Evidence from Siberian caves suggests that a global temperature rise of 1.5 degrees
Celsius could see permanently frozen ground thaw over a large area of Siberia, threatening release of carbon from
soils, and damage to natural and human environments.
A thaw in Siberia's permafrost (ground frozen throughout the year) could eventually release over 1,000 giga-tonnes
of the greenhouse gases carbon dioxide and methane into the atmosphere, potentially enhancing global warming.
The data comes from an international team led by Oxford University scientists studying stalactites and stalagmites
from caves located along the 'permafrost frontier', where ground begins to be permanently frozen in a layer tens to
hundreds of metres thick.
Icy cave in Baikal
Because stalactites and stalagmites only grow when liquid rainwater and snow melt drips
into the caves, these formations record 500,000 years of changing permafrost conditions, including warmer periods
similar to the climate of today.
Icy cave in Baikal, Olkhon island
Records from a particularly warm period (Marine Isotopic Stage 11) that occurred around 400,000 years ago suggest
that global warming of 1.5 degrees Celsius compared to the modern (pre-industrial) climate is enough to cause
substantial thawing of permafrost far north from its present-day southern limit.
'The stalactites and stalagmites from these caves are a way of looking back in time to see how warm periods similar
to our modern climate affect how far permafrost extends across Siberia,' said Dr Anton Vaks of Oxford University's
Department of Earth Sciences, who led the work.
'As permafrost covers 24% of the land surface of the Northern hemisphere significant thawing could affect vast
areas and release giga-tonnes of carbon.
'This has huge implications for ecosystems in the region, and for aspects of the human environment. For instance,
natural gas facilities in the region, as well as power lines, roads, railways and buildings are all built on
permafrost and are vulnerable to thawing. Such a thaw could damage this infrastructure with obvious economic implications.'
Icy cave in Baikal
The team used radiometric dating techniques to date the growth of cave formations (stalactites and stalagmites).
Data from the Ledyanaya Lenskaya Cave – near the town of Lensk, at a latitude of 60 degrees North – in the coldest
region showed that the only period when stalactite growth took place occurred about 400,000 years ago, during a
period with a global temperature 1.5 degrees Celsius higher than today.
Periods when the world was 0.5-1 degrees Celsius warmer than today did not see any stalactite growth in this
northernmost cave, suggesting that around 1.5 degrees Celsius is the 'tipping point' at which the coldest
permafrost regions begin to thaw.
'Although it wasn't the main focus of our research our work also suggests that in a world 1.5
degrees Celsius warmer than today, warm enough to melt the coldest permafrost, adjoining regions would see
significant changes with Mongolia's Gobi Desert becoming much wetter than it is today and, potentially, this
extremely arid area coming to resemble the present-day Asian steppes,'Dr Vaks said.
A report of the research is published in this week's Science Express. The team included scientists from Britain,
Russia, Mongolia and Switzerland.
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