Scientists attribute the thaw to
climate warming. As the air temperature warms, so does
the frozen ground beneath it.
Data quest
The observations reiterate the recent
findings of the Arctic Climate Impact Assessment report,
which attributed the northern polar region's summer sea-ice
loss and permafrost thaw to dramatic warming over the
past half-century.
Thawing permafrost can cause buildings
and roads to droop, and pipelines to crack.
Natural features are also affected.
Scientists reported an increased frequency in landslides
in the soil-based permafrost of Canada, and an increased
instability and slope failures in mountainous regions,
such as the Alps, where ice is locked in bedrock. With
the exception of Russia and its long history of permafrost
monitoring, global records are insufficient - often too
brief or scattered - to determine the precise extent of
ice loss, said Dr Nelson.
However, monitoring programmes that
are now much larger in scope, such as the Global Terrestrial
Network for Permafrost (GTNP), indicate a warming trend
throughout the permafrost zone. Boreholes
in Svalbard, Norway, for example, indicate that ground
temperatures rose 0.4C over the past decade, four times
faster than they did in the previous century, according
to Charles Harris, a geologist at the University of Cardiff,
UK, and a coordinator of Permafrost and Climate in Europe
(Pace), which is contributing data to the GTNP.
"What took a century to be
achieved in the 20th Century will be achieved in 25 years
in the 21st Century, if this trend continues," he
said.
Slip and slide
In Ellesmere Island, Canada, a combination
of warmer temperatures and sunny days has triggered an
increasing frequency of detachment events, or landslides,
over the past 25 years, compared with the previous 75,
according to Antoni Lewkowicz, professor of geography
at the University of Ottawa.
A detachment event occurs on a slope
when the bottom of the active layer - the layer of thawing
and freezing ground above permafrost - becomes slick with
melted ice, causing it to slide off from the permafrost
below.
But in this case, the amount of temperature increase
is not so important as the rate of increase, Dr Lewkowicz
found. Meltwater from ice that warms slowly drains away.
When ice warms quickly, water pools, creating a frictionless
surface between the active layer and the permafrost. Like
a stroll across a sloping icy sidewalk, a fall is almost
certain. "We have records from this particular site
for about 10 or 12 years," said Dr Lewkowicz. "The
years when active layer detachments have taken place have
been times when we've had this rapid thaw down at the
bottom of the active layer."
The slides may cut a wide swath hundreds of metres across,
but extend only 50 or 60cm deep. "They're almost
skin-like landslides, moving across the permafrost,"
said Dr Harris. The exposed permafrost, warmed by the
air, now produces a new active layer.
Sink to source
In steep mountainous regions, permafrost thaw can lead
to slope failure and rock falls. In these areas, the permafrost
ice is in hard rock. Where rocks are jointed, the ice
serves as a kind of cement holding them together.
When it melts, the rock loses its strength and falls.
A dramatic example of this occurred during the European
heatwave of 2003 when a huge block of the Matterhorn broke
off suddenly, leaving Alpine climbers stranded.
It's not just the general warming trend we need to worry
about," said Dr Harris, "but these extreme seasonal
events as well." Dr Nelson says that with human-built
structures, proper engineering and land use can mitigate
permafrost loss.
Tingun Zhang, a researcher at the US National Snow and
Ice Data Center in Boulder, Colorado, reported at the
AGU on the particular challenge slumping ground presents
to the construction of the Qinghai-Xizang railroad across
Tibet, perhaps the most ambitious permafrost-zone project
since the Trans-Alaskan pipeline.
Nearly half the railroad will lie across permafrost,
and temperatures in the region are expected to rise during
this century. Engineers are using a simple - and long
established - trick of cooling the permafrost with crushed
rock. Rocks minimise heat intake in summer and promote
heat loss in winter.
It is the first time a large-scale project is using the
crushed-rock method as its primary solution, according
to Dr Zhang.But not all outcomes of permafrost thaw have
precedent, or an immediate solution. One considerable
variable is the possible release into the air of organic
carbon stored in the permafrost.
In the drier areas, most of the emissions would be in
the form of carbon dioxide (CO2). But in the wetter areas,
it would be methane, a more effective greenhouse gas.
Scientists do not know exactly how much carbon is sequestered
in the permafrost regions, but estimates show it could
be up to a quarter of the sequestered carbon on Earth,
14% of it in the Arctic, alone.