"When you think about it, car accidents happen all the time. But they become more likely based on some environmental conditions, like when it's raining or icy. [...] What we found is quite a bit stronger than a mere correlation."
- Solomon Hsiang
Changes in climate - including anthropogenic global warming - have been linked to conflict by historical analysis (see for example Climate change and conflict frequency), security analysts (see "Climate Change and National Security" - presentation by US Navy Admiral David Titley (video)), politicians (see UN Security Council concerned climate change may aggravate threats to peace) and in reviews of existing research (see Just read "Climate Conflict" by Jeffrey Mazo and What does climate change have to do with conflict?).
According to Solomon M. Hsiang and colleagues, however, all of the above evidence is 'anecdotal'. And it certainly isn't as concrete as their own work: Running a wide selection of statistical analysis methods on on the one hand a data set of occurrences of organized political violence of more than 25 battle-related deaths and on the other climatological records of the El Niño / Southern Oscillation (ENSO) phenomenon.
Looking into their analysis, the authors note how some conflicts seem to be re-occurring events and some seem to be merely 'displaced' in time, accelerated by the climatic shifts. Also, low-income countries are most strongly affected - but are they more at risk of climate induced conflict because they are poor and lack the means to mitigate the effects of environmental impacts? or are they poor because their political stability is sensitive to the ENSO cycle, perhaps even inherently unstable, conflict prone and poor because they are located where the ENSO climate pattern is strongest? Or is a third factor, not considered in this study, influencing or guiding the sensitivity to climate and poverty in relation to conflict proneness?
The increase in conflict risk during the about three degrees Celsius warmer El Niño years roughly corresponds to the decrease observed when average income is increased tenfold. So, could climate wars 'easily' be eliminated by raising the incomes in tropical countries by a mere 1000%? Hsiang and colleagues warn against generalization of their results to apply to global anthropogenic climate change without throughout discussions of climate-conflict links. Yet, perhaps they need not look far: weather directly influences the effectiveness of agriculture, the output of which determines food supply; and since food demand is not exactly down the recently identified link from food prices to risk of riots appears to be rather significant in this respect.
Hsiang, S., Meng, K., & Cane, M. (2011). Civil conflicts are associated with the global climate Nature, 476 (7361), 438-441 DOI: 10.1038/nature10311
This study covered elsewhere: ars technica / El Niño appearances tied to civil conflicts in tropical countries, TIME / Does El Nino—and Climate Change—Really Cause Civil Wars?, International Business Times / El Nino Doubled Chances of Civil War in Tropical Countries, Scientific American / El Nino Ups Conflict Odds, Huffington Post / Does Climate Drive Warfare? A New Study Suggests There's No Question.