800, and socio- economic figures from that period.
They found a strong link between temperature changes and grain price changes, population growth and war fatalities. Colder conditions cut agricultural output, which pushed up prices and sparked unrest.
'It seems to us that the climate-war association is not only valid in the past, but also valid in present days,' Zhang said.
Europe experienced a cool period during the Late Middle Ages, which was marked by the Black Death and other upheavals, while the Renaissance took place during a warmer period, the study said. Another 2007 study by the University of Minnesota found a correlation between rainfall and the fate of Chinese dynasties. Analysing stalagmite from a cave in Gansu province in northwest China, it concluded that weak rainfall coincided with the downfall of the Tang, Yuan and Ming dynasties.
Kyle Meng, from the Columbia University team, told Scientific American magazine that since weather patterns like El Nino could be predicted, 'national governments and international institutions should be ready for [potential conflicts]'.