Sustainability Issue #2 July 2009

Permafrost, ground that is permanently frozen for at least two years in succession, is a hot subject that has attracted a lot of attention in the media in recent years, and with full justification. Permafrost is present below 25 per cent of the land regions of the northern hemisphere. Global warming, which is most intensive in the Arctic where most of the permafrost is situated, leaves a lot of question marks as to what will happen.

When permafrost begins to thaw, ice lenses in the soil melt and give rise to settlements and thermokarst. In regions with infrastructure, such as cities in Siberia and regions with pipelines, thawing of permafrost is a serious and costly problem. Thawing permafrost is also a problem on a global level. When permafrost thaws out, carbon previously bound in the form of organic matter becomes available to microorganisms which degrade it. Depending on oxygen conditions during this degradation, the greenhouse gases carbon dioxide and methane are released and further aggravate the greenhouse effect.

Permafrost in Sweden

In contrast to Siberia which has large cities built in regions with permafrost, Sweden has very little infrastructure in regions covered by permafrost. Permafrost is found in the Swedish alpine region and, in the most northerly regions, also in mires in the low lying areas. In Sweden permafrost is at the edge of its geographical range, which makes it extra susceptible to the climate changes that are taking place.

In Abisko, permafrost has been studied for several decades. Work was initiated in 1978 by Associate Professor Jonas Åkerman, Lund University. He began to measure the active layer which is the topmost layer that thaws and freezes annually on top of the permafrost. Nine mires in the Torneträsk area from Katterjokk in the west to Bergfors in the east all show the same trend. The active layer has become much thicker, from 50 to 90 cm in the westernmost mires. This growth of the active layer is chiefly due to a higher air temperature and increased winter precipitation.

In the same mires where we have seen the greatest changes in the active layer, the permafrost has also completely disappeared from parts of the mires. In ten years, between 1996 and 2006, four fifths of the permafrost has completely disappeared from a mire in Katterjokk. The reason that the permafrost completely disappeared is that this mire had very thin permafrost from the beginning. It was only about 2 metres, which made it more sensitive to the measured rises in temperature and a thicker snow cover in the winter.

Thaws from both the top and bottom

Soil temperature is another parameter that can be investigated to see what conditions are like for the permafrost. It has been measured in four mires from 1980 to 2002. During this period temperatures increased in the upper part of the ground and also at the lower edge of the permafrost. The permafrost has thus become thinner because it is thawing from both directions.

From the top, it is mostly air temperature and thicker snow cover that are the causes, but we are not sure why it is thawing from below. Most probably it is a consequence of greater groundwater flow, and the increased temperature of this.

Greater emission of greenhouse gases

In areas with thawing permafrost, emission of greenhouse gases is on the increase in many places. Interdisciplinary studies are needed to acquire a greater understanding of the changes in carbon dioxide release and the higher methane emissions. Methane is a very active greenhouse gas which is the one of the reasons that it has attracted great attention. There is often talk of a latent methane bomb.

However, a lot of fundamental scientific questions as to what governs the properties and dynamics of methane in regions where permafrost is thawing have not yet been answered. In December 2008 we published in Nature fundamental new findings from north-east Greenland concerning methane emissions from the tundra. These findings concern previously unknown large bursts of methane which occur when the ground freezes in the autumn.

Discovered by chance

When we investigated natural emissions of methane gas from wetlands, we found that as much methane is emitted in a few weeks in late autumn as during all the summer months. This is presumably due to changes in pressure in the topmost soil layer when the soil begins to freeze.

We made this discovery by pure chance when the research station remained open two months longer than usual because of the International Polar Year. We could therefore measure methane emissions right into late autumn when the soil began to freeze. Overall, emissions of methane during autumn weeks imply an increase of approximately 3-4 per cent in the annual methane emissions from the wetlands in the world.

Now we are in full swing with studies that will help us find what governs these large freeze-in emissions, by using radar techniques to document gas dynamics in the soil and measure emissions to the atmosphere.

Rivers become channels

One interesting aspect of the freeze-in emissions we have documented in continuous permafrost regions in Greenland (where permafrost is present below 90-100 per cent of the ground) does not occur in northern Subarctic Sweden where we have discontinuous permafrost (where permafrost is present below 50-90 per cent of the ground). This is presumably due to a critical change in ecosystem function when the permafrost disappears. Important physical processes in the ground are radically altered.

It is possible that the extensivee freeze-in emissions are absent in northern Sweden because the large rivers are still not frozen when the ground freezes. The rivers can then serve as channels for the same emissions as those we observed in Greenland when the ground was freezing.

Thaws at a faster rate

Future scenarios for the Abisko region predict further rises in temperature and also further increases in precipitation. In the low lying areas that still have permafrost this will result in accelerated warming of the ground and increased thaw of the permafrost. This is shown by experiments that have gone on for almost five years in a mire at Mielljokk outside Abisko. Here, snow fences have been erected to simulate a future climate with a thicker snow cover.
What was surprising was that after only three years' treatment there was a great difference in soil temperature between the control areas and the experimental areas. Large differences were also measured in the thickness of the active layer. The experiment had only altered one of the two important climatic parameters that influence the future of permafrost in this area.

Bearing in mind that air temperature is also predicted to rise in future, it is not likely that any permafrost will be left in the low lying areas of northern Sweden by 2050.

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