Desert dig uncovers caches of missing CO₂

The wet undersides of deserts may stash as much as a trillion metric tons of climate-altering carbon, more than stored in all land-based plants, new research suggests.

Human activities such as burning fossil fuels spew carbon dioxide into the atmosphere. Scientists, however, can’t account for where as much as 30 percent of this CO₂ ends up.

“We’ve found a carbon sink in the most unlikely place,” says Yan Li, an ecologist at the Chinese Academy of Sciences in Urumqi. Up to a fifth of this missing carbon may end up beneath irrigated Earth’s deserts, Li and colleagues propose online July 28 in Geophysical Research Letters. In arid regions, water from agricultural irrigation can flush carbon into underground aquifers, reducing atmospheric CO₂ concentrations and combating greenhouse warming, the researchers report.

“Almost nobody paid attention to these desert regions,” Li says. That’s because desert regions lack the abundant plant life needed to pull in significant amounts of carbon via photosynthesis, he says. In the last decade, several studies have measured desert regions absorbing unexpectedly large amounts of CO₂, but these findings were controversial. No one could explain where the absorbed carbon went.

Li and colleagues hunted for this vanished carbon around northwest China’s Tarim Basin, home to the Taklamakan Desert. The researchers collected 170 groundwater samples from aquifers beneath the basin, plus samples from nearby streams and irrigation channels that feed the farms that straddle the desert’s perimeter.

Farmers in arid climates typically overwater their crops to flush out large amounts of salt from the soil. As the water passes through the salty soil, the amount of dissolved carbon in the water more than doubles, the researchers found. Salty, alkaline water can hold more carbon than pure water. Some water makes its way into underground aquifers, locking carbon that would normally escape back into the atmosphere underground. This process boosts the annual amount of CO₂ absorbed by each square meter of desert from 1.34 grams to 20 grams or more, akin to the amount of CO₂ absorbed by forestlands, the researchers estimate. If present in other arid regions with farming, the mechanism could mean that desert aquifers are among the top three largest active carbon sinks on land, Li says.

The Tarim Basin carbon sink is probably relatively new. Carbon dating of the groundwater samples revealed an uptick in its carbon collection roughly 2,000 years ago, around when Silk Road trade routes opened the region to farming. Unlike most other groundwater systems, water typically stays trapped in desert aquifers and is too salty for use as drinking or irrigation water. The carbon will remain underground indefinitely, Li says.

“The carbon goes into the ground and stays there,” he says. The process could be utilized to help combat climate change by irrigating more of the desert to purposely lock up carbon, he proposes.

The new work demonstrates how little we know about arid lands, says ecologist R. Dave Evans, of Washington State University in Pullman. Researchers can now go out and look for the mechanism in other irrigated deserts, he says.

Further study is definitely needed, says biogeochemist Akihiro Koyama, from Algoma University in Sault Ste. Marie, Canada. Finding relatively young carbon in the aquifers isn’t definitive proof that desert irrigation serves as a carbon sink, he says. The new carbon may simply push the old out through a yet-to-be-discovered outlet, resulting in no net effect on the atmosphere. “This is worth looking into,” he says, “but I’d be really cautious.”