Siberian Permafrost Decomposition and Climate Change

Permafrost is soil that remains below 0 degrees Centigrade for at least two consecutive years. In Siberia, over 80 percent of the soils are permafrost to depths of as much as 1.5 km. These deeply frozen soils sequester a reservoir of organic carbon rivaling all of the carbon in the atmosphere. This permafrost comes in two main types. One consists of peat soils that formed in bogs during the last 10,000 years. These moss-dominated wetlands consist of up to 50 percent organic carbon. The other kind of permafrost soil, called ‘yedoma’, is an organic-rich Pleistocene-age loess deposit with ice content 50-90 percent by volume. Siberian yedoma occupies over 1 million square kilometers, and bear carbon contents of 2-5 percent–about 10 times greater than the carbon content of non-permafrost mineral soils. Yedoma permafrost averages 25 metres in depth.

This Siberian deep freeze sequesters enormous stores of buried soil carbon. The yedoma reservoir has been estimated to contain 500 gigatons of carbon; another 400 gigatons of carbon are buried in non-yedoma Siberian permafrost; and some 60 gigatons reside in unfrozen peat bogs. By comparison, the amount of carbon in the atmosphere as carbon dioxide is about 730 gigatons, having increased from 560 gigatons in pre-industrial times (Zimov et al. 2006).
The trouble is, the deep freezer has come unplugged. The arctic is warming. Summertime arctic sea ice cover is down some 20 percent from two decades ago, and the remaining ice is thinner. Industrialists are contemplating the opening of sea lanes above the Canadian Arctic territories. Climate change is greatest at the planet’s high latitudes, and Siberia is no exception, warming at an annual rate of 0.02 to 0.05°C over the last 40 years. At this rate, the overall temperature increase in Siberia will be 1-2°C by 2050, relocating the southern border of the permafrost area northward by 300 to 400 km. There is considerable evidence that the permafrost has already begun to thaw.

Methane bubbles along the shores of Siberian thermokarst lakes with sufficient vigour to maintain open holes in the ice, even in winter.     © Chanton

Yedoma permafrost typically forms lakes when it thaws, as it consists in large part of massive ice wedges and blocks with a smattering of soil mixed in (Figure 1, Walter et al. 2006). As this permafrost thaws it collapses in size, since ice consumes considerably more volume than water. This process of thaw and collapse results in a pock-marked lake-ridden landscape. Thermokarst lakes are a dominant lake type in Siberia’s permafrost zone. Both their number and their aerial extent have grown significantly in recent decades. Using satellite imagery, Walter et al. (2006) reported a 14 percent increase in lake area for a 12,000 km2 study area near Cherskii, Russia. Smith et al. (2005) reported a 12 percent increase in lake area in continuous permafrost zones in Western Siberia over the same time period. The growth of thermokarst lakes in Siberia has produced an estimated 58 percent increase in lake methane emissions over the last 30 years (Walter et al. 2006). 

Upon lake formation and expansion, the thawed organic-rich soils sink to the lake bottoms and accumulate as sediments. Now, like chicken left out of the freezer on the kitchen counter, these soils start to rot, releasing copious quantities of methane gas. Unlike carbon dioxide (a second major product of this decomposition), methane is only slightly soluble in water. When produced at a rapid rate, methane comes to the surface and forms bubbles which can be observed when canoeing in the still waters of lakes or rivers, especially when the sediments are stirred up by a paddle. Gas bubbles can consist of up to 90 percent methane. 

Along the margins of thermokarst lakes, methane bubbles up freely. In some locations (‘hotspots’), the bubbling from the decomposing permafrost is sufficient to prevent the lake surface from freezing, even in winter (Figure 2). Hotspots are visible as ‘black holes’ in the frozen lake ice and can be seen in aerial surveys. Half of the 60 thermokarst lakes surveyed by Walter et al. (2006) exhibited modest thermokarst erosion with gently sloping banks, stable vegetation along the edges, and hotspots distributed over a 15-metre wide edge along the thaw margin. The other half of the lakes exhibited more active thermokarst erosion with steep banks, exposed ice, and more than 30 metre wide belts of hotspot black holes.

This decomposing permafrost releases some 4 teragrams of methane into the atmosphere annually (Walter et al. 2006), increasing present estimates of methane emissions from northern wetlands by up to 63 percent. This has very serious consequences, as methane is a greenhouse gas 23 times more powerful than carbon dioxide on a molecular basis. It has been recently estimated that some 50 gigatons of methane–10 times the current amount of methane in the atmosphere–could be released by this lake-bubbling mechanism if the entire Siberian yedoma-ice complex were to melt (Walter et al. 2007b). The methane released by the thawing permafrost creates a positive feedback loop. The methane added to the atmosphere results in additional climate warming, which produces additional permafrost decomposition, more methane releases, and even more warming. Where will this cycle end? 

Permafrost and policy conundrums

One approach to stabilizing greenhouse gas emissions is through a cap-and-trade system whereby countries are credited for sequestering carbon and charged for emitting it. Russia has the world’s largest standing forests, some 22 percent of the world’s total (Filipovich and Sekhpossian, 2002). Russia also has the potential to store large quantities of carbon in newly planted forests. But if Russia is to be credited for this forest carbon sink, should it also be docked for the methane emitted through permafrost degradation? To what extent is carbon dioxide emitted in China or the United States responsible for the melting Russian permafrost?

In the Siberian tundra massive ice blocks mixed with soil form “yedoma,” here shown collapsing.     © Chanton

The problem is not limited to Russia: northern peatlands across Scandinavia, Canada and the United States also contain large reservoirs of soil carbon, both in permafrost and non-permafrost environments. Climatic warming threatens to liberate this sequestered peatland carbon as well. A similar mechanism of bubble transport has recently been shown to be important in releasing methane from the decomposition of Minnesota peatlands (Glaser et al. 2004). Worldwide, northern high latitude soil carbon stores represent an enormous supply of sequestered organic carbon. These stores are vulnerable to mobilization in the face of climate warming. Their fate will become an increasingly important issue both in climate change models and in cap- and-trade agreements for greenhouse gases. 

Jeffrey Chanton is the John Widmer Winchester Professor of Oceanography at Florida State University. Katey M. Walter is Assistant Professor of Northern Lakes at the Institute of Northern Engineering and the International Arctic Research Centre, University of Alaska Fairbanks.