Environment

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 CO2 dissolved 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]

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/environment/page/2/