Sunday, 6 December 2015

Climate change, violence and crime

Last year I wrote a post about climate change and violence, based on this paper (ungated here) by Hsiang et al. (the same Solomon Hsiang who was a co-author on the paper in Nature I discussed in my last post). Like most papers, the Hsiang et al. paper looks at cross-country differences in conflict. Within-country evaluations are much less common. Which makes this recent paper by Jean-Francois Maystadt (Lancaster University), Margherita Calderone (World Bank), and Liangzhi You (IFPRI and Chinese Academy of Agricultural Sciences) of interest. In the paper, Maystadt et al. look at local warming and conflict in North and South Sudan.

The authors use data measured at the 0.05 degree level (latitude and longitude) over the period 1997 to 2009. I strongly recommend reading the data section of the article, as it has pointers to a number of excellent sources of global spatially-explicit data that would be useful for a number of projects, not just in the context of climate change.

They use time and grid-cell fixed effects to "be able to draw causal inferences", but I probably wouldn't characterise their findings as necessarily causal. Or at least not definitively so. They find:
A change in temperature anomalies of 1 standard deviation is found to increase the frequency of violent conflict by 32%... temperature variations may have affected about one quarter (26%) of violent events in Sudan. On the contrary, no significant impact is found for rainfall anomalies...
Temperature anomalies (deviations from mean temperature) were associated with over a quarter of conflict events in the Sudan, which is a large number. The authors investigate the mechanisms for this (it is unlikely to be water stress because rainfall is not significant), and find that pastoralist areas (where livestock are an important source of income) are particularly affected by temperature. The authors conclude that conflict over natural resources (i.e. water) is exacerbated in these areas, but if that were the case you would expect rainfall to be a bigger factor. I'd be more inclined to believe that heat stress affects livestock in negative ways (weight loss, dehydration, mortality), that affects income security for pastoralists.

On a different but related note, earlier in the year I read this 2014 paper (ungated earlier version here) by Matthew Ranson (Abt Associates), but I hadn't had a chance to blog about it until now. In the paper, Ranson looks at the relationship between monthly weather patterns and crime in the U.S. This paper is interesting because Ranson doesn't stop at looking simply at the relationship, but projects the change in crime over the rest of the century and estimates the social costs of the additional crimes. He finds a number of interesting things:
Across a variety of offenses, higher temperatures cause more crime. For most categories of violent crimes, this relationship appears approximately linear through the entire range of temperatures experienced in the continental United States. However, for property crimes (such as burglary and larceny), the relationship between temperature and crime is highly non-linear, with a kink at approximately 50 °F... the year 2090, crime rates for most offense categories will be 1.5-5.5% higher because of climate change... The present discounted value of the social costs of these climate-related crimes is between 38 and 115 billion dollars.
I would suggest that, if you did a similar analysis for New Zealand, we might see something similar (with the magnitude of social costs being much lower of course due to smaller population). Both papers provide additional reasons to hope for some agreement in Paris.

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