Carbon ‘sponge’ found beneath desert

Irrigating farms in dry parts of the globe may provide an unplanned climate benefit. This water appears to have washed enormous amounts of carbon deep underground, a new study indicates. Locked away there — in the form of the climate-warming carbon dioxide — this carbon has not had an opportunity to contribute to global warming.

Over the past century, human activities have been spewing huge amounts of carbon dioxide, or CO₂, into the air. Much of it comes from the burning of fossil fuels and of forests. In recent decades this air pollution has been fueling a low-grade fever in Earth’s atmosphere. But this global warming has not been as big as emissions would had suggested it should be. For some reason, as much as 30 percent of the CO₂ seems to have gone missing. And the new study now finds evidence that farm irrigation may have stored up to one-fifth of it beneath deserts.

The amount of carbon in this stash appears huge — up to one trillion metric tons, the new study finds. If true, it would be equal to more than all of the carbon now held by trees and other land-based plants.

“We’ve found a carbon sink in the most unlikely place” — under irrigated deserts, says Yan Li. He’s an ecologist at the Chinese Academy of Sciences in Urumqi. At least this is what Li and his colleagues proposed online July 28 in Geophysical Research Letters.

“Almost nobody paid attention to these desert regions,” Li says. That’s because desert regions lack abundant plant life. Through photosynthesis, these green plants suck up and store huge amounts of carbon in their tissues, he says.

In the last decade, several studies had measured deserts absorbing unexpectedly big amounts of CO₂. Such findings were controversial, however. Scientists could not explain where the absorbed carbon had gone.

Li and colleagues decided to hunt for this vanished carbon around northwest China’s Tarim Basin. It’s home to China’s largest desert. Eighty-five percent of this Taklamakan Desert consists of little more than sand dunes. The researchers sampled groundwater at 170 sites beneath the basin. They also sampled nearby streams and irrigation ditches. This surface water quenches the thirst of farms that straddle the desert’s perimeter.

Farmers in dry climates tend to overwater their crops. This helps to flush out large amounts of salt from the soil (which would poison any crops they might want to grow in that soil). As the water passes through the salty soil, the amount of dissolved carbon in the water more than doubles, Li’s team found. Salty, alkaline water can hold more carbon than pure water. Some of the water percolating down through the ground will end up in underground aquifers. These can then lock away carbon that would otherwise escape back into the atmosphere.

This process boosts the annual amount of CO₂ absorbed by each square meter of desert from 1.34 grams to 20 grams or more, Li’s team finds. That’s an amount of CO₂ comparable to what forest lands absorb, the researchers estimate. And the same thing might be happening in other desert regions with farming — such as California and the American Southwest. If this does occur, then this irrigation wash water could mean that desert aquifers are among the top three ongoing carbon sinks on land, Li says.

The Tarim Basin carbon sink is probably relatively new. Scientists have been able to use carbon dating to calculate the age of its groundwater. Tested samples revealed a sharp climb in the water’s collection of carbon. This started roughly 2,000 years ago, when Silk Road trade routes opened the region to farming.

Water collects in groundwater below non-deserts too. However, people often pump those supplies for drinking and irrigation. They don’t tend to remove water from desert aquifers because is too salty for such uses. That means the carbon in this water could remain underground indefinitely, Li says.

“The carbon goes into the ground and stays there,” he suspects. As such, countries might consider irrigating more of the desert to purposely lock up carbon, he proposes, to help combat climate change.

The new work demonstrates how little we know about arid lands, says R. Dave Evans. He’s an ecologist of Washington State University in Pullman. Researchers now can go out and look for signs this also occurs in other irrigated deserts, he says.

But further study is definitely needed, says Akihiro Koyama. A biogeochemist, he works at Algoma University in Sault Ste. Marie, Canada. “This is worth looking into,” he says, “but I’d be really cautious.” Finding relatively young carbon in the aquifers does not prove that desert irrigation will lock up carbon underground, he explains. The new carbon might simply push the old out through some yet-to-be-discovered means. Then there would be no climate benefit effect.

Power Words

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alkaline  An adjective that describes a chemical that produces hydroxide ions (OH-) in a solution. These solutions are also referred to as basic — as in the opposite of acidic — and have a pH above 7.

aquifer  Rock that can contain or transmit groundwater.

arid   A description of dry areas of the world, where the climate brings too little rainfall or other precipitation to support much plant growth.

basin   (in geology) A low-lying area, often below sea level. It collects water, which then deposits fine silt and other sediment on its bottom. Because it collects these materials, it’s sometimes referred to as a catchment or a drainage basin.

biogeochemistry  A term that covers processes that cycle (or eventually deposit) pure elements or chemical compounds (including minerals) between living species and nonliving parts (such as rock or soil or water) within an ecosystem. A scientist who works in this field is a biogeochemist.

carbon  The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

carbon dioxide  A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide, abbreviated as CO₂, acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

climate   The weather conditions prevailing in an area in general or over a long period.

ecology  A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

fossil fuel   Any fuel — such as coal, petroleum (crude oil) or natural gas —  that has developed in the Earth over millions of years from the decayed remains of bacteria, plants or animals.

global warming  The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

groundwater  Water that is held underground in the soil or in pores and crevices in rock.

irrigation  The supply of water to land or crops to help growth.

perimeter   The outer border or edge of some defined area. For instance, the perimeter of some people’s property is set off by a fence.

photosynthesis  The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water.

sink     (in biology or geology) Some part of an ecosystem or the environment that serves as a storage depot for some chemical. For instance, trees or the soil can become a sink for the carbon released into the atmosphere.

tissue   Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.