Extra CO2 the cause of dryland greening trends

"We now understand why that's occurring, but we don't necessarily know if that's a good thing or not," researcher Lixin Wang said of the phenomenon known as dryland greening.
By Brooks Hays   |   Feb. 16, 2016 at 2:43 PM

INDIANAPOLIS, Feb. 16 (UPI) -- Earth's drylands are getting greener. The phenomenon has been documented in satellite images over the last several years, but scientists haven't been able to explain the trend.

New evidence suggests higher atmospheric concentrations of carbon dioxide are responsible for dryland greening. Between 1960 and 2015, CO2 levels increased 27 percent.

After analyzing the findings of several regional studies on the subject, researchers determined that rising CO2 levels have increased the availability of soil water. Researchers believe higher CO2 levels have also diminished plant stomatal conductance -- the exchange of carbon dioxide and water vapor in and out plant leaves.

Though other factors -- like a shift in grass-eating species or the encroachment of bigger, greener bushes on grasslands -- may explain localized instances of greening, CO2 is the only explanation that fits the global trend.

"We know from satellite observations that vegetation is greener than it was in the past," lead reseracher Lixin Wang, an assistant professor of earth sciences at Indiana University-Purdue University Indianapolis, said in a press release. "We now understand why that's occurring, but we don't necessarily know if that's a good thing or not."

Drylands are the most abundant type of terrestrial biome on the planet, encompassing 41.3 percent of Earth's land surface and serving as home to more than 2 billion people. Drylands are defined by the counterbalance between precipitation and evaporation or transportation. In other words, drylands lose most of the water they gain.

The new analysis -- detailed in the journal Scientific Reports -- revealed a strong correlation between CO2 concentrations and soil water levels.

Rising CO2 levels had less of an effect on non-drylands, which saw a soil water increase of 9 percent. Soil water levels among drylands, however, increased 17 percent.

Increasingly, climate scientists are trying to understand the ways global warming will alter the planet's hydrological cycles, and determine how those changes will alter various ecosystems.

Wang and his colleagues aren't sure how long the heightened soil water levels will remain. At what point will rising temperatures offset the benefits of more abundant water reserves?

"Importantly, the observed response lends weight to the hypothesis that any additional soil water in the root zone is then available to facilitate vegetation growth and greening under enhanced carbon dioxide," Wang said. "Future studies using global-scale process-based models to quantitatively assess the carbon dioxide impact on soil moisture is needed to further validate the hypothesis."

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