Last year, I ran across this paper in Science (ungated version here), entitled "Quantifying the Influence of Climate on Human Conflict", by Solomon Hsiang of Princeton University, and Marshall Burke and Edward Miguel both of University of California, Berkeley. Essentially, the authors conduct a meta-analysis to investigate whether climate variables cause conflict. Meta-analysis is a method of combining the results of many previous studies to generate a single (and usually more precise) estimate of the effect size. As a method, it is common in medicine and rapidly gaining currency in quantitative social science.
As an aside, definitively identifying causality is a problem (as I've outlined before). At least in this case, reverse-causality (conflict causing climate change) is a little implausible (having said that, of course, it depends on the nature of the conflict and these would probably have some impact on climate), and contemporaneous causation (where some other variable causes both climate change and conflict) is also fairly unlikely.
The authors found that:
...large deviations from normal precipitation and mild temperature systematically increase the risk of many types of conflict, often substantially, and that this relationship appears to hold over a variety of temporal and spatial scales... The standardized effect of temperature is generally larger than the standardized effect of rainfall, and the effect on intergroup violence (e.g. civil war) is larger than the effect on interpersonal violence (e.g. assaults).In other words, higher or lower than normal rainfall, and warmer temperatures increase conflict. How much more conflict? From the paper:
Nearly all studies suggest that warmer temperatures, lower or more extreme rainfall, or warmer El Niño-Southern Oscillation (ENSO) conditions lead to a 2 to 40% increase in the conflict outcome per 1σ in the observed climate variable.This is quite a substantial effect. Which leads us to climate change, where the pattern of rainfall is expected to change (increase in some areas and decrease in others), and where temperature is expected to increase significantly. Again, from the paper:
These large climatological changes, combined with the quantitatively large effect of climate on conflict - particularly intergroup conflict - suggest that amplified rates of human conflict could represent a large and critical impact of anthropogenic climate change.So, as climate change kicks in we can expect more violent conflict. What does that mean for New Zealand? Matching the effects up with this MFE publication by Brett Mullan of NIWA and co-authors leads to some interesting effects. More violence and intergroup conflict on average across the country (through increases in winter and summer temperatures), and more violence in the west of the North Island (which is expected to have both wetter winters and drier summers). Look out New Plymouth!