Scientists expect the changing climate to bring on more drought; there's going to be less rainfall in the already arid regions.
That alone would be bad news for denizens of the planet's dry zones—in some places in North Africa, the American Southwest, India, and the Middle East, water shortages could well become an existential threat to societies built there. But new research shows that in addition to less rain, the rate of evaporation is likely to rise, too. Combined, the two forces could dry out up to a third of the planet.
The study, published in the journal Climate Dynamics last month, estimates that climate change will cause reduced rainfall alone to dry out 12 percent of the Earth's land by 2100. But if evaporation is factored in, the study's authors say that it will "increase the percentage of global land area projected to experience at least moderate drying by the end of the 21st century from 12 to 30 percent."
“We know from basic physics that warmer temperatures will help to dry things out,” the study’s lead author, Benjamin Cook, a climate scientist with Columbia University and NASA's Goddard Institute for Space Studies, said in a statement. “Even if precipitation changes in the future are uncertain, there are good reasons to be concerned about water resources.”
Writing in a 2011 literature review in the science journal Nature, the physicist Joe Romm elaborates on how increased heat and evaporation can lead to a vicious cycle: "Precipitation patterns are expected to shift, expanding the dry subtropics. What precipitation there is will probably come in extreme deluges, resulting in runoff rather than drought alleviation. Warming causes greater evaporation and, once the ground is dry, the Sun’s energy goes into baking the soil, leading to a further increase in air temperature."
Disappearing soil moisture is likely to be a greater problem than previously thought, and the occasional downpour won't sate year-round crops. As Columbia University notes, "An increase in evaporative drying means that even regions expected to get more rain, including important wheat, corn, and rice belts in the western United States and southeastern China, will be at risk of drought."
If it becomes too dry to cultivate crops on one-third of the planet's surface, there's little doubt that crisis will follow. For millions of people who depend on food grown in vulnerable regions, the future is literally evaporating. More