Drylands are not getting drier as temperatures rise

In a surprising new study, experts at Columbia Engineering have found that drylands are not getting drier. The researchers discovered that soil moisture exerts a negative feedback on surface water availability in drylands, which offsets some of the anticipated decline.

It was previously expected that as a result of global warming, surface water availability would increase in wet regions and decrease in dry regions.

As a result of rising temperatures, the atmosphere will become more humid. This climate factor is expected to amplify the existing pattern of water availability, causing the “dry-get-drier, and wet-get-wetter” responses to global warming.

However, the Columbia team wondered why climate model predictions do not project significant “dry-get-drier” responses over drylands, even when researchers use the high emissions global warming scenario. 

Study co-author Sha Zhou theorized that soil moisture-atmosphere feedbacks may play an important part in future predictions of water availability in drylands.

The researchers discovered a long-term soil moisture regulation of atmospheric circulation and moisture transport that largely offsets the potential decline of future water availability in drylands.

“These feedbacks play a more significant role than realized in long-term surface water changes,” said Zhou. “As soil moisture variations negatively impact water availability, this negative feedback could also partially reduce warming-driven increases in the magnitudes and frequencies of extreme high and extreme low hydroclimatic events, such as droughts and floods. Without the negative feedback, we may experience more frequent and more extreme droughts and floods.”

By combining a multi-model land-atmosphere coupling experiment with a new statistical approach developed for the study, the researchers found strong declines in surface water availability in response to global warming. The drying of soil moisture actually negatively feeds back onto water availability.

“Our work finds that soil moisture predictions and associated atmosphere feedbacks are highly variable and model dependent,” said study co-lead author Pierre 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.”

The study is published in the journal Nature Climate Change.

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By Chrissy Sexton, Earth.com Staff Writer