Siberian Caves Reveal Advancing Permafrost Thaw
Melting of significant portions of Arctic permafrost
could accelerate climate change into a catastrophe
By
David
Biello
PERMAFROST CAVE: The frost
crystals at the entrance to the Ledyanaya Lenskaya cave in Russia denote
the region's permafrost, which has been in place for roughly 400,000
years, according to the cave's speleothems.
Image: Vladimir V Alexioglo
Permafrost is not so permanent. Across the Arctic, swathes of
once-frozen-solid ground have begun to thaw. If the records
preserved in Siberian caves are accurate, much more of the region could
melt if temperatures continue to warm.
Geoscientist Anton Vaks of the University of Oxford led an international
team of experts—including the Arabica Caving Club in Irkutsk—in sampling
the spindly cave growths known as stalagmites and stalactites across
Siberia and down into the Gobi Desert of China. Taking samples of such
speleothems from six caves, the researchers then reconstructed the
last roughly 500,000 years of climate via the decay of radioactive
particles in the stone. When the ground is frozen above a cave no
water
seeps into it, making such formations "relicts from warmer periods
before permafrost formed," the researchers wrote in a
study published online in Science on 21 February.
The
details of the study reveal that conditions were warm enough even in
Siberia for these mineral deposits to form roughly 400,000 years ago,
when the global average temperature was 1.5 degrees Celsius higher than
present. It also suggests that there was no permafrost in the Lena River
region at that time, because enough water seeped into the northernmost
cave to enable roughly eight centimeters of growth in the formations.
That was, in fact, the last time the formations in the
Ledyanaya
Lenskaya Cave grew, although other caves further south showed
multiple periods of growth coinciding with other warmer periods. "That
boundary area of continuous permafrost starts to degrade when the mean
global temperature is 1.5 degrees C higher than present," Vaks explains.
"Such a warming is a threshold after which continuous permafrost zone
starts to be vulnerable to
global warming."
Since Vaks's present is the "preindustrial late Holocene," that means
the planet is already more than halfway there, having experienced
0.8 degree C warming to date. Such a thaw is no small matter, given
that permafrost covers nearly a quarter of the land in the Northern
Hemisphere and holds roughly 1,700 gigatonnes of carbon—or roughly twice
as much carbon as is currently trapping heat in the atmosphere. Much of
that carbon would end up in the atmosphere if the
permafrost was to thaw further.
That may not have occurred during the warm period 400,000 years ago,
known as
Marine Isotope Stage 11 to scientists, which featured elements such
as boreal forest on Greenland and higher sea levels. "The thawing was
probably very brief because the layer deposited in the northernmost cave
stalactite was relatively thin," Vaks says—too thin in fact to determine
how long the warm period lasted. "We don't see any extraordinary
increase in atmospheric CO2 or methane during MIS-11." And the Gobi
Desert might benefit, enjoying wetter conditions in the future if the
record in these caves is accurate.
It's not clear
how far north such thawing might extend if global average
temperatures continue to warm until they match those from long ago. "Now
we are looking for caves with speleothems in northern Siberia to answer
this question," Vaks notes, adding that the northernmost cave is already
much warmer than in the late 18th century based on historical reports.
Further research could be done by taking sediment cores from Arctic
river deltas or lakes, though this remains an epic task given the
vastness and remoteness of the region. But, already, it is clear
that global climates not much warmer than present are enough to thaw
even more permafrost—as far north as 60 degrees latitude.
"The potential impact of these results extends to global policy: these
results indicate the potential release of large amounts of carbon from
thawed permafrost
even if we attain the 2 degree [C] warming target under
negotiation," says Kevin Schaefer, a scientist at the U.S. National Snow
and Ice Data Center, who has
also
studied permafrost but was not involved in this, in his words,
"great science" effort. "Permafrost thaws slowly and the carbon will be
released into the atmosphere over two to three centuries."
Already, such thawing Arctic ice—whether underground or at sea—has
further opened up the territory to exploration for resources,
particularly
oil. At the same time, the big thaw will make getting the oil out
more expensive—billions of dollars in infrastructure investments in
pipelines, roads and the like will be damaged as the ground shifts
beneath them.
© 2013 Scientific American,
a Division of Nature America, Inc. All Rights
Reserved.
http://www.scientificamerican.com/article.cfm?id=siberian-caves-reveal-permafrost-thaw
|