Much of Black Earth is a description of how Germany ruthlessly destroyed neighboring countries and their political institutions, leading to the mass murder of Jews in those regions. But then, in the conclusion, Snyder makes a disturbing “warning” based on the lessons of the Holocaust. As the benefits of the Green Revolution peter out and the risks of climate change increase, he suggests, we are once again becoming vulnerable to fears of food insecurity—and, thus, once again in danger of battling over agricultural lands. “Another moment of choice, a bit like the one Germany faced in the 1930s, could be on the way,” Snyder writes. He adds: “We have changed less than we think.”

“It will be the largest redistribution of wealth from the poor to the wealthy in history.”

Snyder argues that the changing climate has already played a role in conflicts in Africa, such as the civil war in Sudan that began in 2003. But his real fears are for the future. China, he points out, is unable to feed itself with domestic agricultural production, and many of its people have personally faced the terror of starvation: the famine caused by Mao’s Great Leap Forward between 1958 and 1962 killed tens of millions. Much as Germany in the 1930s envied the agricultural resources of Eastern Europe, China is increasingly attempting to control those of Africa and eyeing the vast resources of its neighbor Russia, says Snyder.

Invoking the haunting evils of Nazi Germany to warn of future dangers ignores the unique perversion of Hitler’s thinking. And, as Snyder readily acknowledges, China is not Nazi Germany; its rulers have embraced science and technology in addressing climate change. Nevertheless, Snyder’s fundamental point remains: climate change—even the prospect of it—has the power to grotesquely transform global politics. And if history is any guide, governments and rulers may not respond to the threats in a rational manner.

Syria and the Mideast

The suspicion that climate change will contribute to conflict is not new. ­Nicholas Stern, a former chief economist of the World Bank and advisor to the British government, predicted in his 2006 report “Economics of Climate Change” that “higher temperatures will increase the chance of triggering abrupt and large-scale changes that lead to regional disruption, migration and conflict.” Over the last decade, many researchers have tried to document the connection.

In 2011, Solomon Hsiang, then at Princeton and now a professor at the Goldman School of Public Policy at the University of California, Berkeley, coauthored a paper showing that instances of civil war doubled in the tropics during times when the El Niño effect produced unusually warm temperatures at those latitudes. The paper was the first to demonstrate that a global climate effect could be linked to conflict. A few years later, Hsiang and his colleagues at Berkeley and Stanford analyzed the growing literature on climate and conflict and found a consistent result in 60 research papers: rising temperatures and changes in precipitation patterns increased the risk of conflicts. Not only is there evidence that climate is connected to conflict, says his coauthor Marshall Burke, a Stanford professor, but the effects can be substantial. He says, “In sub-Saharan Africa, when temperatures are a degree warmer, we see 20 to 30 percent increase in civil conflict. That’s a huge number.”

One explanation might lie in the way climate changes affect agriculture. Take the war in Syria, for example. Beginning in the winter of 2006-2007, the Fertile Crescent, which runs across northern Syria and provides the country with much of its food, experienced a three-year drought that was the most severe on record. It prompted as many as 1.5 million people to migrate to the country’s urban centers. These formerly rural people joined more than a million refugees from Iraq’s war of the mid-2000s, who were already living in the areas surrounding Syria’s cities. There, growing crime, inadequate infrastructure, overcrowding, and a lack of response from the government all contributed to unrest. Widespread uprisings in these urban outposts quickly spun into today’s civil war, which began in early 2011.

Climate change made the drought far more severe, and the subsequent widespread crop failure and resulting mass migration contributed to the conflict, says Colin Kelley, a climate scientist at the University of California, Santa Barbara, who has specialized in the Mediterranean region. In a recent paper, Kelley and his coauthors document how rising levels of greenhouse gases disrupted the normal patterns of wind that bring moisture from the Mediterranean during the winter rainy season. It’s part of a long-term drying effect in the region and consistent with predictions from climate-­change models, he says. In general, he adds, subtropical regions around the world, such as the Fertile Crescent, are expected to become more arid.