Permafrost refers to soil that has a temperature below 0° C for more than two consecutive years. With global warming, the thaw of permafrost is waking up its constituents. Their contact with water could have consequences for our climate. Sophie Opfergelt, an FNRS research associate and a professor at the Earth and Life Institute (UCL), is trying to determine the consequences.
We hear about the effect of climate change on sea ice or ice caps. Its effect on terrestrial ecosystems is less well known. In the Arctic, most soils are permafrost. Their warming has a significant impact on their stock of organic carbon, which is a source of greenhouse gases. Prof. Opfergelt decided to study permafrost. From 29 April to 16 May, she and her team (Catherine Hirst and Elisabeth Mauclet) travelled to Alaska’s Eightmile Lake to study its extreme environment. Returning a few weeks later, she explains the reasons for the study as well as the first observations.
Focus on permafrost
Prof. Opfergelt’s research is part of the five-year (2017-22) WeThaw project funded by the European Research Council (ERC), which studies the Arctic permafrost. This frozen ground represents a quarter of the earth’s surface. Its top layer, which thaws every year, is called the ‘active layer’. With global warming, permafrost has been deteriorating for some 30 years. As a result, the active layer’s depth increases every year and its preserved constituents become exposed to water. These are the constituents Prof. Opfergelt went to sample.
A region highly sensitive to global warming
The reason she and her team travelled specifically to Alaska, south of Fairbanks, is that the region is particularly sensitive to global warming. The temperature of the permafrost under the active layer oscillates between -0.5 and -2° C. An increase in soil temperature of 1 to 2° C would be enough to thaw it. Recent studies have shown that the rise in Arctic air temperatures is twice that of temperatures on a global scale. Why? Because of a magnifying effect at the North Pole. Eightmile Lake is also a great place to work because its permafrost temperatures have been monitored for 30 years. In reaction to their increase, vegetation changes were observed on this tundra, a kind of discontinuous carpet of grasses, moss, lichens and small trees. In addition, permafrost cores were drilled at this site a year ago and will be used in collaboration with Northern Arizona University, in the US.
Sampling water, rock, soil and vegetation
On 29 April – the end of winter – Prof. Opfergelt and her team arrived at the site. The spring thaw started abruptly a week after their arrival. The mission’s goal was to take samples from the ecosystem (rock, soil, ground water, snow, vegetation, river) before, during and after the thaw. Before the thaw, the researchers drilled a meter of auger ice to collect water from the river and obtain a pre-thaw sample. The thaw then broke up the ice and the river began to flow freely and they sampled it. Every evening, the team carried out water filtration work to ensure that no solid particles persisted in the samples. To sample the soil, or more precisely the active layer still frozen at the beginning of the season, a hammer and chisel were needed. These samples provide a valuable indicator of the frozen soil mineral reserve since October 2017, at the beginning of the previous winter. The objective is to characterise this reserve just before it comes into contact with the water released by melting snow in the spring.
A large reserve of organic carbon
Permafrost contains minerals and organic materials, including organic carbon. The amount is huge: three times the organic carbon in the world’s forests. As long as the permafrost remains frozen, this organic carbon can’t decompose. But by exposing these soils to thawing, their decomposition increases greenhouse gases (CO2 and methane) in the atmosphere. As a result, today the amount of greenhouse gases emitted by permafrost thaw is equivalent to the greenhouse gases emitted by the United States. This explains the scientific community’s efforts to monitor the permafrost’s organic carbon reservoir. However, an aspect not yet considered is the influence of permafrost mineral constituents. Exposed to water, minerals can dissolve and trigger abundant reactions in the soil. Mineral elements such as calcium, magnesium, potassium and phosphorus could be released and serve as nutrients for soil plants or microorganisms.
Possible impacts on the environment
This is the question for Prof. Opfergelt and her team. Two outcomes are possible:
- The release of mineral elements could trigger the decomposition of organic carbon by providing nutrients to soil microorganisms. This would increase the amount of greenhouse gases released into the atmosphere, and atmospheric temperatures would increase further.
- Minerals exposed by the thaw could provide surfaces onto which organic carbon could attach, which would limit decomposition and minimise the impact of permafrost thaw on rising global air temperatures.
It’s only been weeks since the team returned to Belgium, so it can’t yet draw conclusions. However, Prof. Opfergelt stresses the urgent nature of understanding this phenomenon given that 30% of the total permafrost surface is doomed to disappear by the end of this century. This field mission allowed the team to see the thaw’s rapidity and abruptness. In less than a week, vegetation hidden under a blanket of snow emerged, rivers lost their ice, and insects, birds, elks and caribou appeared in a previously silent landscape. The region’s nature seems to respond very quickly to slight changes in temperature.
For the rest, all samples taken from the field arrived at the UCL laboratory. The team continues the work by quantifying chemical elements in samples in order to trace their origin: Do they come from minerals? Vegetation? Other questions are also posed, and Prof. Opfergelt will try to answer them in the months and years to come. The main one: What are the processes triggered by the thaw that control the release of mineral elements? Water analysis conclusions will come soon, while soil characterisation will take longer. The next step will be to conduct soil thaw experiments in the laboratory. By using blocks of frozen soil brought back from Alaska, the objective is to control more thoroughly what happens during thawing. Finally, the team is already planning to return to the field later in the season to monitor the impact of thawing on the ecosystem’s various reservoirs.
A glance at Sophie Opfergelt's bio
2002 : Bachelor of Geology, UCL
2008 : PhD in Agronomy and Bioengineering, UCL
2009-2010 : Postdoctorate, Earth Science Department, Oxford University
2011-2013 : FNRS Postdoctoral Researcher, ELIE/UCL
2014 : Ebelmen Award of the International Association of Geochemistry
2014 : FNRS Research Associate (ELIE/UCL)
2017 : ERC Starting Grant – WeThaw