Future droughts worse than expected

A new study is helping astrobiologists understand how climate change may shape the future of life on Earth.

As we approach the end of the next century, many scientists believe that we could be facing increasingly severe droughts due to changes in rainfall. The new study shows that things could be worse than originally thought.

Previous theories have predicted that changes in rainfall due to global warming could cause the Earth's land area to experience increased drying. It turns out, however, that rainfall is just one part of the story.

"If you only account for precipitation changes in the future, we can expect increased drought on about 12% of the global land area," said Dr. Benjamin Cook of Columbia University's Lamont-Doherty Earth Observatory and the NASA Goddard Institute for Space Studies. "If you include warming and increased evaporation in these calculations, the drying intensifies and spreads to ~30% of the global land area."

Cook and his colleagues found that higher evaporation rates could result from a warmer climate. Evaporation would 'wring' water out of the soil, even in some areas that receive a decent amount of rain. And while global warming is forcing water out of the ground, it will also increase the moisture content of our planet's atmosphere. This, too, could have global consequences.

"In the global average, evaporation is expected to increase atmospheric moisture content by about 7% per degree Celsius of warming. This is just the Clausius-Clapeyron relationship," said Cook. "The expectation is that, at the global level, this increase in atmospheric moisture content will act to amplify greenhouse gas warming trends because water vapor is also an important greenhouse gas. Regionally, it gets more complicated because you have some areas where humidity will increase and other areas where it will decrease."

The Intergovernmental Panel on Climate Change (IPCC) has already warned that declining soil moisture is going to cause problems for agriculture in many regions of the globe. Today's 'Marginally wet' areas at mid-latitudes on Earth – like the Great Plains of the United States – are expected to become arid. This increasing aridity could force ecosystem change in these regions.

"I certainly expect that we will see impacts on ecosystems and agriculture at the regional level with increased aridity," says Cook. "You can already see this happening with some of the severe droughts in the southwest and California."