As you report, the research of Richard Tol of the Economic and Social Research Institute in Dublin and Sebastian Wagner of GKSS, a research institute near Hamburg, “throws into question the assumption that rising temperatures and violence go hand in had” (“Cool Heads or Heated Conflicts?” October 10, 2009). The research concerns the correlation between low temperatures and conflict in Europe over the last millennium. It shows that in the more remote past the effects of cold weather on harvests led to supply shortages, and that these indeed increased the likelihood of conflict over food, but that the correlation vanishes in the mid-Eighteen century, when the industrial revolution began. Systematic plant breeding, the introduction of new crops, and new forms of crop rotation, as well as improvement of irrigation and transport of food began in the same period. The lesson, you argue, is that the way to minimize the likelihood of climate-induced conflict in the future is to continue the process of crop improvement. As an example, you offer genetic engineering leading to heat- and drought-tolerant varieties. If farmers are made aware of these varieties and encouraged to use them, and if free trade and non-agricultural development are promoted, “people will have no cause to fight, and tyrants no excuse to stir them up.” Splendid. Assuming that climate change will not affect our ability to accomplish all you propose, climate-induced wars can surely be averted. But the old assumption that rising temperatures and violence go hand in hand will spring back to life as soon as your assumption comes into question. This is what the research by Tol and Wagner actually shows.