Permafrost melt poses long-term threat, says study

Yahoo News 27 May 09;

PARIS (AFP) – Melting permafrost could eventually disgorge a billion tonnes a year of greenhouse gas into the atmosphere, accelerating the threat from climate change, scientists said Wednesday.

Their probe sought to shed light on a fiercely-debated but poorly-understood concern: the future of organic matter that today is locked up in the frozen soil of Alaska, Canada, northern Europe and Siberia.

The fear is that, as the land thaws, this material will be converted by microbes into carbon dioxide, which will seep into the atmosphere, adding to the greenhouse effect.

This in turn will stoke warming and cause more permafrost to thaw, which in turn pushes up temperatures, and so on.

But how and when this vicious cycle could be unleashed is unclear.

Indeed, some voices have argued that it will not present a significant threat, as plants will start to grow on the soggy, warmer earth and suck in carbon dioxide (CO2) from the atmosphere through photosynthesis, thus blunting the problem.

A team led by University of Florida ecology professor Ted Schuur investigated an area of tundra at Eight Mile Lake in central Alaska, where permafrost thaw has been monitored since 1990 but had begun to start many years before.

Schuur's team used hand-built, automated chambers, which they deployed at three sites that represented minimal, moderate and extensive amounts of thaw.

From 2004 to 2006, the chambers measured how much carbon was escaping from the soil and how much was being absorbed by any vegetation.

In areas that had thawed for the previous 15 years, there was a net uptake of carbon, meaning that the newly-established plants sucked up more CO2 than was lost from the soil.

But in areas that had begun to thaw decades before, the reverse was true.

There was a net loss of CO2, the principal greenhouse gas blamed for global warming, as older stocks of carbon were gradually released to the atmosphere.

"At first, with the plants offsetting the carbon dioxide, it will appear that everything is fine, but this actually conceals the initial destabilisation of permafrost carbon," Schuur said in a press release.

"But it doesn't last, because there is so much carbon in the permafrost that eventually the plants can't keep up."

Most of the 13 million square kilometres (five million square miles) of permafrost remain frozen, but thawing is already under way around the region's southern fringes and is thought likely to expand this century.

In that scenario, the permafrost could release around a billion tonnes a year of carbon, roughly equivalent to the contribution to greenhouse emissions each year by deforestation in the tropics, the paper said.

Even as the Arctic greens, the rising loss of older carbon "could make permafrost a large biospheric carbon source in a warmer world," it said.

Burning fossil fuels adds about 8.5 gigatonnes of emissions each year, but it is a process that can theoretically be controlled.

Permafrost thaw, though, would be self-reinforcing and could be almost impossible to brake.

"It's not an option to be putting insulation on top of the tundra," Schuur said.

"If we address our own emissions either by reducing deforestation or controlling emissions from fossil fuels, that's the key to minimising the changes in the permafrost carbon pool."

Study cites 'slow-motion' threat from permafrost
Seth Borenstein, Associated Press Yahoo News 27 May 09;

WASHINGTON – Global warming's "slow-motion time bomb" of trapped greenhouse gases in the Arctic's thawing tundra may not go off quite as fast as once feared, a new study found.

Even so, it remains a problem that in the long run is still likely to worsen global warming in an uncontrollable way, researchers reported.

The study, published in Thursday's journal Nature, looked at thawing parts of Alaska and found that greenhouse gas releases initially are sucked up by new plants as the Arctic gets warmer and greener. But that helpful effect doesn't last.

Eventually, between 15 and 50 years, those plants "can't keep up" and get overwhelmed, said study lead author Ted Schuur, a University of Florida ecologist.

At that point, a billion tons of carbon a year can be released into an atmosphere that is already warming because of carbon dioxide emissions from power plants, cars, and other industrial activities, Schuur said. That would contribute the same amount to global warming as the deforestation of the tropics, he said.

"I call it a slow-motion time bomb," Schuur said.

Making matters worse is that much of the gas trapped in permafrost is methane, which is more than 20 times more potent than carbon dioxide.

It's a vicious cycle. As the world warms, more permafrost thaws. As more permafrost thaws, more greenhouse gases are released and the world warms even more, scientists say.

For the long-term, the new paper "heightens rather than lessens the concern" about the effects of trapped greenhouse gases in permafrost, said Stanford University ecologist Chris Field, who wasn't part of the study.

Permafrost Meltdown May Bog Down Global Warming--For Awhile
Thawing permafrost may soak up greenhouse gas before it begins to spew it out
Scientific American 27 May 09;

When permafrost thaws, microbes convert ancient organic matter in the frozen soil into climate-warming gases, like carbon dioxide and methane, potentially triggering a positive feedback loop that further melts the Arctic.

But the once-barren soil also spouts new—and larger—shrubs that can act as a carbon sink, and scientists have wondered what the net effect of permafrost thawing would be on atmospheric carbon balance. A new study published this week in Nature suggests that changing landscape can counterbalance the release of permafrost carbon—but only for a little while.

"The greening of the Arctic will not compensate for the huge amount of permafrost carbon that will be released," says lead author Ted Schuur, an ecosystem ecologist at the University of Florida in Gainesville.

Permafrost contains approximately 98 petagrams (98 quadrillion grams, or 108 billion tons) of carbon—or one sixth the total amount currently in the atmosphere. In order to understand how rapidly this carbon would be released into the air, Schuur and his group have been measuring carbon dioxide absorption and emission by plants and soil at the Eight Mile Lake watershed in the northern foothills of the Alaska Range across what they call a "gradient of thaw." Then, they use radiocarbon dating to determine how long the greenhouse gas had been trapped as carbon in the permafrost.

Not surprisingly, the researchers found that more extensively thawed soils emit carbon at a higher rate than recently thawed soils. But when the researchers looked at sites that had been thawing for about 15 years, they found that the increased plant growth more than compensated for their old carbon emissions. After 15 years, however, these soils were losing much more old carbon than the new plants were able to soak up.

"This is the first real study to put numbers on these things," says David Lawrence, an Earth systems scientist at the National Center for Atmospheric Research who was not involved in the study. "Data like this are critical." He is eager to incorporate the new findings in the Community Land Model used for climate simulations.

Lawrence points out, however, that not all permafrost will thaw in the same way. The study was conducted on a type of ground called thermokarst, which allows water to drain away as it melts. In wetter areas, Lawrence explains, permafrost has the potential to turn into a peat land and remain a long-term carbon sink, but that could also lead to increased methane emissions. "From a global warming perspective," he says, "that could still turn around and be positive feedback."

Schuur agrees with this assessment. "We can't say its going to be 15 years in every place," he says, "but we can say [that] in the initial decade of thaw permafrost will be acting as a carbon sink, and by 30 years it will be a carbon source."