Permafrost’s ticking “carbon bomb” may never truly detonate

New research suggests that carbon stored in frozen soil will be released gradually.

 

Remember last year, when NASA scientists freaked out because the amount of carbon dioxide in the atmosphere reached the threshold of 400 parts per million for the first time in recorded history? It was a big deal, because carbon dioxide is a greenhouse gas and the more of it in the atmosphere, the hotter the earth gets. 

But what if I told you that the carbon stored in the frozen permafrost of Alaska, Canada, Russia and other northern latitudes is twice the amount that’s currently in the atmosphere? And that permafrost has warmed by an average of nearly 11 degrees in the past 30 years, from 18 degrees Fahrenheit to over 28 degrees Fahrenheit? 

You might be inclined to surmise — as scientists have for decades — that thawing permafrost is about to become a huge contributor to global warming. Scientists have long floated a “carbon bomb” hypothesis, postulating that a swift conversion of permafrost-stored carbon into atmospheric carbon dioxide will significantly accelerate climate change. I wrote about it myself last year, when boreal forests were burning with unprecedented intensity; and just last week, The Washington Post warned readers about thawing permafrost, calling it “the Arctic climate threat that nobody’s even talking about yet.” 

Yet a study released Wednesday by the Permafrost Carbon Network (led by Northern Arizona University, the U.S. Geological Survey, and the University of Alaska Fairbanks) suggests that the carbon bomb will never truly detonate. “What our syntheses do show is that permafrost carbon is likely to be released in a gradual and prolonged manner,” says A. David McGuire, a senior scientist with the U.S. Geological Survey’s Alaska Cooperative Fish and Wildlife Research Unit and a climate modeling expert with the Institute of Arctic Biology at the University of Alaska Fairbanks. “The rate of release through 2100 is likely to be of the same order as the current rate of tropical deforestation in terms of its effects on the carbon cycle.” 

Coastal erosion reveals the extent of ice-rich permafrost underlying active layer on the Arctic Coastal Plain in the Teshekpuk Lake Special Area of the National Petroleum Reserve - Alaska.
Brandt Meixell, USGS

That doesn’t mean that the carbon dioxide released from thawing permafrost is insignificant. There’s still a whole lot of carbon stored in frozen soils — up to 30 percent of all the land-based carbon on earth — and though we may not see the effects of its release in a single explosion over the next few decades, it’ll nonetheless impact climate change within the next century or so. In all, the amount of carbon predicted to be released within the next 85 years is the equivalent of 10 percent of the current emissions from fossil fuels.

But let’s back up for a second. Why were scientists so wrong about the rate at which permafrost will emit carbon dioxide? 

McGuire and his co-authors suggest the answer lies in the variety of soils that form a mosaic over permafrost-covered regions. The rate at which permafrost releases carbon dioxide depends on the rate at which thawed organic matter in the soil decomposes, which in turn depends on the soil’s composition. Some newly thawed soils may indeed release a lot of carbon over a short time period, but the rate at which carbon is released slows down with time. Plus, carbon in well-drained soils is primarily released as carbon dioxide, while carbon in poorly-drained soils is generally converted  to methane, an even more potent greenhouse gas.

In short, there’s more variation than previous researchers accounted for. “They were basically saying, ‘Well, we looked in this one small area and saw that you get a lot of permafrost thaw and carbon release in the first few years,’ and then extrapolating that this would occur everywhere,” McGuire says. “I don’t think people really understood a lot about the potential timescale they were making inferences about.”

McGuire and his co-authors emphasize that though it may be happening more gradually, climate modelers nonetheless need to start incorporating the effects of thawing permafrost into models predicting global climate change. He also suggests that future research could pair existing soil composition maps with laboratory tests to calculate rates of release for different soil types, allowing scientists to identify carbon-release “hot spots” in the vast permafrost landscape.  

So is this a tiny bright spot in an otherwise dismal climate change forecast? In terms of the immediacy of the problem, yes, McGuire says. But given that current climate models may not be accounting for a carbon influx equivalent to 10 percent of current fossil fuel emissions, the long-term answer is less rosy. 

Krista Langlois is a correspondent at High Country News. 

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