Why you should care about thawing permafrost
24% of all exposed land in the Northern Hemisphere is permafrost. It contains an estimated 1400-1700 billion tonnes of carbon, twice the amount in the atmosphere. And it’s thawing at an unprecedented rate, releasing vast quantities of this carbon into the atmosphere, in one of the most important yet unknown positive feedback mechanisms in climate change.
Permafrost is defined as soil, rock, and ice which has been frozen for two or more years; in many areas it has been frozen for millennia. Unable to support trees, it results in the tundra ecosystem, which underlies the Russian Far East, northern Siberia, most of Alaska, northern Canada, and the fringes of Greenland, although as much as a quarter of this vast landscape could thaw out for a single degree rise in global temperatures.
left the muddy active layer is visible, whilst the ice wedges and ice pores in the soil are permanently frozen below it; right the distribution of permafrost across the Northern Hemisphere, with the darkest purple representing continuous permafrost
Permafrost comprises of three layers: the top layer – called the active layer, which is 0.3-4m thick – thaws and freezes with the seasons. Beneath the permafrost table, the soil is frozen, in some places to depths of 1400m. It acts as an impermeable cap, trapping the organic matter from peatbogs under ice and frozen soil (crysol), isolating it from decomposers such that dead plants and animals cannot decay, and natural gas deposits are locked away.
Now, though, due to climate change, the active layer is expanding in size as the Arctic warms at a rate of twice the global average. Thawing is bringing millennial-old organic matter into contact with decomposers, break it down to release ancient sequestered carbon into the atmosphere.
Permafrost is the wild card of climate change because it is far from certain how this organic matter will decompose. The ratio between carbon dioxide to methane produced is difficult to estimate, yet hugely significant: in the first two decades after its emission, methane is, by some estimates, 100 more potent at re-emitting infrared radiation back to Earth, although it decays much more quickly.
The decomposition of organic matter in permafrost
If the plant and animal matter decays in the presence of oxygen, in the exposed layers of the active zone, the decomposing microorganisms will produce carbon dioxide; whereas if it occurs in anoxic conditions, methane will form as the microorganisms use anaerobic respiration.
This process, known as methanogenesis, happens if the thawed permafrost is trapped in marshy conditions under lakes in the newly-thawed landscape, for example, or if the material is trapped underground. One study (published in 2006), which looked at such lakes in northern Siberia, found that rates of methane bubbling out of lakes, due to anaerobic decomposition in talik (the thawed permafrost under the lakes) estimated that this bubbling could be responsible for 3.8 x1012 g of carbon per year entering the atmosphere alone.
What is contentious, and almost impossible to estimate, is which pathway the frozen organic matter will take when it thaws. To a large extent this is determined by the hydrology of the region, which is constantly changing as the ice in the cryosol melts. When ice wedges in particular, buried close to the surface of the permafrost melt, it can cause mass movement – ground slumping – due to the fact that ice has a greater volume than water, leading to the development of a landscape known as thermokarst: marshy depressions in the ground filled with water, some forming huge lakes, interspersed with grassy hummocks.
The water courses in the landscape differ wildly from the flatness of the permafrost tundra, and hence it is difficult to predict the ratio of CO2:CH4 released. Secondly, carbon from the soil can be leached into the groundwater, and will decompose at an even faster rate in the warmer water of watercourses. And lastly, when permafrost is eroded at coastlines it decomposes in the water to produce methane, and this erosion is happening at accelerated rates due to climate change.
left thawing permafrost has led to mass movement in this area as ice has melted and turned to water; above the lakes of a thermokarst landscape
What could be more significant, though, is the fact that thawing permafrost provides channels for geological methane seeps to reach the surface: fossil natural gas which was previously capped by permafrost. A recent German study (2017) in the Canadian Mackenzie Delta showed that this was potentially the most important factor: the aerial study found that 17% of all methane emissions in the area were concentrated over just 1% of the surface area of the study, often over lakes where ebullition (bubbling) was happening.
In some areas of discontinuous permafrost, boreal fires are becoming more common, correlating with carbon change, and as well as releasing carbon dioxide into the air, and decreasing the albedo of snow, these fires remove the top part of the active layer. This exposes more deeply buried organic material to microbial decay, but also to increased solar radiation, leading to greater thawing. Plant growth in periglacial areas increases with warmer temperatures, and their roots also draw up deeper organic material, incorporating them into the plant tissues, which then decompose each autumn. In discontinuous permafrost areas, peat-rich soil called yedoma also release carbon with increased temperatures, one study suggesting yedoma releases 1012kg methane a year alone.
A tipping point…
The cycle of thawing permafrost and increased carbon emissions is likely to cause an uncontrollable positive feedback cycle, poised to be one of the most important tipping points for our climate, which we cannot climb back from. It’s uncertain where the tipping point lies, or exactly how much carbon thawing permafrost will emit to the atmosphere, but every year of increased warming traps us in its cycle, which will only amplify and accelerate climate change.
We are already seeing the effects of this vast ecological and periglacial change in the Arctic, as permafrost turns to thermokarst, and we are now standing at the precipice of its effects at a global level. It is essential that permafrost is not viewed as an out-of-the-way side-line issue in climate change, shelved because of its complexity and uncertainty. It is at the heart of the global climate debate, and it will only become more important as a positive feedback mechanism.