The Long-Run Effects of Climate Change on Conflict, 1400-1900∗ (Preliminary) Murat Iyigun

Nathan Nunn

Nancy Qian

November 5, 2015

Abstract This paper investigates the long-run effects of climate change on conflict. We construct a geo-referenced and digitized database of historical conflicts in Europe, North Africa and the Near East during 1400-1900, which we merge with historical temperature data. The results provide novel evidence of offsetting long-run forces. On the one hand, consistent with adaptation, we find that climate change that occurred more than fifty years ago has no direct effect on conflict. On the other hand, consistent with intensification, earlier climate change can indirectly increase current conflict by interacting with more recent climate change. Conflict increases monotonically with the duration of climate change. Our results are driven by conflicts that are part of midscale wars, and regions which contain a border or with a high level of political fractionalization in the base period. Keywords: Environment, Development, Political Economy. JEL Classification: D74; Q34; P16.

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We thank Dan Keniston, Nicholas Ryan and Joseph Shapiro for their many helpful comments; Nicola Fontana, Anna Hovde, Eva Ng, Brittney Stafford-Sullivan and Jaya Wen for excellent research assistance. Please send comments and suggestions to murat.[email protected], [email protected], [email protected].

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Introduction

This study aims to make progress on understanding the long-run effects of climate change on conflict. Several prominent recent studies provide theoretical and empirical evidence that conflict plays a central role in state formation, state capacity and economic development (e.g., Besley and Persson, 2008, 2010; Gennaioli and Voth, 2015). The impact of climate change on conflict is particularly salient today as climate change continues in the 21st century and researchers provide increasingly more evidence of how weather shocks, which disrupt economic activity such as agriculture, lead to conflict in the short and medium run. In their review of the existing evidence, Burke et al. (2015a) show that the finding that deviations in weather increases conflict is ubiquitous across temporal and geographic contexts. At the same, they point out that our understanding of the long-run effects are at best preliminary. In principle, the long-run effects can be very different from short- and medium-run effects. On the one hand, general equilibrium effects can offset adverse short-run effects. Afflicted populations relocate, or adopt new technologies such that climate change which occurred in the past will have limited effect on current levels of conflict. The existing climate change literature refers to this as “adaptation”. Similarly, afflicted regions may adopt new institutions to help them deal with environmental change so that the adverse effect of recent climate change is smaller in regions with a history of climate change. We will refer to this as “institutional adaptation”. On the other hand, long-run effects may be larger if there is “intensification”, a term coined by (Dell et al., 2014) to refer to the positive interaction effect of climate change over different lagged periods. For example, continued famine, conflict and migration may erode state capacity and increase political instability such that the adverse effect of recent climate change on conflict is larger in regions which have a long history of environmental disruptions. Thus, the long-run effects of climate change on conflict is an empirical question. To understand the magnitude of the underlying mechanisms driving the cumulative long-run effect of climate change, we need to compare the effect of climate change which occurred recently to that which occurred further back in the past, and allow these two effects to interact. The former comparison will reveal the importance of general equilibrium effects. The interaction will capture the net o