This weekly newsletter delves into a different captivating topic from the world of geoscience, exploring Earth’s fascinating phenomena in bite-sized, easy-to-understand segments each week.
Want this newsletter delivered to your email, then click to get started.
Monday 26th of February, permafrost is a permanently frozen layer of soil, gravel and sand consolidated by ice and lies below the Earth’s surface. Permafrost remains at or below 0°C and it covers approximately 22.8 million square kilometres in Earth’s Northern Hemisphere. When does soil become permafrost? If a layer of soil freezes for more than 15 days per year it is called seasonally frozen ground and a layer of soil which freezes for less time than this is called intermittently frozen ground. Permafrost is the layer of soil which is frozen for a minimum of 2 years but it can be millions of years old. When temperatures get warmer permafrost is said to thaw, we will get onto why this is so significant on global climate later.
Distribution of Permafrost
Permafrost is found in Arctic regions such as Greenland, Russia and Alaska and doesn’t always form in a single sheet of ice. There are four ways in which the distribution of permafrost can be characterised: Isolated, Sporadic, Discontinuous and Continuous. Continuous permafrost is a single continuous sheet of permafrost which underlies larger than 90% of the surface. Discontinuous permafrost can also be split up into sporadic and isolated permafrost. Sporadic permafrost means 10%–50% of the surface has permafrost underneath it and isolated permafrost means less than 10% of the surface has permafrost underneath it. So far we have only talked about permafrost on the land whereas sometimes permafrost can cover the sea floor. This permanently frozen sea floor is called subsea permafrost and was formed over 11,000 years ago during the start of Holocene at the end of the last major glacial epoch. Subsea permafrost only exists under the Arctic Ocean does not exist in the Southern Hemisphere.
The Structure of Permafrost
As seen in the diagram below, there are four layers of permafrost. These are: ground surface, active layer, permafrost and unfrozen ground. The ground surface is obvious — it is the uppermost layer of permafrost and it is exposed to temperature variations the most. The active layer is the top layer of permafrost that thaws during summer and freezes again during autumn and many plants that exist in permafrost regions grow in the active layers as the roots can’t penetrate the permafrost below. The active layer is very thin around 15 cm deep. Below the active layer, it is the permafrost layer which consists of organic matter like frozen plants and animals storing huge amounts of carbon. This is very important because when this layer of permafrost thaws, large amounts of methane are produced through the process of methanogenesis which occurs in rotting biomass stored in the permafrost. Finally, below the permafrost is the unfrozen ground which remains above freezing usually due to geothermal heat.
Climate Change and Permafrost
So far we have looked at the distribution and structure of permafrost but let’s get onto why the thawing of permafrost enhances climate change. But first let me tell you about a study from Dartmouth College. The new study provides evidence that the Arctic's frozen soil is the dominant force shaping Earth’s northernmost rivers but as climate changes weakens this permafrost, the polar waterways expand which churn up the thawing soil of the previous permafrost releasing carbon dioxide stored in the permafrost. The researchers calculated that for every 1°C increase, this process could release as much carbon as 35 million cars emit in a year. However, this is nothing compared to what I am about to tell you. As I mentioned earlier, the permafrost layer consists of large amount of organic matter made up from dead plants and animals. When the permafrost thaws the process of methanogenesis occurs. Methanogenesis also known as biomethanation is the formation of methane by microbes known as methanogens. These organisms are capable of producing methane in anaerobic environments. When these organisms break down dead plant and animal matter methane is produced in large quantities as a bioproduct of the digestion. This methane can be observed in permafrost and in terrestrial ice on the surface as small bubbles of gas.
Why is methane a problem? Methane is responsible for more than 25% of global warming and it is released in the energy sector and the agricultural sector particularly from livestock and rice farming. Methane is 28 times more potent as carbon dioxide is at trapping heat in the atmosphere. Because of its structure, methane absorbs frequencies of infrared radiation emitted from the Earth’s surface in a process similar to carbon dioxide however at a stronger level. This creates a positive feedback loop as when organic carbon is frozen it cannot decompose but as it thaws the release of methane increases global temperatures which leads to more thawing of permafrost hence enhancing the greenhouse effect. This is a dangerous cycle and could potentially be a climate disaster. Scientists believe there will be widespread thawing by as soon as 2100. As permafrost thaws, ancient bacteria and viruses preserved in the ice and soil also thaw. These reanimated microbes have the potential to cause illness in humans and animals. Scientists have identified microbes that are over 400,000 years old in thawed permafrost. Arctic permafrost alone holds an estimated 1,700 billion metric tons of carbon which is roughly 51 times the amount of carbon the world released as fossil fuel emissions in 2019. The scary part is we don’t know the effects of this. For example, the Earth’s polar regions help stabilise the planet’s climate by driving heat away from the equator but a permafrost-free Arctic could be catastrophic.
Furthermore, another factor to consider is the effect of eustatic sea level rise from the melting of cryospheric stores of ice such as permafrost which will increase sea levels leading to higher levels of erosion which will negatively affect coastal towns and cities. An increase in sea levels is also accompanied by thermal expansion of the oceans where the oceans expand in volume due to an increase in temperature which further raises sea levels. Thermal expansion is then therefore influenced by permafrost thawing.
Conclusion
When I say permafrost might kill us I do exaggerate however the threat of permafrost thawing is very real. Collectively, we need to investigate more into permafrost and try to reduce the un-natural thawing of this phenomena. This will allow us to prepare for the future and try to mitigate the negative effects of global warming enhanced by greenhouse gases such as methane. In conclusion, the release of methane is caused by the digestion of organic carbon stored within the permafrost by methanogens in which methane is a biproduct of the digestion.
Thank You
Thank you for reading, all information is taken from reputable sources and are linked below. All images are free to use under copyright laws.
National Geographic ‘ Energy Education ‘ EarthHow ‘ Polar Pedia ‘ Jet Propulsion Laboratory