Sheltered by snow-spattered mountains, the Stordalen mire is a flat, marshy plateau, pockmarked with muddy puddles. A whiff of rotten eggs wafts through the fresh air. 

Here in the Arctic in Sweden's far north, about 10 kilometers east of the tiny town of Abisko, global warming is happening three times faster than in the rest of the world.

On the peatland, covered in tufts of grass and shrubs dotted with blue and orange berries and little white flowers, looms a moonlander-like pod hinting at this far-flung site's scientific significance.

Researchers are studying the frozen – now shapeshifting – earth below known as permafrost.

As Keith Larson walks between the experiments, the boardwalks purposefully set out in a grid across the peat sink into the puddles and ponds underneath and tiny bubbles appear.

The distinct odor it emits is from hydrogen sulfide, sometimes known as swamp gas. But what has scientists worried is another gas rising up with it: methane. 

Carbon stores, long locked in the permafrost, are now seeping out.

Between carbon dioxide (CO2) and methane, permafrost contains some 1,700 billion tons of organic carbon, almost twice the amount of carbon already present in the atmosphere.

Methane lingers in the atmosphere for only 12 years compared to centuries for CO2 but is about 25 times more potent as a greenhouse gas over a 100-year period.

Thawing permafrost is a carbon "time bomb," scientists have warned.

In the 1970s, "when researchers first started showing up and investigating these habitats, these ponds didn't exist," says Larson, project coordinator for the Climate Impacts Research Center at Umea University, based at the Abisko Scientific Research Station.

"The smell of the hydrogen sulfide, that's associated with the methane that's being released – they wouldn't have smelled that to the extent we do today," adds Larson, who measures how deep the so-called active layer is by shoving a metal rod into the ground.

Permafrost – defined as soil that stays frozen year-round for at least two consecutive years – lies under about a quarter of the land in the Northern Hemisphere.

In Abisko, the permafrost beneath the mire can be up to tens of meters thick, dating back thousands of years. In parts of Siberia, it can go down over a kilometer and be hundreds of thousands of years old.

With average temperatures rising around the Arctic, the permafrost has started to thaw. 

As it does so, bacteria in the soil begin to decompose the biomass stored within. The process releases the greenhouse gases carbon dioxide and methane – further accelerating climate change in a vicious circle.

A few minutes drive away at the much smaller Storflaket mire, researcher Margareta Johansson has tracked the thawing permafrost since 2008 by measuring the active layer, the part of the soil that thaws in summer.

"In this active layer, where measurements started in 1978, we have seen it become between seven and 13 centimeters thicker every decade," says Johansson, from Lund University's department of physical geography and ecosystem science.

"This freezer that has kept plants frozen for thousands of years has stored the carbon that then can be released as the active layer gets thicker," she adds.

By 2100, the permafrost could have significantly thawed if CO2 emissions are not reduced, experts on oceans and the cryosphere from the UN's Intergovernmental Panel on Climate Change (IPCC) have warned.

The Arctic's average annual temperature rose by 3.1 degrees Celsius from 1971 to 2019, compared to 1C for the planet as a whole, the Arctic Monitoring and Assessment Programme said in May.

So could the permafrost reach a tipping point? That is, a temperature threshold beyond which an ecosystem can tip into a new state and risk disturbing the global system.

It's feared, for example, that the Amazon tropical forest could turn into a savannah or that the ice sheets atop Greenland and West Antarctica could melt entirely.

"If all the frozen carbon would be released, it would almost triple the concentration of carbon in the atmosphere," Gustaf Hugelius, from Stockholm University who specializes in the carbon cycles of permafrost, tells AFP.

"But that will never happen," he quickly adds. The thawing of the permafrost, he says, will not take place all at once, nor will all the carbon be released in a giant puff.

Rather, it will seep out over decades, even hundreds of years.

The big issue with permafrost is that the thawing and accompanying carbon release will continue even if human emissions are cut.

"We have just begun activating a system that will react for a very long time," Hugelius says.