About 40 percent of the carbon dioxide released from deforestation and burning fossil fuel stays in the atmosphere. Another 30 percent is taken up in the oceans. It was thought that the remaining 30 percent of anthropogenic carbon was absorbed by plants on land, but subsequent measurements didn't bear that out, leading to the question of the "missing carbon sink."
The American Geophysical Union announced a new study accepted for publication in its journal Geophysical Research Letters that shows that massive aquifers underneath deserts may hold more carbon than all the plants on land.
While this may not account for all of the "missing" carbon sinks around the world, the research suggests at least some of this carbon could be disappearing underneath the world's deserts, a process amplified by irrigation. By examining the flow of water through a Chinese desert, scientists found that crops absorb atmospheric carbon, releasing it into the soil where it is transported underground in groundwater. According to the research, aquifers store the dissolved carbon deep below the desert floor where it can't escape back into the atmosphere.
“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. “It is basically a one-way trip.”
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.
Even if desert aquifers are not the only source of missing carbon sinks around the world, they could be important in understanding future land management, says soil ecologist Michael Allen from the Center for Conservation Biology, at the University of California, Riverside, who was not part of the study. When 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. “This means [managers] can take practical steps that could play a role in addressing carbon budgets,” said Allen.
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.
Based on the various rates that carbon entered the desert throughout history, the study’s authors estimate 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.
The study’s authors estimate the world’s desert aquifers contain roughly 1 trillion metric tons (1 trillion U.S. tons) of carbon—about a quarter more than the amount stored in living plants on land. Li said 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.
Allen said 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,” he said.
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