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Study Pinpoints Process That Eases Drying in Drylands

brown hills and sparse trees and shrubs
A dryland ecosystem in Northern California shows decreasing soil moisture but little changes in surface water availability. Photo: Columbia Engineering

Climate change is making drylands drier, but scientists have identified a natural process that has been helping to limit water loss. A new study published in Nature Climate Change finds that soil moisture’s influence on evapotranspiration and wind patterns may help to ease the loss of surface water in arid areas.

Scientists have thought that global warming will increase the availability of surface water in wet regions, and decrease water availability in dry regions. As air temperatures rise, more water evaporates into the air from the ocean and land, causing drier areas to lose more water, and wetter areas to receive more precipitation.

However, climate models projected that drylands wouldn’t become as dry as expected in the near future, providing an exception to the “dry-get-drier, and wet-get-wetter” rule. A team led by Pierre Gentine, Maurice Ewing and J. Lamar Worzel professor of earth and environmental engineering and an affiliate of the Earth Institute, wondered why.

Sha Zhou, a postdoctoral fellow at Lamont-Doherty Earth Observatory and the Earth Institute who studies land-atmosphere interactions and the global water cycle, thought that soil moisture-atmosphere feedbacks might play an important part in future predictions of water availability in drylands.

The new study is the first to show that long-term soil moisture changes and feedbacks between soil moisture and the atmosphere play an important and previously underestimated role in these predictions about the future of drylands. The researchers found that long-term soil moisture helps to regulate atmospheric circulation and moisture transport. These effects largely ameliorate the potential decline of future water availability in drylands. Although drylands will continue to become drier with climate change, the effect would be much worse without the feedbacks identified in the study.

“These feedbacks play a more significant role than realized in long-term surface water changes,” says Zhou. The feedbacks from soil moisture could also partially reduce the frequency and severity of extreme weather events such as droughts and floods, she adds.

The team combined satellite and on-the-ground measurements with models and a novel statistical approach in order to examine the role of soil moisture-atmosphere feedbacks in future water availability changes over drylands. They also investigated the mechanisms underpinning future water availability changes due to these feedbacks.

They found, in response to global warming, strong declines in surface water availability (which equals precipitation minus evaporation) in dry regions over oceans, but only slight declines over drylands. Although global warming is expected to slightly reduce water availability and hence soil moisture in drylands, the drying of soil actually limits other changes in water availability—declining soil moisture reduces evapotranspiration and evaporative cooling, and enhances surface warming in drylands relative to wet regions and the ocean. This land-ocean warming contrast strengthens the air pressure differences between ocean and land, driving more wind to blow from the ocean to land, and with it, more water vapor.

“Our work finds that soil moisture predictions and associated atmosphere feedbacks are highly variable and model-dependent,” says Gentine. “This study underscores the urgent need to improve future soil moisture predictions and accurately represent soil moisture-atmosphere feedbacks in models, which are critical to providing reliable predictions of dryland water availability for better water resources management.”

Adapted from a press release by Columbia Engineering.

Correction, 01/06/2021: An earlier version of this story incorrectly stated that the study found that drylands are not getting drier; in fact, they are getting drier with climate change, but the changes would be worse without the mechanisms identified in the new study. We regret the error. 

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