Humans accidentally create hidden carbon sink in desert

Since the dawn of farming, humans have been accidentally creating a huge carbon sink that by now may store more carbon than all of the world's living plants.

But this sink is in the last place that you'd expect to find huge amounts of carbon – under the desert, reports New Scientist.

That is the surprising conclusion of work done in one desert in China. If the findings are confirmed in other deserts around the world, it could present a way of taking carbon out of the atmosphere. But it also means we need to be careful not to disturb the huge carbon sinks stored under desert sands.

"Basically, people thought the whole arid region is totally negligible to the global carbon budget," says Yan Li of the Chinese Academy of Science in Urumqi, China. "We are arguing that that's not the case."

Li and colleagues took samples of water from the Tarim Basin, a salty aquifer under a desert in north-west China. They measured the carbon content of the water there and dated it.

They then repeated the process with water that flows into Tarim Basin from glaciers, and with water that is used to irrigate local farms, which comes from a nearby river. Using that information, they could draw a timeline showing how much carbon got into the basin and at what time.

Remarkably, over human history, the rate at which carbon was sunk into the groundwater rose dramatically, increasing by more than 12 times over the past 8000 years. The particularly high levels of carbon storage in this region began 2000 years ago when the Silk Road opened up, which resulted in increased levels of human activity and farming around the Tarim Basin.

Desert sands have been examined for carbon storage before, but that did not reveal this process because the carbon is not stored in the sand – it is transported down into the groundwater, Li says.

Deep storage

The process begins when humans start to grow crops in the sandy soil. As the plants suck carbon dioxide out of the air, some is released into the sand and more is added by microbes that break down nutrients in the soil.

Normally that CO₂ would escape into the air, but in arid farming, a lot of water is used to combat rising salinity caused by evaporating water. The extra water dissolves the CO₂ and deposits it in the aquifer below.

So long as the aquifer is a closed system – which it usually is – the CO₂ will stay there.

The mechanism makes sense and has been rigorously studied, says Pep Canadell of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Canberra, Australia.

Canadell is executive director of the Global Carbon Project, which aims to develop a complete picture of the world's carbon budget.

But he has doubts about the global importance of desert carbon sinks.

"One needs a lot of irrigation in the desert to make this mechanism globally meaningful," says Canadell. "One region in China doesn't make the world."

Li agrees, saying that this is the first time that this process has been identified, so it has not even been looked for elsewhere yet.

He says that the salty water in these sorts of aquifers is not usually thought of as a resource – it cannot be used for irrigation, for example – so it has barely been studied. "It's basically out of sight, out of mind," he says.

Li expects the process to occur elsewhere, although it will vary with the pH of the soil – the more alkaline it is, the more carbon can be absorbed by irrigation water.

And because between 5 and 10 per cent of arid regions are farmed, there will be about a trillion tonnes of carbon stored in saline aquifers if similar mechanisms are at work across the globe – a quarter more than is stored in living plants.

If confirmed, this has two important implications, according to Li.

The first is that these saline aquifers should be left alone. There have been plans to use them for various things, including geothermal energy and irrigation. "To open up the saline ground water would really be a problem for sure," Li says.

But more positively, Li thinks the mechanism could be exploited as a form of carbon sequestration by expanding irrigated farming on the edges of deserts.