Where Is All The "Missing" Carbon Going?

The rare water bodies on the surface of the Tarim Basin, China, are dwarfed by the underground lakes beneath. These have been found to be a major carbon store. Yan Li.

Assistance in fighting climate change appears to be coming from some very unlikely places: deep aquifers under deserts. The claim could have significant implications for how we try to lock carbon away.

Every year, humans release over 36 billion tonnes (40 billion tons) of carbon dioxide through the burning of fossil fuels and felling of forests. Almost half of this stays in the atmosphere, forming the primary driver of Global Warming. Another third is absorbed by the oceans, with a different set of devastating effects. The rest was assumed to be captured by plants.

However, while both forests and savannas have been shown to be storing some of this carbon in the bodies of plants or in the soil, not all of it can be accounted for in this way. This triggered a search for the “missing carbon sink” that holds the remainder. We need to know what it is to learn whether it is likely to continue to capture carbon, or if it will saturate, as we suspect the oceans will.

Dr Yan Li of the Chinese Academy of Sciences reports in Geophysical Research Letters that inorganic carbon is stored in areas where saline or alkaline soils are being irrigated. The soil type is crucial since, the authors note, “CO2 solubility in water increases exponentially with alkalinity and linearly with salinity.”

These areas are often on the edge of deserts, representing the last farmable territory before conditions become impossible for agriculture, and the carbon can be washed into huge water bodies lying under the deserts.

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

Adding all of the desert aquifers in the world together, they cover an area equal in size to North America, so the potential storage capacity is huge. Li is not claiming that aquifers account for the entire missing sink, but thinks they may be a significant portion.

Li used the Tarim Basin in China’s Xinjang region as a test case. He collected 170 samples from the underground ocean beneath the Basin, which contains ten times as much water as the Great Lakes combined. Using radio isotope analysis, Li established the age of the carbon at each point, demonstrating that it can take 10,000 years for carbon to reach the center of the Basin from its rim.

Prior to the establishment of agriculture in the region, only a small amount of carbon was being added to the aquifer each year. Li attributes this to most of the carbon dioxide released by plant roots and soil microbes into the earth eventually escaping into the atmosphere.

However, 200 years ago agriculture arrived, bringing irrigation with the thresher’s flail. The extra water washed 12 times as much carbon into the ground beneath the farmland, from where it meandered down to the sunless sea. Li estimates that the waters beneath the Tarim Basin hold 20 billion tonnes (22 billion tons) of carbon.

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