For decades, historians have examined evidence of correlations between climate change and civil conflict. Bad weather and sustained droughts have often been seen as causes of peasant revolts and revolutions, such as the French Revolution of 1789, yet these arguments often consider localized weather factors, rather than global climate in their analyses.
One of the most exciting areas of research on climate change and civil conflict involves global cooling in the early modern period. Some historians have argued that the peak of the Little Ice Age in the seventeenth century contributed significantly to civil unrest and political conflicts such as the Thirty Years’ War, the English Civil Wars, and the Fronde. Various theories of a General Crisis of the Seventeenth Century have been developed to explain links between climate change, economic disruption, and social conflict. For a recent reappraisal of these debates, see a forum on “The General Crisis of the Seventeenth Century Revisited,” published in American Historical Review 113 (October 2008): 1029-1099.
Now, a new scientific study argues that the El Niño climate pattern has significantly influenced civil conflicts in the tropical regions of the world since the 1950s. The research is based on a statistical analysis of civil conflicts and unrest between 1950 and 2004, finding that civil conflicts occurred twice as often in El Niño years.
This new research is published as:
Solomon M. Hsiang, Kyle C. Meng, and Mark A. Cane, “Civil Conflicts are Associated with the Global Climate,” Nature 476 (25 August 2011): 438-441.
This study is controversial because of its definitions of civil conflict and its assumptions about the causation of civil wars. Andrew R. Solow criticizes the Hsiang et al study in Nature 476 (25 August 2011): 406-407, suggesting that Hsiang and his colleagues may be going too far in arguing that climate change actually causes civil conflicts (as opposed to worsening them). Solow asserts that “Whether global climate variations give rise to new civil conflicts or modulate existing ones would seem to have somewhat different implications for improving the situation.”
Historians of civil conflicts and revolutions will be interested in this new research, although I suspect many will find the study’s assumptions and methods problematic.
NPR provides a brief report on the new study and other news media are reporting this story.
Brian Sandberg: Historical Perspectives 
Having read a bit of the literature on the crisis of the seventeenth century, I guess that I have to disagree with Parker’s conclusions about cause and effect. While I agree that European unrest was exacerbated by repeated crop failures and other agricultural disruptions, the period of the fourteenth and fifteenth centuries were actually much more agriculturally challenging. Yet during these times, Europeans were able to adjust to new climatic conditions. This is especially salient in light of the tremendous losses caused by repeated outbreaks of plague and associated massive death tolls. Parker’s textual sources certainly imply a sense of crisis and disruption, but I think we need to look to political ambitions, religious tensions, societal structures and social change to better understand this “crisis.” The same holds true today.