Ever since Bernie Sanders' remarks about climate change causing terrorism, a lot of folks have been asking about what we know on this and related issues. I worked with Marshall and Tamma Carleton to put together this short brief for those interested in knowing what we know quickly. (For those looking for a long answer, see here.)
- Research clearly demonstrates that hotter temperatures cause more individual level violence (e.g. homicides in the US) and more large-scale violence (e.g. civil wars in Africa), and that extreme rainfall leads to violence in agrarian contexts.
- Climate change to date, via warmer temperatures, has likely increased the risk of conflict, although this has not yet been empirically proven.
- Attributing the Syrian conflict to climate change is difficult. What we can say is that drought and hot temperatures increase the likelihood of these types of conflict.
- There is currently little evidence for or against a systematic relationship between climate and terrorism.
Background - Historically, many social philosophers, historians, and social scientists have hypothesized that large-scale climatic changes were responsible for various types of social conflict, societal collapse, or conversely, golden ages of peace. Until recently, there was no systematic quantitative understanding of this question. A key challenge in establishing causality is determining if the likelihood of a given conflict event was influenced by climatic events. This is analogous to determining if car accidents become more likely during rainstorms; no accident can be fully attributed to the rain, but each accident was probably made more likely because of a rainstorm. Advances in computing, statistical analysis, data collection, and measurement of climatic conditions now make a systematic understanding of these relationships possible, with findings published in the world’s leading peer-reviewed scientific journals (e.g. Nature, Science).
Basic facts – Around the world and throughout history, up to the present, research now clearly shows that climatic events are associated with changes in rates of violence and conflict. This relationship is observable for many types of violence: from intra-personal violence (suicide) and inter-personal violence (rape, murder, domestic violence, police violence) to organized inter-group violence (ethnic riots, civil conflict), political change (leadership transitions, coup d’états) and institutional/societal collapse in historical periods (described below). In general, hotter conditions produce more violence, and more extreme rainfall patterns (either too much or too little) generate violence in agrarian contexts. In historical epochs when regions were much colder (e.g. the Little Ice Age in Europe), then cooling events tended to produce more violence. In general, warming by one standard deviation (in local historical conditions) increases baseline rates of interpersonal violence 2.5% and intergroup violence 11% (ref here).
Historical “collapse” and state failure – Research shows that climatic changes likely played a key role in the disintegration of many historical political institutions and societies, such as the collapse of Angkor Wat (Cambodia), Chinese Dynasties, Maya, various European systems, and Akkadia (Mesopotamia). Such events are analogous to modern “state failure” and were likely caused by a confluence of factors, of which climatic shifts were only one. Drawing on these events as analogs to modern populations requires caution but is not entirely unwarranted. While modern technologies differentiate the present from these civilizations, the resources available to many modern populations are similar to resources available to historical societies. For example, per capita GDP in Dem. Rep. Congo is lower than average incomes of the Maya at the time of their collapse, and Chad, Haiti, Sierra Leone, Afghanistan, Guinea, and Tanzania have GDPs similar to the Ming Dynasty at the time of its breakdown.
Mechanisms – Establishing what mechanisms link climate to violence is more difficult than establishing the existence of the link. This is analogous to establishing the harmful effects of smoking tobacco in the 1940s before understanding why they occurred. Psychologists and military organizations have shown that interpersonal violence (e.g. homicide) is almost certainly driven by psychological channels related to increased aggression, cognitive errors, or reductions in impulse control under high temperatures. Many hypotheses could explain climate effects on intergroup violence. Climatic changes have been shown to weaken local economies, leaving young men underemployed and/or frustrated. Weakening economies inhibit state capacity to enforce the rule of law and suppress challenges to power. Shifting climates also alter income distributions, possibly generating inter-group resentment. Deteriorating economic conditions generate migrants that stress social systems in recipient locations. Cognitive effects, like those generating interpersonal violence, might also play a role in escalating intergroup violence, but the evidence is unclear. Some results, such as increased likelihood of inter-group retaliation or police errors on hot days, suggest this mechanism may contribute.
What we do not know well – The evidence linking (i) international conflict to climate, (ii) terrorism to climate, and (iii) conflict to natural disasters is particularly weak. These are active areas of research and there is extremely limited evidence either in favor or against the existence of a linkage. The existence of such relationships would perhaps be unsurprising, given the strength of evidence on other categories of violence, but it remains unproven.
Effects of climate change to date – Surface temperatures have risen detectably since 1980, although no studies have rigorously tested the idea that current rates of violence are higher today than they would be in the absence of climate change.
Syrian conflict – It is widely thought that anthropogenic climate change contributed to the current Syrian conflict, although this has not been definitively proven. What is known is that the drought in Syria preceding the conflict would have likely been weaker in the absence of anthropogenic climate change, and this drought has been qualitatively implicated in the conflict. Abnormally high temperatures likely further increased conflict risk.
El Niño – Forecasts predict that the current 1-in-100 year El Niño event will continue through this winter. El Niño generally causes tropical and sub-tropical regions to become dramatically drier and hotter, and has been implicated in generating over 20% of historical civil conflict risk around the globe since 1950. Based on current forecasts, the risk of new civil conflicts in the tropics and sub-tropics this year is roughly double the risk in normal years. It is possible that some of this temporary risk could be mitigated through temporary policy actions. This effect and forecast are unrelated to anthropogenic climate change.