Research tracks path of carbon dioxide via agriculture to underground storage
FRISCO — Vast aquifers beneath the world’s deserts may be significant carbon sinks, scientists with the University Corporation for Atmospheric Research said.
In a new study, the UCAR researchers estimated that those aquifers may store more carbon than all the plants on land.
About 40 percent of the carbon dioxide produced by people stays in the atmosphere and heats the planet. About 30 percent is taken up by oceans, where it is rapidly acidifying the water to the detriment of shellfish and other marine species.
The other 30 percent is partially absorbed by land plants, but when scientists ran CO2 models, it didn’t add up, so they started searching for additional carbon sinks.
The new study suggests some of this carbon may be disappearing underneath the world’s deserts – a process exacerbated by irrigation. Scientists examining the flow of water through a Chinese desert found that carbon from the atmosphere is being absorbed by crops, released into the soil and transported underground in groundwater, in a a process that accelerated when farming started in the region 2,000 years ago.
Underground aquifers store the dissolved carbon deep below the desert where it can’t escape back to the atmosphere, according to the new study.
The new study estimates that, because of agriculture, about 14 times more carbon than previously thought could be entering these underground desert aquifers every year. These underground pools that taken together cover an area the size of North America may account for at least a portion of the “missing carbon sink” for which scientists have been searching.
“The carbon is stored in these geological structures covered by thick layers of sand, and it may never return to the atmosphere,” said Yan Li, a desert biogeochemist with the Chinese Academy of Sciences in Urumqi, Xinjiang, and lead author of the study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union. “It is basically a one-way trip.”
Knowing the locations of carbon sinks could improve models used to predict future climate change and enhance calculations of the Earth’s carbon budget, or the amount of fossil fuels humans can burn without causing major changes in the Earth’s temperature, according to the study’s authors.
Although there are most likely many missing carbon sinks around the world, desert aquifers could be important ones, said Michael Allen, a soil ecologist from the Center for Conservation Biology at the University of California-Riverside, who was not an author on the new study.
If farmers and water managers understand the role heavily-irrigated inland deserts play in storing the world’s carbon, they may be able to alter how much carbon enters these underground reserves, he said.
“This means [managers] can take practical steps that could play a role in addressing carbon budgets,” said Allen.
The scientists tracked the carbon with a close study of the Tarim Basin, a Venezuela-sized valley in China’s Xinjiang region. Water draining from rivers in the surrounding mountains support farms that edge the desert in the center of the basin.
The researchers measured the amount of carbon in each water sample and calculated the age of the carbon to figure out how long the water had been in the ground.
The study shows the amount of carbon dioxide dissolved in the water doubles as it filters through irrigated fields. The scientists suggest carbon dioxide in the air is taken up by the desert crops. Some of this carbon is released into the soil through the plant’s roots.
Although this process of carbon burial occurs naturally, the scientists estimate that the amount of carbon disappearing under the Tarim Desert each year is almost 12 times higher because of agriculture. They found that the amount of carbon entering the desert aquifer in the Tarim Desert jumped around the time the Silk Road, which opened the region to farming, begin to flourish.
After the carbon-rich water flows down into the aquifer near the farms and rivers, it moves sideways toward the middle of the desert, a process that takes about 10,000 years.
The study estimates that about 20 billion metric tons (22 billion U.S. tons) of carbon is stored underneath the Tarim Basin desert, dissolved in an aquifer that contains roughly 10 times the amount of water held in the North American Great Lakes. Globally, carbon storage in the sub-desert aquifers could be as high as 1 trillion metric tons (1 trillion U.S. tons) of carbon–about a quarter more than the amount stored in living plants on land.