Civil wars are twice as likely to break out in countries that see severe weather fluctuations because of El Niño, according to a new study published in the journal Nature.

El Niño Southern Oscillations occur every four to seven years when winds warm the surface of the ocean, subjecting tropical countries to effects that range from heavy rains to drought. Solomon M. Hsiang of Princeton University and his colleagues studied the correlation between El Niño and political unrest in 175 different countries from 1950 to 2004, and found that civil war broke out nearly six percent of the time when countries were affected by the climate phenomenon, compared to the non-El Niño rate of three percent.

This study shows a systematic pattern of global climate affecting conflict and shows it right now, Hsiang told The Washington Post.

Hsiang acknowledged that the results are not enough to establish a direct causal link, but he said that the data suggests that El Niño could contribute to a multiplicity of factors that might ignite strife. Researchers noted, for example, that agricultural production usually suffers during El Niño and that unemployment can rise, creating restive populations.

Different hypotheses have been proposed as to how one phenomenon causes the other, and we aren't sure yet what the correct narrative is, Hsiang said. 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.

In the 234 civil wars the researchers studied, 21 percent of civil conflicts overall and 30 percent in tropical nations were linked to the presence of El Niño. Tropical nations where civil war and El Niño overlapped included Sudan, El Salvador, the Phillipines, Uganda, Angola, Haiti, Burma, Eritrea, Indonesia, Cambodia and Rwanda.

I would love to say we would be able to predict conflicts, but this study falls short of that, co-author Kyle Meng said. We are able to predict strong El Niño years, and based on the result of our study the likelihood of violence breaking out in the tropics increases dramatically. At a minimum, national governments and national institutions should be ready for such a thing.