While politicians around the world debate how to reduce human-caused greenhouse gas emissions, scientists are making some unsettling discoveries about another developing greenhouse gas problem: nature's own emissions.
A study published this week shows that the amount of carbon locked in the Arctic permafrost is more than double previous estimates. Additionally, other research shows that the permafrost is thawing, meaning this enormous amount of carbon could be released into the atmosphere as the greenhouse gases carbon dioxide and methane.
The thawing of the permafrost is especially dangerous because it could cause a domino effect of more warming that, for now, cannot be checked by human engineering or policy.
"We now estimate the deposits contain over 1.5 trillion tons of frozen carbon, about twice as much carbon as contained in the atmosphere", said Dr. Charles Tarnocai, Agriculture and Agri-Food Canada, Ottawa, and lead author of the study, published in Global Biogeochemical Cycles.
As long as permafrost is frozen, the carbon in the soil is locked up.
But when it thaws, the carbon becomes exposed, and microbes called methanogens break down the carbon and release methane, a greenhouse gas that is 20 times more effective at trapping heat than carbon dioxide.
"A little bit of methane goes a long way with respect to warming," says Breck Bowden, a University of Vermont professor conducting research in Alaska on the effects of thawing permafrost on ecosystems.
The release of methane could create a domino effect, explains Vladimir Romanovksy, professor in geophysics at the University of Alaska Fairbanks.
The thawing of the permafrost produces methane, which causes further warming, which in turn thaws more permafrost, producing more methane. This cycle, known as a positive feedback system, mirrors a similar feedback loop in the Arctic involving ice. Ice normally reflects the sun's light and heat, but global warming has caused ice to melt, leaving more of the darker Arctic land and water open to absorb the sun's heat, leading to more polar ice loss.
It's possible that, once started, one or both of these feedback systems could continue by themselves – even if human greenhouse gas emissions were to be significantly reduced or eliminated, Romanovsky says. Alone, the permafrost could contribute to the atmosphere an amount of greenhouse gases comparable to the amount humans have emitted, and possibly more.
Bowden emphasizes that both the Arctic and global climate are complex systems whose future behavior is difficult to predict, and he cautions that scientists do not know how much carbon dioxide and methane might be produced and precisely what feedbacks will result. However, he also warns,
"It's hard to envision how the [positive feedback] scenario could be stopped, though in time it might be slowed or muted by reducing [human] greenhouse gas emissions."
At the moment, the only way to prevent this cycle is to prevent global warming and stop greenhouse gas emissions, Bowden says:
"For us to consider keeping permafrost throughout the Arctic from thawing as a consequence of global warming is beyond possible. The solutions and the technologies we have to engineer permafrost to keep it from thawing simply are way too expensive to consider employing across the Arctic. The only solution is for the Arctic to stay cold – and that won't occur unless we stop greenhouse gas emissions."
"The only way to prevent thawing of the permafrost is to prevent warming of the climate."
How the thawing of the permafrost will influence climate change remains to be seen, but evidence shows that the permafrost has been getting warmer over the past several decades.
Romanovsky and other scientists have recorded the temperature of the permafrost in 300 Arctic locations. In a dozen of these spots, the data spans three decades, such as in Fairbanks, where the permafrost temperature has increased. In many places where Romanovsky is recording temperatures, the shallow permafrost is near 0 degrees Celsius, at which point it thaws.
Romanovsky says there is a consistent, if not constant, warming trend:
"We can definitely say that there was significant warming from the '70s and the early '80s towards the '90s and maybe in some ways in the 2000s. Of course, it's not a linear process. At some times it's faster or slower or it pauses. Permafrost can be very inertial and it takes time for it to thaw."
There is not yet enough data to know when a massive thaw of the permafrost and the release of this sequestered carbon will occur, but current estimates say as early as mid-century or at the latest, the end of the century, he says.
"The timeline is very important," Romanovsky says. "I said mid- or end of the century, which is a 50-year difference, and that could be a very big difference."
The thawing of the permafrost could have other environmental implications beyond climate change. Bowden is studying thermokarsts, the pits left in the ground after ice from once-frozen ground melts away and the soil left behind collapses.
Bowden, who has been working in Alaska for 25 years, first turned his attention to thermokarsts in 2003, when he was flying in Alaska with a colleague.
"We happened to notice that this one river had been turned chocolate brown by sediment being introduced into the [Toolik] river by one very small thermokarst feature.
"A light went off in our heads that there wouldn't need to be too many of these small themorkarst features like this to have a big impact on the landscape, because this one tiny little feature was affecting a 40 km stretch downstream with very heavy loads of sediment."
In comparing aerial photographs of northern Alaska from the 1980s with aerial photographs from 2006, they found that the number of thermokarsts had doubled in two decades.
When a thermokarst occurs, nutrients necessary for life, such as carbon, nitrogen and phosphorus, are released from the formerly frozen soil, allowing microbes to interact with these nutrients, and possibly stimulating life in the land or waters where it is released.
Bowden is investigating how thermokarsts affect the community structures of ecosystems, noting that in one experiment at the Toolik Field Station in Alaska, adding a tiny amount of phosphorus to a river converted what was a community based on diatoms – microscopic algae – to a community dominated by aquatic mosses.
"It raises the question of what are we doing to our aquatic systems? Are these changes we want to see happen in those aquatic systems?"
(Photos: Permafrost (top) by Lakerae/Flickr Creative Commons; Themokarst by Andrew Balser/Institute of Arctic Biology)