The Missing Sink: Carbon storage in the world’s deserts

Deep-rooted and completely erroneous preconceptions of our planet’s arid lands as sterile bit-players in the great game of the earth’s dynamic systems have long inhibited our scientific enthusiasm for, and understanding of, the desert. We are now beginning to catch up – take, for example, this recent headline from the American Geophysical Union:

The world's deserts may be storing some of the climate-changing carbon dioxide emitted by human activities, a new study suggests. Massive aquifers underneath deserts could hold more carbon than all the plants on land, according to the new research.

As described in a summary of this research on Science Daily:

Humans add carbon dioxide to the atmosphere through fossil fuel combustion and deforestation. About 40 percent of this carbon stays in the atmosphere and roughly 30 percent enters the ocean, according to the University Corporation for Atmospheric Research. Scientists thought the remaining carbon was taken up by plants on land, but measurements show plants don't absorb all of the leftover carbon. Scientists have been searching for a place on land where the additional carbon is being stored--the so-called "missing carbon sink."

The lead author of the report in the AGU publication, Geophysical Research Letters, is Yan Li, a desert biogeochemist with the Chinese Academy of Sciences in Urumqi, Xinjiang; he and his team examined the character of groundwaters in the gigantic closed system of the arid Tarim Basin, and came up with some fascinating – and provocative – results. Runoff waters from the surrounding mountains pick up, as a normal part of the carbon cycle, some CO₂dissolved from the rocks and soils through which the rivers flow. However, by the time that water ends up in aquifers, the underground reservoirs beneath the desert, it contains substantial amounts of DIC, dissolved inorganic carbon.

Being able to date the carbon, Li and his colleagues could distinguish between old carbon originating from the rivers and very young carbon added to the water as it seeped through the soils of the irrigated oases along the desert margins. These are poor soils, not in themselves sources of much CO₂- it originates from the respiration of the roots of crops and microbes in the soil. And because these crops are irrigated almost constantly, not only to keep them growing but to wash out the salts that, as in all desert agriculture, accumulate in the soil, most of the CO₂ is transported downward into the groundwater moving out below the desert to be trapped in the deep aquifers. Importantly, because of the salts, these waters are saline and alkaline and the solubility of CO₂ in saline/alkaline water is much higher than in pure or acidic water – the desert groundwater is a very significant CO₂ sink.

Because of their ability to date the carbon dissolved in the waters, the researchers were able to establish that the levels jumped substantially in historical times as the development of the Silk Road enabled the beginnings of oasis agriculture. Man’s activities – irrigation and over-irrigation – have augmented the efficiency of this carbon sink by, it is estimated, a factor of twelve:

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 approximate 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.

And because this is a saline and alkaline aquifer, the water is completely unsuitable for agriculture – it will likely remain below the desert as essentially permanent carbon storage - undoubtedly not the only missing sink, but a hitherto unidentified one. As Li remarks: “The fact that such a huge carbon pool and active sink has been unstudied for so long may simply be because it is remote and hidden under deserts: out of sight, out of mind.”

[Image at the head of this post is of agriculture and dunes along the northern edge of the Tarim Basin, Google Earth; diagram of the process of carbon storage from Yan Li, Yu-Gang Wang, R. A. Houghton, Li-Song Tang. Hidden carbon sink beneath desert. Geophysical Research Letters, 2015; DOI:10.1002/2015GL064222]