Drylands Are Not Getting Drier As Predicted by Local weather Scientists – Watts Up With That?

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Drylands Are Not Getting Drier As Predicted by Climate Scientists – Watts Up With That?

Columbia Engineering press release

A study by New Columbia Engineering, which first examined the long-term effects of feedback from soil moisture and atmosphere in arid areas, shows that soil moisture exerts negative feedback on the availability of surface water in arid areas and offsets some of the expected decline

04 JANUARY 2021 | BY HOLLY EVARTS | PHOTO CREDIT: COLUMBIA ENGINEERING

An arid ecosystem in Northern California shows decreasing soil moisture, but little changes in surface water availability.

New York, NY – January 4, 2021 – Scientists have assumed that global warming will increase the availability of surface water – freshwater resources created by precipitation minus evapotranspiration – in humid regions and decrease water availability in arid regions. This expectation is mainly based on atmospheric thermodynamic processes. As air temperatures rise, more water evaporates from the sea and land into the air. Because warmer air can hold more water vapor than dry air, a more humid atmosphere is expected to amplify the existing pattern of water availability, resulting in atmospheric responses to global warming that become drier and wetter.

A Columbia engineering team led by Pierre Gentine, Maurice Ewing and J. Lamar Worzel, professor of earth and environmental engineering affiliated with the Earth Institute, wondered why coupled climate model predictions did not have significant "dry-dry" responses to arid areas cause. tropical and temperate areas with a drought index less than 0.65, even when researchers use the high-emission global warming scenario. Sha Zhou, a postdoctoral fellow at the Lamont-Doherty Earth Observatory and the Earth Institute who studies land-atmosphere interactions and the global hydrological cycle, believed that soil moisture-atmosphere feedback could play an important role in future predictions of water availability in arid regions .

The new study, published today by Nature Climate Change, is the first to show the importance of long-term soil moisture changes and the associated soil moisture-atmosphere feedback in these predictions. The researchers identified long-term soil moisture regulation of atmospheric circulation and moisture transport that improves the potential decline in future water availability in arid areas beyond what is expected in the absence of soil moisture feedback.

"This feedback plays a more important role than long-term changes in surface water," says Zhou. “Since soil moisture fluctuations have a negative impact on water availability, this negative feedback could also partially reduce the increases in the sizes and frequencies of extremely high and extremely low hydroclimate events such as droughts and floods caused by warming. Without the negative feedback, we can experience more frequent and extreme droughts and floods. "

The team combined a unique, idealized land-atmosphere coupling experiment with multiple models with a novel statistical approach they developed for the study. They then applied the algorithm to observations to investigate the critical role of soil moisture and atmosphere feedbacks in future changes in water availability in arid areas and to examine the thermodynamic and dynamic mechanisms underlying future changes in water availability due to these feedbacks.

They found, in response to global warming, a sharp decrease in the availability of surface water (precipitation minus evaporation, P-E) in arid regions over oceans, but only a small decrease in P-E over arid regions. Zhou suspected that this phenomenon is connected to land atmosphere processes. “Over arid areas, soil moisture is expected to decrease significantly under the influence of climate change,” she explains. "Changes in soil moisture would further influence the atmospheric processes and the water cycle."

Full press release here.

Full study, here with paywall.

HT / Clyde Spencer

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