The Arctic Now Traps 25% of World's Carbon
The arctic could potentially alter the Earth’s climate by becoming a possible source of global atmospheric
carbon dioxide. The arctic now traps or absorbs up to 25 percent of this gas but climate change could
alter that amount, according to a study published in the November,2009 issue of Ecological Monographs.
In their review paper, David McGuire of the U.S. Geological Survey and the University of Alaska at Fairbanks
and his colleagues show that the Arctic has been a carbon sink since the end of the last Ice Age, which
has recently accounted for between zero and 25 percent, or up to about 800 million metric tons, of the
global carbon sink. On average, says McGuire, the Arctic accounts for 10-15 percent of the Earth’s carbon
sink. But the rapid rate of climate change in the Arctic – about twice that of lower latitudes – could
eliminate the sink and instead, possibly make the Arctic a source of carbon dioxide.
“This study is another example of the important role played by USGS and its partners in providing the scientific
research that must be the backbone of any actions related to climate change,” said Secretary of the Interior Ken Salazar..
Permafrost Currently Stores Carbon Dioxide
Carbon generally enters the oceans and land masses of the Arctic from the atmosphere and largely accumulates in
permafrost, the frozen layer of soil underneath the land’s surface. Unlike active soils, permafrost does not
decompose its carbon; thus, the carbon becomes trapped in the frozen soil. Cold conditions at the surface have
also slowed the rate of organic matter decomposition, McGuire says, allowing Arctic carbon accumulation to exceed its release.
Warmer Temperatures Count Liberate the Carbon Dioxide
But recent warming trends could change this balance. Warmer temperatures can accelerate the rate of surface organic
matter decomposition, releasing more carbon dioxide into the atmosphere. Of greater concern, says McGuire, is that
the permafrost has begun to thaw, exposing previously frozen soil to decomposition and erosion. These changes could
reverse the historical role of the Arctic as a sink for carbon dioxide.
“In the short term, warming temperatures could release more Arctic carbon to the atmosphere,” says McGuire. “And
with permafrost thawing, there will be more available carbon to release.”
More Methane from a Warmer Arctic
On the scale of a few decades, the thawing permafrost could also result in a more waterlogged Arctic, says McGuire,
a situation that could encourage the activity of methane-producing organisms. Currently, the Arctic is a substantial
source of methane to the atmosphere: as much as 50 million metric tons of methane are released per year, in
comparison to the 400 million metric tons of carbon dioxide the Arctic stores yearly. But methane is a very
potent greenhouse gas – about 23 times more effective at trapping heat than carbon dioxide on a 100-year time
scale. If the release of Arctic methane accelerates, global warming could increase at much faster rates.
“We don’t understand methane very well, and its releases to the atmosphere are more episodic than the exchanges of
carbon dioxide with the atmosphere,” says McGuire. “It’s important to pay attention to methane dynamics because of
methane’s substantial potential to accelerate global warming.”
How Will the Arctic Respond to Climate Change?
But uncertainties still abound about the response of the Arctic system to climate change. For example, the authors
write, global warming may produce longer growing seasons that promote plant photosynthesis, which removes carbon
dioxide from the atmosphere. Also, the expansion of shrubs in tundra and the movement of treeline northward could
sequester more carbon in vegetation. However, increasingly dry conditions may counteract and overcome these effects.
Similarly, dry conditions can lead to increased fire prevalence, releasing even more carbon.
McGuire contends that only specific regional studies can determine which areas are likely to experience changes in
response to climate change.
“If the response of the arctic carbon cycle to climate change results in substantial net releases of greenhouse
gases, this could compromise proposed mitigation efforts for controlling the carbon cycle,” he says.
The article, Sensitivity of the Carbon Cycle in the Arctic to Climate Change, was published online in Ecological
Monographs. The coordinating lead author is David McGuire, USGS, and the co-authors include internationally renowned
scientists from Canada, Germany, Sweden, and the United States. This study was sponsored by the Arctic Monitoring and
Assessment Program, the Climate in the Cryosphere Program, and the International Arctic Science Committee.
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| This figure shows the mean extent of permafrost in the Arctic, estimated for (a) the years 1990-2000 and (b) the years 2090-2100. In (c), the estimation of loss of permafrost by 2100 is overlaid on estimates for the year 2000. Credit: A. David McGuire, USGS. |
| A lot of old carbon is stored deep in the tundra where it is locked in permafrost. As these areas start to thaw over about 15 years, large ice wedges in the soil get smaller causing pot-holing and soil depression. The newly available water prompts faster plant growth, and the carbon taken out of the atmosphere by the plants photosynthesizing is greater than the carbon released back into the atmosphere by plants respiring and microbes decomposing carbon. However, after about 50 years, as thawing continues and the soil settles even more, plants are growing faster yet, however the rate of plant respiration and old carbon release through microbes grows even bigger netting more carbon out into the atmosphere than into the soil. Reported in "Arctic Tundra May Contribute to Warmer World". Image by Zina Deretsky, National Science Foundation |
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