And the amounts of carbon involved are enormous — one common estimate is that there’s more than twice as much carbon stored in northern permafrost as there is currently wafting about the planet’s atmosphere.

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Now, though, a major and surprising new report from the U.S. Geological Survey would appear to undercut, significantly, this worry, at least for one key northern region: the U.S. state of Alaska. In the process, the document raises deep questions about what the true carbon consequences of Alaska’s ongoing warming will be — a mystery whose solution may also implicate still greater carbon stores across Arctic regions in Canada and Siberia.

Alaska alone, though, is massive enough. While it makes up 18 percent of the United States’ total area, the state stores 53 percent of all of the nation’s carbon, much of it in permafrost below the ground, the USGS study reports. Indeed, it adds that wildfires in Alaska give off more greenhouse gases to the atmosphere annually than all fires in the Lower 48 states.

Clearly, any change to this carbon in one direction — shifting it from the land to the atmosphere — would be disastrous. But is that happening?

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To better understand the issues at stake here, it may help to have a quick refresher on some concepts that climate scientists live and breathe but that the rest of us do not. Researchers say that a particular region — in this case, Alaska — is a carbon “sink” if its lands, plants, waters and so on are pulling more carbon dioxide and other greenhouse gases out of the atmosphere than they are putting into it. And conversely, researchers say that an area is a carbon “source” if the net result of everything happening there, across often very diverse types of landscapes and ecosystems, leads to more carbon ending up in the atmosphere.

Here’s where the USGS report comes in — and what’s new about it. Attempting to quantify the vast amounts of carbon stored in Alaska’s trees, waters and soils, the study finds that Alaska is a net carbon repository, or sink. The state is subtracting a relatively small amount of carbon from the atmosphere at the moment, about 3.7 million metric tons per year.

That’s good news for both the United States and the planet. But it’s just the beginning. The report also finds — perhaps more controversially — that heading out to the year 2100, the state of Alaska as a whole should wind up pulling even more carbon back out of the atmosphere than it is now, rather than expelling more of it there. In other words, the sink should strengthen.

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The reason is that while Alaska’s boreal forest region is expected to see bigger and more intense wildfires that send up large amounts of carbon stored in trees and soils — and while permafrost will certainly degrade to some extent — other parts of Alaska are simultaneously expected to green up. There will be more carbon dioxide in the air (plants dig it), and less really cold weather, among other factors, leading to a phenomenon that has often been dubbed “Arctic greening.”

“The simulations show that the tundra increases in biomass, and carbon storage. And that’s both in northern and western Alaska,” said David McGuire, a U.S. Geological Survey researcher who edited the extensive report, which contains contributions from multiple scientists, along with the USGS’s Zhiliang Zhu.

For those who have been following the climate debate closely, it’s an unexpected conclusion — and one that climate change skeptics and doubters might trumpet as a classic case of an alarmist climate scenario not coming to pass after all. If you dig down into the fine print, though, there remain many uncertainties — and many continuing reasons for concern about what will happen to stored carbon in Alaska and across the frozen north.

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“What I don’t want people to think is, everything’s all rosy,” McGuire said. “We’re not saying that.”

First of all, McGuire and Zhu noted in an interview, the study necessarily omitted some carbon sources that are not well understood, such as methane emissions from lakes. Their inclusion could, conceivably, tip the balance back into one where Alaska is adding carbon to the atmosphere, at least when it comes to assessing the state’s carbon balance in the present.

Or as the study puts it: “It is important to recognize that [methane] emissions from lakes have not been considered in this assessment, and it is likely that Alaska would be a source of greenhouse gases under all climate simulations if these emissions were considered in the assessment.”

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Similarly, even though Alaska is storing carbon as a whole, some of the carbon that goes up into the atmosphere does so in the form of methane, a particularly powerful greenhouse gas. And there’s enough methane coming from wetlands in Alaska that it causes a modest net warming effect on the atmosphere anyway, McGuire and Zhu said. Thus, paradoxically, in this case a carbon sink can cause the planet to warm up slightly.

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Some other Alaska and permafrost researchers, not involved in the current study, also said that there remain reasons for concern here.

“It’s important to remember that these models are predicting both losses of soil carbon as well as new plant uptake and so it’s going to be critical to assess whether stimulated plant uptake by rising CO2 and the other factors really will compensate for soil carbon losses, because that’s the process that offsets emissions to the atmosphere,” said Ted Schuur, a permafrost researcher at Northern Arizona University in Flagstaff. “This is the same question in global scale models too — and the jury is still out whether plant stimulation by rising CO2 will maintain over this century.”

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“In my view, nobody really has a great understanding of what will happen to permafrost carbon as permafrost thaws over the coming century,” added Max Holmes, a permafrost researcher at the Woods Hole Research Center. “What is clear, however, is that the amount of carbon in permafrost in the Arctic and boreal region dwarfs the amount of carbon in vegetation.”

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Thus, Holmes said that he remains concerned that permafrost thaw could outpunch plant growth from a carbon perspective. “I think it would be a huge mistake for policymakers to bank on the Arctic mitigating global warming over the coming century — the opposite still seems more likely to me,” he added.

Indeed, an expert assessment of the views of a large number of Arctic researchers recently found they tend toward the belief that “Arctic and boreal biomass should not be counted on to offset permafrost carbon release and… that the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario.” Clearly, this tug of war — between permafrost pouring carbon into the atmosphere and new plant growth pulling it back again — will be a crucial determinant of how the Arctic shapes our planetary future.

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Thus, the new U.S. Geological Survey study certainly does present surprisingly good news for Alaska’s role in climate change — but it also raises a lot of questions, as its authors fully admit.

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“This is big progress that we’ve made in terms of getting this assessment out, but it shouldn’t be the last word on the Alaska problem,” McGuire said.

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