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DUVANNY YAR, Russia (Reuters) - Sergei Zimov bends down, picks up a handful of treacly mud and holds it up to his nose. It smells like a cow pat, but he knows better.
"It smells like mammoth dung," he says.
This is more than just another symptom of global warming.
For millennia, layers of animal waste and other organic matter left behind by the creatures that used to roam the Arctic tundra have been sealed inside the frozen permafrost. Now climate change is thawing the permafrost and lifting this prehistoric ooze from suspended animation.
But Zimov, a scientist who for almost 30 years has studied climate change in Russia's Arctic, believes that as this organic matter becomes exposed to the air it will accelerate global warming faster than even some of the most pessimistic forecasts.
"This will lead to a type of global warming which will be impossible to stop," he said.
When the organic matter left behind by mammoths and other wildlife is exposed to the air by the thawing permafrost, his theory runs, microbes that have been dormant for thousands of years spring back into action.
As a by-product they emit carbon dioxide and -- even more damaging in terms of its impact on the climate -- methane gas.
According to Zimov, the microbes are going to start emitting these gases in enormous quantities.
Here in Yakutia, a region in the north-eastern corner of Siberia, the belt of permafrost containing the mammoth-era soil covers an area roughly the size of France and Germany combined. There is even more of it elsewhere in Siberia.
"The deposits of organic matter in these soils are so gigantic that they dwarf global oil reserves," Zimov said.
U.S. government statistics show mankind emits about 7 billion tonnes of carbon a year.
"Permafrost areas hold 500 billion tonnes of carbon, which can fast turn into greenhouse gases," Zimov said.
"If you don't stop emissions of greenhouse gases into the atmosphere ... the Kyoto Protocol (an international pact aimed at reducing greenhouse emissions) will seem like childish prattle."
It might be easy to dismiss the 52-year-old, with his beard and shock of wavy hair, as an alarmist crank. But his theory is grabbing attention in the scientific community.
"There's quite a bit of truth in it," Julian Murton, member of the International Permafrost Association, told Reuters.
"The methane and carbon dioxide levels will increase as a result of permafrost degradation."
A United Nations report in June said there was at yet no sign of widespread melting of permafrost that could stoke global warming, but noted the potential threat.
"Permafrost stores a lot of carbon, with upper permafrost layers estimated to contain more organic carbon than is currently contained in the atmosphere," the report said.
"Permafrost thawing results in the release of this carbon in the form of greenhouse gases which will have a positive feedback effect to global warming."
Zimov is chief scientist at the Russian Academy of Science's North Eastern Scientific station, three plane rides and eight times zones away from Moscow.
At Duvanny Yar on the shores of the Kolyma River, the phenomenon that Zimov describes in speeches at scientific conferences can be seen first hand.
The steep-sided river bank, until now held up by permafrost, is collapsing as the ice melts. Every few minutes, a thud can be heard as another wedge of soil and permafrost comes tumbling down, or a splash as a chunk falls into the river.
Nearby, Zimov points to an area so far unaffected by the melting -- a forest of larch trees with berries and mushrooms and covered with a soft cushion of moss and lichen.
Further down the slope though, the landscape is covered with trees toppled over as the soil disintegrates. Brooks murmur down the slope carrying melted water.
Elsewhere, places that five or 10 years ago were empty tundra are now dotted with lakes -- a result of thawing permafrost. These 'thermokarst' lakes bubble with methane, over 20 times more potent as a greenhouse gas than carbon dioxide.
The permafrost thaw affects those rare outposts where humans have settled. In Chersky, a town of 3,000 people, apartment blocks have cracks running through their walls as the earth beneath them subsides. Many have been demolished as unsafe.
So few people live in or visit this wilderness that the changing landscape on its own is unlikely to worry people on the other side of the world.
But Zimov warns that people everywhere should take notice, because within a few years, the knock-on effect of the permafrost melting in Siberia will be having a direct impact on their lives.
"Siberia's landscape is changing," he said. "But in the end local problems of the north will inevitably turn into the problems of Russia's south, the Amazon region or Holland."
Additional reporting by Alister Doyle