According to a new study published online in the journal Geophysical Research Letters, massive aquifers underneath deserts contain approximately one trillion tons of carbon, about a quarter more than the amount stored in plants on land.

Humans add carbon dioxide to the atmosphere through fossil fuel combustion and deforestation. About 40% of this carbon stays in the atmosphere and roughly 30% enters the ocean.

Researchers thought the remaining carbon was taken up by plants on land, but measurements show plants don’t absorb all of the leftover carbon. They have been searching for a place on land where the additional carbon is being stored.

The new study suggests some of this carbon may be disappearing underneath the world’s deserts – a process exacerbated by irrigation.

The study’s authors – Dr Yan Li from the Xinjiang Institute of Ecology and Geography and his colleagues from China and the United States – examined the flow of water through the Tarim Basin, a Venezuela-sized valley in China’s Xinjiang region.

The scientists measured the amount of carbon in water samples from this valley and calculated the age of the carbon to figure out how long the water had been in the ground.

They found that around 20 billion metric tons of carbon is stored underneath the Tarim Basin desert, dissolved in an aquifer that contains roughly ten times the amount of water held in the North American Great Lakes.

According to the team, carbon from the atmosphere is being absorbed by crops, released into the soil and transported underground in groundwater – a process that picked up when farming entered the region 2,000 years ago.

“The carbon is stored in these geological structures covered by thick layers of sand, and it may never return to the atmosphere. It is basically a one-way trip,” said Dr Li, who is the lead author on the study.

He added: “more information about water movement patterns and carbon measurements from other desert basins are needed to improve the estimate of carbon stored underneath deserts around the globe.”

“The new study is an early foray into this research area. It is as much a call for further research as a definitive final answer,” said Dr Michael Allen from the University of California-Riverside, who was not involved in the study.

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Yan Li et al. Hidden carbon sink beneath desert. Geophysical Research Letters, published online July 28, 2015; doi: 10.1002/2015GL064222