Permafrost Thaw in a Subarctic Peatland – Which Factors are Most Important?

Abstract:

Permafrost peatlands cover extensive areas in the northern circumpolar permafrost region and are important soil organic carbon reservoirs. To better understand how permafrost peatlands will respond to future climate change and assess the carbon-climate feedback as a result of permafrost thaw, more knowledge about the impact and relative importance of different meteorological parameters on ground temperatures is needed. In Tavvavuoma located in the sporadic permafrost zone in northern Sweden, meteorological parameters and ground temperatures have been monitored in a peat plateau complex since 2005. Various landscape units within the peatland have different ground temperatures. In fen deposits of drained thermokarst lakes and in lake sediments permafrost is absent. In the central part of the peat plateau permafrost is present, but close to 0 °C. Alongside a thermokarst lake shoreline, a maximum thermo-erosion rate of ~0.2 m/yr has been registered and ground subsidence of 0.5 m has taken place from 2102 to 2013. Despite a slight cooling trend in the air temperature record 2006-2012, ground temperatures at 2 m depth have increased by 0.05 °C/yr and at 6 m depth the permafrost has started to thaw from below. According to simple linear regression analyses the mean summer temperature the present and preceding year and mean annual temperature the preceding year are variables affecting the thaw depth (p<0.1). For the ground temperature at 1 m depth the number of thawing degree-days in the summer, mean summer temperature the preceding year, mean winter temperature and snow depth are possible contributing factors (p<0.2). However, it is difficult to explain why the permafrost is getting warmer despite the overall cooling trend in air temperature (2006-2012). An explanation could be that the permafrost is relict and not in equilibrium with the current climate. A long-term increase in air temperature has been recorded at surrounding meteorological stations since the mid 20^th century. If the ongoing increase in ground temperature in Tavvavuoma is a result of this continuing warming trend, short-term variability in meteorological parameters can still have an impact on the rate of permafrost degradation, but unless pronounced climate cooling would take place the overall long-term thawing of the peat plateau is inevitable.