In addition to being a warming hot spot, the Arctic plays a pivotal role in the movement of carbon between atmosphere, land and sea.

But the degree to which Arctic regions are a carbon sink, versus a source of greenhouse gas, is still a matter of debate. Permafrost holds vast amounts of carbon long stored in cold conditions, for example, but scientists are trying to pinpoint the pace at which the carbon will be released into the atmosphere because of thawing of frozen soil.

Similarly, there are uncertainties about the degree to which reduced cover of Arctic sea ice will change the amount of carbon sequestered in the Arctic Ocean.

Now two studies are offering some preliminary answers, and their results raise new concerns about the factors playing into future warming. In the first, a chief conclusion is that there needs to be more of a focus on the Arctic winter when it comes to thawing permafrost.

"We have a potential [carbon] release that could be similar to deforestation, but far less energy is going into measuring or monitoring it," said Edward Schuur, a professor of ecosystem ecology at the University of Florida and co-author of the study, which appeared in Ecology.

Arctic permafrost holds a vast pool of carbon. With climate change, it is estimated that more of this vast pool of carbon will be released to the atmosphere, as long-term frozen soil thaws, explained Schuur. However, warming also is expected to increase plant growth in the tundra, a factor that sucks up carbon from the air and provides a counter to warming.

Via a first-of-its-type experiment over three years, the researchers compared how this increased plant growth in the summer offset the release of carbon from permafrost thawing.

They found that induced warming indeed spurred plant growth in the summer in a region of the Alaskan tundra near Denali National Park, increasing carbon uptake from the atmosphere. But this uptake from vegetation was completely offset by carbon release in the winter, an Arctic season that has been less of a focus for researchers in the past.

Although the researchers found that increased carbon release essentially countered all the carbon uptake from increased plant growth, the results likely represent a minimum effect, said Schuur. In real conditions, where warming would likely be much more uniform throughout the year, the net loss of carbon from tundra to the atmosphere could be expected to "more than double," according to the study.

Carbon release occurs because microbes in the permafrost feed on organic matter, releasing greenhouse gas along the way, explained Schuur.

Simulating a warming Arctic
The researchers simulated future warming at plots by using a combination of miniature greenhouses and man-made snowdrifts, which counterintuitively warm permafrost like a blanket. Schuur said the warming conditions -- which were simulated in both the air and the tundra -- were similar to what could occur in the next few decades in the region.

As the active layer of the permafrost thawed every summer over the study period, the induced warming spurred tundra plants to grow taller, and at a faster pace, Schuur said.

"From a climate change perspective, this is a good thing, because the vegetation is taking carbon out of the air," Schuur said.

However, the researchers went a step further than many prior studies and tried to simulate warming in winter conditions, via the snowdrift blanketing. "Once you put on that put blanket [of a snowdrift], the soils are warmer throughout the whole year," said Schuur.

In a warmer winter in the tundra, the "middle" layer of the permafrost, just underneath the frozen surface, takes a little longer to freeze after the summer season. This delayed freezing allows microbes in the permafrost to have a longer time to feed on organic matter than they had in the past. The microbes are active, even as vegetation activity shuts down in winter months.

Climate change is "increasing [the microbe] food source," said Schuur.

Schuur said he expected the next phase of research at the sites, over another three-year period, to show that the carbon uptake of plants eventually will lose pace to the carbon release from microbes in soil. Plants only have so much room to grow taller and faster, and at some point will not continue to keep sucking up carbon on a linear trend, he said.

"Soil doesn't have this kind of limit," said Schuur.

Dueling studies
The study fits into a growing body of work examining the degree to which the Arctic and regions in the far north will be a carbon source versus a carbon sink. Another study, also released this week, examined a different aspect of Arctic carbon flux, with different conclusions about the region's role in absorbing carbon.

In that study, which was supported by the National Science Foundation and the National Oceanic and Atmospheric Administration, scientists reported that the Arctic Ocean is becoming more of a carbon sink, because decreased sea ice cover is spurring phytoplankton blooms that absorb carbon.

"The reason the Arctic [Ocean] as a whole is becoming more of a sink is because the biological drawdown of carbon is getting larger," said Stephanie Dutkiewicz, a principal research scientist at the Massachusetts Institute of Technology and co-author of the study.

From 1996 to 2007, the modeling concluded that the amount of carbon taken up by the Arctic Ocean increased by 1 megaton annually.

This was so even though a few studied regions -- in the Greenland and Barents seas -- bucked the carbon sink trend during "severe" years of ice loss. They bucked the trend partially because they receive very warm water from the Atlantic, and carbon is less soluble in very warm water, explained Dutkiewicz.

Although more research is needed, it is likely that the Arctic Ocean as a whole will continue to be a carbon sink in the near future, she said. While that is good news in a sense, more regions, like the Barents Sea, are likely to shift from sink to source in the next 50 years. Additionally, it will not help organisms in the Arctic Ocean sensitive to acidification caused by more carbon uptake.

"So, though the oceans have mediated some of the climate issue, they have done so at their own peril," said Dutkiewicz.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500