Reykjavik, Iceland - Over the past year, a number of giant, mysterious holes have emerged in Siberia, some as deep as 200 metres.
Scientists say the craters may be emerging because the frozen ground, or "permafrost," that covers much of Siberia has been thawing due to climate change, allowing methane gases trapped underground to build up and explode.
Permafrost is ground that is permanently frozen, where the ground temperature has remained below 0 °C (32 °F) for at least two years. It covers about a quarter of the northern hemisphere's land surface.
When permafrost thaws, microbes digest the plant and animal remains that were locked in the permafrost and release greenhouse gases, carbon dioxide and methane into the atmosphere.
The phenomenon is a self-feeding cycle, explained Sarah Chadburn, from the University of Exeter.
"Permafrost soils contain vast amounts of carbon, nearly twice as much as is currently in the atmosphere. As the permafrost thaws in a warming climate, the soil decomposes and releases carbon to the atmosphere as carbon dioxide and methane. These are greenhouse gases, and they warm the Earth even more. This leads to more permafrost thawing, more carbon release, and so the cycle continues," Chadburn said.
At the recent Arctic Circle Assembly in Reykjavik, Iceland, Max Holmes from the US-based Woods Hole Research Center (WHRC) said in a presentation that the Siberian sinkholes "are an additional indication that vast changes are under way in the Arctic."
"I don't worry about them too much in and of themselves," the researcher said. "But they do reinforce the notion that big changes are already happening, and that we are likely to have more unpleasant surprises in the future."
Recent research has found that a third greenhouse gas, nitrous oxide (N2O), is also emitted in some areas covered by thawing permafrost.
"We now know that a lot of nitrogen is released during permafrost thaw and that the microbes responsible for N2O production are present in virtually all Arctic and boreal systems," said Ben Abbott, a France-based scientist who studies permafrost in Alaska.
He added that it was unclear whether nitrogen gas emissions from thawing permafrost are significant compared with those of carbon dioxide and methane.
Despite scientists' concern that thawing permafrost could exacerbate global warming, Chadburn noted that "most climate models do not include the warming aspect of permafrost emissions," including the models used by the Intergovernmental Panel on Climate Change (IPCC).
Although the IPCC has acknowledged that permafrost contributes to global warming, a lack of data on the phenomenon has meant that they have not been able to include it in their reports.
Chadburn estimated that thawing permafrost would raise global temperatures by an average of 0.3 °C but could be as much as 0.7 °C.
Given predictions that permafrost thaw could cause warming, Hugues Lantuit from the Alfred Wegener Institute in Germany said that "the objective for the COP21 climate summit should really be a temperature increase of no more than 1.7 °C to take account of emissions from permafrost," referring to the annual global conference on climate change to be held next month in Paris.
Walter Oechel from San Diego State University and the Open University and Donatella Zona from the University of Sheffield have been measuring methane fluxes in the Arctic for more than a decade. "We expect methane emissions from the Arctic to increase dramatically with warming of the Arctic," they said.
"And, the potential is there for this release to become catastrophic."
Meanwhile, the frequency of fires has been intensifying in Arctic areas, noted Scott Goetz from WHRC. More than two million hectares of land have burned in Alaska this year, he said in his presentation at the Arctic Circle Assembly.
"Climate warming and drying are intensifying the fire regime. These fires burn roots and the trees then fall over… Fire disturbance deepens thaw depth and mobilises permafrost carbon," Goetz said.
In addition to contributing to global warming, thawing permafrost also affects wildlife and indigenous populations in the Arctic.
Courtney Price, of the Arctic Council's Conservation of Arctic Flora and Fauna organisation, said continued thawing of permafrost is one factor endangering thermokarst lakes. These lakes are formed by the thawing of permafrost and accumulation of surface water in the depression.
But if permafrost continues to thaw, there is no structure to hold the water, and the lakes can drain completely, Price said.
"Thermokarst lakes act as 'hot spots' of biological activity in northern regions… Such biologically productive systems are important to Arctic peoples for supporting traditional lifestyles, and for providing water to rural/urban communities and development, especially where groundwater resources are unavailable," she explained.
The phenomenon also affects public safety: Around 70% of the world's permafrost is found in Russia and, in Siberia, entire cities, of which Yakutsk is the largest, are built on permafrost. When permafrost thaws, buildings can tilt and become uninhabitable.
The solution? WHRC scientist Sue Natali said that "to save permafrost, we have to reduce fossil fuel use and manage forests globally to enhance carbon dioxide uptake by the biosphere."
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