Chinese
scientists say a huge ocean underneath Xinjiang's Tarim basin acts as a
major carbon sink, protecting us from even greater global warming.
Photo: Nasa
There
could be an “ocean” hidden under one of the driest areas on Earth,
according to a breakthrough discovery by Chinese scientists.
The amount of salt water beneath the Tarim basin in Xinjiang could be
equivalent to ten times the water of all five Great Lakes in the United
States.
“This is a terrifying amount of water,” said professor Li Yan, who
led the study at the Chinese Academy of Sciences’ Xinjiang Institute of
Ecology and Geography in Urumqi.
“Never before have people dared to imagine so much water under the sand. Our definition of desert may have to change,” he said.
The Tarim is the world’s largest landlocked basin and home to
Taklimakan, the largest desert in China. The basin is known for its rich
oil reserves, but to access them requires large amounts of water.
For a long time scientists had suspected that melt water from high
mountains nearby had sipped beneath the basin, but the exact amount of
water reserves there remained unknown.
Precise estimates are difficult because surface water in the region,
such as seasonal rivers and lakes appear at random times in inconsistent
locations, making direct measurement impossible.
Li’s team stumbled on the discovery by accident.
“We were after carbon, not water,” he said. Xinjiang contains major oil and gas reserves. Photo: Reuters
Greenhouse gas carbon dioxide can be absorbed in certain regions
known as "carbon sinks", such as forests and oceans. Locating these
sinks may help scientists better understand climate change.
Around ten years ago, Li’s team discovered large amounts of carbon
dioxide disappearing in Tarim, with no explanation over where it could
be going.
In a paper in the journal Geophysical Research Letters, Li’s
team reported that there could be a large amount of water under Earth's
largest deserts which serve as carbon sinks as important as forests and
oceans.
Under the Tarim desert, over a depth stretching thousands of metres,
exists an enormous amount of saline water fll of carbon dioxide, they
found.
The team obtained deep underground water samples from nearly 200
locations across the desert. By measuring the amount of carbon dioxide
in these samples, and comparing them to the carbon dioxide in melt
water, the scientists were able to calculate how much water had flown
into the basin.
“Our estimate is a conservative figure. The actual amount could be larger,” Li said.
Melt water has been used by people in Xinjiang for agricultural irrigation for thousands of years.
The soil of farmland in the region is alkaline, helping the
dissolving of carbon dioxide into the water. By dating the age of the
carbon Li's team "recorded a jump of 'carbon sinking' after the opening
of the ancient Silk Road more than two thousand years ago."
“CCS [carbon capture and storage] is a 21st century idea, but our
ancestors may have been doing it unconsciously for thousands of years,"
he said.
However, Li emphasised that the "ocean" under Tarim would not be much immediate use for Xinjiang's economic development.
The water is not just salty, but contains a large amount of carbon
dioxide. “It’s like a can of coke. If it is opened all the greenhouse
gas will escape into the atmosphere,” he said.
The biggest question now is whether similar “oceans” can be also be
found under other large deserts, such as Sahara. Li said they would work
with research teams around the world to find out the answer.
The chance of water under these deserts is high because the amount of
carbon these “oceans” carried could reach a trillion tonnes, which
matches the amount of “missing carbon” on the planet, according to Li's
team's calculations.
The confounding case of the missing carbon
Scientists followed the journey of
water through the Tarim Basin from the rivers at the edge of the valley
to the desert aquifers under the basin. They found that as water moved
through irrigated fields, the water gathered dissolved carbon and moved
it deep underground.
For several decades, scientists have been searching for about
a billion tons of carbon dioxide generated from the burning of fossil
fuels.
Of the roughly 11 billion tons of carbon produced each
year, it has been estimated that more than 5 billion remains in the
atmosphere. Another 3 billion is stored in the oceans. The rest of it,
it seems, is sequestered in tropical and northern forests, but there's
some -- perhaps a billion tons' worth, perhaps even more -- that can't
quite be accounted for.
Scientists have struggled to explain the "missing carbon sink," with some arguing it's actually in the forests after all, and others believing it's elsewhere.
Now,
an international team from the United States and China is suggesting
there is another possible repository for some of this missing carbon:
below the deserts of the world.
In a paper in the Geophysical
Research Letters, a journal of the American Geophysical Union, the
scientists contend that massive aquifers underneath deserts could hold
more carbon than all the plants on land. Thanks to agriculture in these
areas, they estimated these underground deserts aquifers are storing 14
times more carbon every year than previously thought.
Researchers
gathered groundwater flowing under the desert sands. The amount of
carbon carried by this underground flow increased quickly when the Silk
Road, which opened the region to farming, began 2,000 years ago.
Yan Li
"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 on the study. "It is basically a one-way
trip."
The idea of deserts as carbon sinks has been suggested in
the past. Some scientists have argued that the carbon was taken up by
desert plants and transferred deep into the root system where it
remains. But that was controversial due to the lack of plants and the
poor state of soils.
To test their aquifer theory, the authors of
the new research examined the flow of water through the Tarim Basin, a
Venezuela-sized valley in China's Xinjiang region. Farmers on the edge
of the desert are supported by water draining from rivers in the
surrounding mountains.
They found a complex process, whereby the
amount of carbon dioxide dissolved in the water doubles as it filters
through irrigated fields. They also found that carbon dioxide in the air
was being absorbed by the desert crops through the plant's roots and
that microbes add carbon dioxide to the soil when they break down sugars
in the dirt.
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. The amount entering the Tarim Desert increased
significantly about 2,000 years ago, when the Silk Road opened and
farming began to flourish.
After the carbon-rich water flows down
into the aquifer near the farms and rivers, it moves sideways toward the
middle of the desert, a process that takes roughly 10,000 years.
And
over its history, the desert has been a huge and long standing carbon
sink. The study's authors estimate that over its lifetime, the desert
has held an aggregate 22 billion tons of carbon, dissolved in an aquifer that contains roughly 10 times the amount of water held in the Great Lakes.
R.A.
Houghton, a co-author on the study from the Woods Hole Research Center
whose expertise is in land use and carbon cycles, said the findings
could help produce better climate models into the future and force
scientists to rethink the idea that increased emissions were leading to
greater growth rates in forest, and thus, more carbon being stored in
them.
"If you are proposing a mechanism to explain the carbon
sink, you have to have a process that is increasing over time," Houghton
said. "So, what is neat about this study is that they propose this
increasing sink in these desert brines is due to agriculture. The sink
is getting bigger as we irrigate."
While the study could help
solve the mystery of the missing carbon, Houghton warned against using
the findings to justify increased farming in arid regions as a benefit
to the climate.
"Many places in the world are drawing water from
clean aquifers. It's not being replaced," Houghton said. "So once it is
used up, what do you do to keep all that land watered?"
Xinjiang contains major oil and gas reserves. Photo: Reuters
Xinjiang contains major oil and gas reserves. Photo: Reuters
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