Zone of Peace
Environment China: The third pole Climate change is coming fast and furious to the Tibetan plateau.Jane Qiu reports on the changes atop the roof of the world.
Nature 454, 393-396
(2008)July 23, 2008
The Tibetan plateau gets a lot less attention than the Arctic or
Antarctic, but after them it is Earth’s largest store of ice. And the
store is melting fast. In the past half-century, 82% of the plateau’s
glaciers have retreated. In the past decade, 10% of its permafrost
has degraded. As the changes continue, or even accelerate, their
effects will resonate far beyond the isolated plateau, changing the
water supply for billions of people and altering the atmospheric
circulation over half the planet.
The plateau’s pivotal role is due almost entirely to its height.
Being an average of 4 kilometres above sea level makes it peculiarly
cold for its latitude—colder than anywhere else outside the polar
regions. Lhasa, capital of the Tibet Autonomous Region, is by Tibetan
standards relatively low-lying, at 3,650 metres — yet it is higher
even than La Paz, Bolivia, the highest capital city of a country.
Lhasa’s year-round average temperature is 8 °C; at the same latitude
Houston, Texas, has an average temperature of 21°C. The altitude
makes Tibet cold, especially in winter; its snow and ice cover, by
reflecting sunlight, make it colder still. The very bulk of the
plateau affects how winds circulate above it, and its altitude also
places the surface simply closer to the stratosphere than is normal.
The proximate cause of the changes now being felt on the plateau is a
rise in temperature of up to 0.3 °C a decade that has been going on
for fifty years—approximately three times the global warming rate.
The questions are how much more change to expect in the future, and
how severe the effects will be on the planet’s climate as a whole.
“Our understanding of global climate change would be incomplete
without taking into consideration what’s happening to the Tibetan
plateau,” says Veerabhadran Ramanathan, an atmospheric scientist at
the Scripps Institution of Oceanography in La Jolla, California.
Perhaps surprisingly given its significance, the potential impact of
the Tibetan plateau is still unfamiliar to many climatologists. One
reason is that there are far fewer data available compared with the
Arctic and Antarctic, which have seen a far greater number of
scientific expeditions to plumb their secrets. Although fieldwork
there can be tough, the plateau offers the same physical isolation
coupled with political challenges, at least for Western researchers.
“The plateau’s remoteness, high altitude and harsh weather conditions
make any research on the region very challenging,” says Yao Tandong,
director of the Institute of Tibetan Plateau Research, headquartered
in Beijing, of the Chinese Academy of Sciences.
Ice cores being carried down to base camp, and Yao Tandong (right)
working on a glacier.Ice cores being carried down to base camp.
Yao and his colleagues should know: in the 1980s, they were among the
few researchers persevering in difficult field conditions to gather
data on the plateau’s past climate history. They drilled ice cores,
up to 300 metres long, from Himalayan glaciers 7,200 metres high.
“It’s all done manually, and we had to carry them down the mountain.
There were no helicopters, no heavy equipment,” he says. “It’s -30
°C, with the wind cutting through us like a knife. It’s no mean
feat.” Such ordeals seem to have paid off: in collaboration with
glaciologist Lonnie Thompson of Ohio State University in Columbus,
the team’s work on oxygen isotopes within the cores yielded the most
comprehensive temperature reconstruction for the plateau, showing a
large-scale warming trend that began in the twentieth century and is
amplified at higher elevations1. Their findings are consistent with
temperature records from meteorological stations that have made
continuous measurements since the 1950s [ref. 2].
Some of this is what you would expect in a world undergoing
greenhouse warming, but there are regional factors on the plateau
that exacerbate the effect. In summer, dust from regional deserts
blows towards and up against the northern and southern slopes of the
plateau. One recent satellite study, for instance, tracked dust
wafting in from the Taklamakan desert to the north3. “We were really
surprised to find this much dust over the plateau,” says Huang
Jianping, an atmospheric scientist at Lanzhou University and lead
author of the study. The dust layers can reach as high as 10
kilometres above sea level, where they both absorb and reflect
sunlight, changing the amount of radiation that reaches the plateau.
Combining with the dust to drive climate change are emissions of
‘black carbon’, the soot that results when people cook with biofuels
such as wood, crop waste or dung. Southeast Asia, including the
Himalayas, is one of the global hotspots for black-carbon emissions4.
Using unmanned aircraft, Ramanathan and colleagues measured the
amount of sunlight absorbed by black carbon, and found that it
contributes as much as 50% of the solar heating of the air5. “It’s
the second-largest contributor to atmospheric warming in the region,
after carbon dioxide,” he says. He estimates that the combined effect
of black carbon and greenhouse gases may be sufficient to account for
a warming trend of 0.25 °C per decade in the Himalayas, roughly what
has been observed so far.
Plumes from dust storms in the Taklamakan desert, such as this one in
June 2005, can reach the Tibetan plateau and affect the climate
there.Plumes from dust storms in the Taklamakan desert, such as this
one in June 2005, can reach the Tibetan plateau and affect the
climate there.NASA/MODIS RAPID RESPONSE TEAM
When black carbon settles on Himalayan glaciers, it darkens the snow
and ice so that they absorb more heat and become warmer. “The melting
seasons on the plateau now begin earlier and last longer,” says Xu
Baiqing of the Institute of Tibetan Plateau Research. Glaciers at the
edge of the plateau tend to melt more than those in the middle; one
study, for instance, showed that glaciers in the eastern part of the
Kunlun Mountains retreated by 17% over the past 30 years, which is
ten times faster than those in the central plateau. If current trends
hold, two-thirds of the plateau glaciers could be gone by 2050, says Yao.
FLOODS AND DROUGHTS
The melting glaciers are starting to leave behind dangerous glacial
lakes, in which meltwater ponds behind a dam of debris left by the
retreating ice tongue. Scientists have identified 34 such glacial
lakes on the northern slopes of the Himalayas, and 20 outburst floods
have been recorded in the past 50 years.
The risk of floods, though, is but a short-term danger far exceeded
by long-term issues with water supplies atop the plateau. Runoff from
the region’s mountains feeds the largest rivers across Southeast
Asia, including the Yangtze, Yellow, Mekong, Ganges and Indus rivers.
If glaciers continue to retreat and snowpack shrinks atop the
plateau, the water supplies of billions of people will be in danger6.
“A large-scale thaw of permafrost would result in the loss of its
water content and trigger an ecological catastrophe.” Ouyang Hua
Permafrost is also at risk, as rising temperatures cause the ‘active’
ground layer—which freezes and thaws every year—to thicken.
That, in turn, affects how heat and moisture flow between the ground
and the atmosphere, further perturbing the system7. Degradation of
permafrost will not only put the Qinghai_Tibet Railway at risk8, but
also endangers the plateau’s alpine ecosystems, which rely on
permafrost to trap water in the topmost layers of soil to allow
plants to thrive at an altitude that would otherwise be too hostile
for them. “A large-scale thaw of permafrost would result in the loss
of its water content and trigger an ecological catastrophe,” says
Ouyang Hua, deputy director of the Institute of Geographical Sciences
and Natural Resources Research in Beijing. As permafrost stores
one-third of the world’s soil carbon, vegetation loss would lead to a
huge amount of carbon entering the atmosphere, exacerbating global warming.
With all the changes the Tibetan plateau is undergoing—a warming
climate, retreating glaciers, degrading permafrost and alpine
ecosystems — what are the implications for the regional and global
climate? The first and most important victim could be the Indian
monsoon. This strong seasonal wind results from differences in the
thermal properties between land and ocean. In summer, the vast land
in Asia heats up more than the Indian Ocean, leading to a pressure
gradient and the flow of the air and moisture from the ocean. The
rise of the Tibetan plateau starting 50 million years ago (see
‘Lifting the roof of the world’) is thought to have strengthened this
effect. As the land surface absorbs more sunlight than the
atmosphere, the plateau creates a vast area of surface warmer than
the air at that elevation, thereby increasing the land_ocean pressure
gradient and intensifying the monsoon.
Some climate models show that global warming would lead to a greater
increase in the plateau’s surface temperature than over the ocean,
thus augmenting the monsoon. On the other hand, some models suggest
that aerosols that absorb solar radiation, and changes in land use in
the region, could weaken the monsoon. “The intensity of the monsoon
is likely to depend on which of these two competing forces
dominates,” says Ramanathan.
No matter what the causes are, some studies indicate that the
weakening force may be prevailing, or has prevailed for at least the
past three centuries. Duan Keqin, of the Cold and Arid Regions
Environmental and Engineering Research Institute in Lanzhou, and his
colleagues reconstructed a 300-year history of snow accumulation by
analysing ice cores from the Dasuopu glacier9. They believe the ice
there preserves an estimate of monsoon variations in the Himalayas.
“We found that the warmer it was, the weaker the monsoon,” says Duan.
On average, a temperature increase of 0.1 °C was associated with a
decrease of 100 millimetres in snow accumulation. But similar studies
on other parts of the plateau are needed to confirm the results, he notes.
“Changes in the Indian monsoon are not the only threat in Asia to the
global climate,” adds Rong Fu of the Georgia Institute of Technology
in Atlanta. Her research shows that convection over the Tibetan
plateau can transport water vapour and pollutants to the
stratosphere, the atmospheric layer that is immediately above the
troposphere and contains most of the Earth’s ozone. “The strong,
horizontal wind in the stratosphere could then spread the water
vapour and pollutants globally,” says Fu.
Plumes from dust storms in the Taklamakan desert, such as this one in
June 2005, can reach the Tibetan plateau and affect the climate
there.Yao Tandong working on a glacier.
Water vapour has a stronger greenhouse effect than carbon dioxide per
molecule, but it normally reaches no higher than 1_2 kilometres below
the stratosphere. The situation is different over the plateau, over
which the convection layer is shifted some 6 kilometres further up so
that its top boundary is around 18 kilometres up, in the lower
stratosphere. In addition, the troposphere is thinner over the
plateau, and the heat emitted by the surface can reach higher and
make the air warmer at the base of the stratosphere. “So more water
vapour is able to get to the stratosphere without being frozen or
precipitated,” says Fu. Warmer temperatures over the plateau can
resulting increased glacial melting and water-vapour transport—
which, in turn, causes strong convection and lifts even more water
vapour up. “It’s very worrying to think that a lot of it may reach
the stratosphere,” she says.
“Worrying,” indeed, best captures the mood of researchers who work on
the Tibetan plateau. They are keen to undertake large-scale,
comprehensive studies and to collect as many data as possible. “We
know so little about it and understand it even less,” says Yao. One
ongoing study is to document all the glaciers in China, recording
characteristics such as their location, area, length, thickness and
the position of the snow line. A similar survey was conducted between
1978 and 2002, which scientists believe could serve as a reference
point to reveal any major changes. In addition, glaciologists
continue to identify and closely monitor potentially dangerous
glacial lakes in hopes of heading off any potential outburst floods.
QUICK WAY OUT
“Reducing emissions of greenhouse gases and black carbon should be
the top priority.” Xu Baiqing
Meanwhile, others focus on the bigger picture of how to tackle
pollution problems in Asia. “Reducing emissions of greenhouse gases
and black carbon should be the top priority,” says Xu. Ramanathan
reckons that cutting down on black-carbon emissions could be a “quick
way out of the mess”, given that its half-life in the atmosphere is
about 15_20 days compared with the century-scale half-life of carbon
dioxide. His simulations suggest that, just by removing traditional
ways of cooking with wood, dung and crop residues, some 40_60% of the
black-carbon emissions would be gone. This could be “a short-term
fix, a low-hanging fruit that is much cheaper and faster” than
reducing carbon dioxide, he says. “The key is to give villagers
access to better forms of energy.”
In the end, the Tibetan plateau may be a crucial testing ground for
how humans and the environment collide in a globally warmed world.
Can the world’s third pole be saved? “Let’s hope that the changes the
plateau is going through are only transient,” says Yao. “What we do
about them probably will determine what’s going to happen to it in the
• Jane Qiu writes for Nature from Beijing. See Editorial, page 367,
and News Feature, page 384. For a podcast and more on China see
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