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An Experimental Climate Change Study of the Effect of Increasing Snow Cover on Active Layer Formation of a Palsa, Finnish Lapland

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An Experimental Climate Change Study of the Effect of Increasing Snow Cover on Active Layer Formation of a Palsa, Finnish Lapland Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 An experimental climate change study of the effect of increasing snow cover on active layer formation of a palsa, Finnish Lapland M. Seppälä Physical Geography Laboratory, University of Helsinki, Finland ABSTRACT: In natural conditions palsas are mainly uncovered by snow. Wind drifts snow from the summits and deposits it on the edges of palsas. The question of this study is what happens if palsas are covered by thick snow for most of the cold season. This should be the case if the surface temperature of the North Atlantic Ocean increases, with increased evaporation and precipitation in the form of snow in Lapland. To study the effects of increasing snow cover a 1.5 m high snow fence was built in 1997 around an instrumented small palsa. The fence produced a snow cover of about 0.5 m on the palsa. Outside the fence another palsa of similar size was instru- mented. It did not have more than a few cm of snow on its summit in the middle of winter. Temperatures were recorded continuously for five years and they show that the snow-covered surface of the palsa could be 3–4°C higher than on the unmanipulated palsa. The surprising result was that the snow-covered palsa thawed slower and the active layer was thinner and often 10 cm less deep than in the palsa in the natural stage. 1 INTRODUCTION Palsas are peat hummocks with a permafrost core. They are characteristic features for the zone of discontinuous permafrost with mean annual air temperature some- what below 0°C. Palsas normally have a very thin snow cover (Seppälä 1988, 1990, 1994). The consequence of the thin snow cover for palsa formation has been exper- imentally demonstrated by Seppälä (1982, 1995). The question addressed in this study is what may happen if palsas are covered by thick snow for most of the cold season. This may happen if the surface tem- perature of the Northern Atlantic Ocean increases caus- ing higher evaporation and more snow in Lapland. The hypothesis is that snow cover insolates the frozen core and it will be warmer at the beginning of the thawing season, the thickness of active layer should increase and ultimately the palsa should totally thaw and disappear. To simulate this proposed change in snow cover a snow fence was installed around a small palsa in 1997 at Vaisjeäggi mire (69°49ЈN, 27°10ЈE, 290 m ASL) in Figure 1. Location of the study site in Finnish Lapland. northernmost Finland (Fig. 1). The presented results are preliminary and inconclu- sive. The data of thawing against the full record of mete- orological observations have not yet been analysed in a side. In early winter a strong storm knocked down this short report. parts of the fence before the wet mire was deeply frozen. The following summer the fence was repaired and the following winters the experiment was more successful. 2 FIELD EXPERIMENT The experimental site was instrumented with tem- perature loggers (Hobos and Hamsters) which recorded A palsa some 80 cm in height, 11 m long and 8 m in the temperatures at 1- and 1.5-hour intervals, respec- width was surrounded with a 1.5 m high snow fence in tively. Air temperature was recorded 1.5 m above the August 1997. The area formed a square about 30 m on mire surface and 30 cm above the palsa surface and at 1013 Figure 2. Snow depth inside the snow fence in April 2000 and 2001. Table 1. Example of the mean (x), maximum and minimum temperatures (C) and their standard deviations (s) from October 2000 to the end of May 2001 at the experimental site: Air ϩ150 cm above the mire surface, air temperature on the palsas (Exp-air, Contr-air), on the surfaces of palsas (Exp-surf, Contr-surf) and 20 cm below the surface (Exp-20, Contr-20). Air ϩ150 Exp-air Contr-air Exp-surf Contr-surf Exp-20 Contr-20 October 2000 x 2.8 2.4 2.6 2.9 2.5 1.9 2.1 max 11.3 11.1 11.8 9.6 9.3 4.5 4.5 min Ϫ15.5 Ϫ16.5 Ϫ16.3 Ϫ3.7 –4.0 Ϫ0.7 Ϫ0.7 s 5.9 6.0 5.9 3.7 3.7 1.8 1.7 November 2000 x Ϫ3.6 Ϫ4.1 Ϫ3.9 Ϫ1.8 Ϫ2.3 Ϫ1.0 Ϫ0.8 max 1.5 1.0 1.0 Ϫ0.2 Ϫ0.5 Ϫ0.7 Ϫ0.5 min Ϫ15.0 Ϫ15.3 Ϫ15.0 Ϫ10.8 Ϫ10.3 Ϫ3.2 Ϫ2.0 s 4.3 4.3 4.3 2.2 2.3 0.5 0.3 December 2000 x Ϫ11.3 Ϫ11.7 Ϫ11.5 Ϫ4.8 Ϫ5.5 Ϫ2.9 Ϫ2.7 max 1.0 0.3 0.5 Ϫ0.2 Ϫ0.7 Ϫ1.0 Ϫ0.7 min Ϫ29.1 Ϫ29.6 Ϫ29.1 Ϫ9.5 Ϫ10.3 Ϫ5.2 Ϫ5.5 s 7.7 7.6 7.6 2.3 2.6 1.3 1.4 January 2001 x Ϫ6.9 Ϫ7.4 Ϫ7.3 Ϫ3.6 Ϫ5.4 Ϫ2.8 Ϫ3.7 max 3.3 2.0 2.0 Ϫ0.2 Ϫ0.5 Ϫ0.7 Ϫ0.7 min Ϫ23.6 Ϫ25.6 Ϫ25.1 Ϫ9.0 Ϫ14.8 Ϫ5.5 Ϫ9.0 s 5.1 5.1 5.1 1.8 2.8 1.1 1.8 February 2001 x Ϫ13.5 Ϫ14.0 Ϫ14.0 Ϫ5.2 Ϫ8.8 Ϫ4.0 Ϫ6.5 max 2.0 1.0 1.0 Ϫ1.0 Ϫ1.7 Ϫ2.0 Ϫ2.2 min Ϫ34.1 Ϫ32.8 Ϫ36.1 Ϫ9.8 Ϫ14.3 Ϫ7.0 Ϫ10.5 s 8.1 7.8 8.1 1.9 3.3 1.2 2.1 March 2001 x Ϫ12.1 Ϫ12.7 Ϫ13.1 Ϫ6.4 Ϫ9.5 Ϫ5.2 Ϫ8.0 max Ϫ0.2 Ϫ2.0 Ϫ2.2 Ϫ3.0 Ϫ4.2 Ϫ3.0 Ϫ4.5 min Ϫ29.6 Ϫ28.1 Ϫ31.1 Ϫ9.3 Ϫ15.3 Ϫ6.7 Ϫ11.8 s 6.5 5.9 6.3 1.4 2.4 0.8 1.6 April 2001 x Ϫ3.2 Ϫ5.1 Ϫ5.0 Ϫ3.4 Ϫ4.7 Ϫ3.6 Ϫ4.8 max 20.3 7.8 10.6 10.8 10.8 Ϫ1.0 Ϫ1.0 min Ϫ25.6 Ϫ22.3 Ϫ26.1 Ϫ7.0 Ϫ12.8 Ϫ5.5 Ϫ8.8 s 7.1 5.5 6.2 2.3 3.7 1.3 2.0 May 2001 x 2.2 1.1 1.4 1.9 1.6 Ϫ0.8 Ϫ0.5 max 16.3 14.6 14.6 18.1 17.8 Ϫ0.7 Ϫ0.2 min Ϫ5.5 Ϫ5.2 Ϫ5.2 Ϫ2.2 Ϫ2.2 Ϫ1.0 Ϫ1.0 s 3.3 2.7 2.8 3.1 2.9 0.1 0.2 1014 the palsa surface, and soil temperature was recorded monthly temperatures increased from 0.2–0.4°C 20 cm below the surface. (October 2000) to 3.6°C (February 2001) and 1.3°C Similar instrumentation was installed on a similar (April). The experimental site was warmer than the size palsa outside the fenced area to record the unma- control palsa (Table 1; Fig. 5). This situation continued nipulated, natural conditions for comparison. in the summer but the difference was only about 1°C. Snow depths were measured in March or April. The Snow on palsas melts very fast at the end of winter. positions of the active layers on the palsas were The sun and rain starts the thawing of the surface peat observed during the summers and especially in August when the surface is free of snow. In April 2002 the and September when the thawing season was almost thawing started unusually early and both palsas were completed (cf. Seppälä 1976, 1983). partly free of snow already by the 25th of April (Fig. 6). At that time the control palsa had from 3 to 6 cm and the experimental palsa from 0 to 4 cm unfrozen 3 RESULTS surface peat. Normally thawing starts in May and progresses The fence collected up to 50 cm snow on the upper very quickly downwards. By the middle of June it has surface of the palsa (Fig. 2) and at the same time the reached the depth of about 20 cm (Table 2). At the end control palsa had only approximately 6 cm snow on its of August the normal depth of active layer on palsas in surface. Finnish Lapland is between 50 and 60 cm, and can still An example of temperature fluctuations for the win- deepen a few cm in September and October (Seppälä ter 2000–2001 is shown in Table 1. Air temperatures in 1976, 1983) but it seldom exceeds 70 cm. three different observation points were very similar (Table 1). The reason probably is that air temperatures were measured 150 cm above the surface (Fig. 3) in summer but in winter the probe was only 30–50 cm above the snow surface (Fig. 4). The differences in surface and soil temperatures are clearly observed (Fig. 5). The differences in the mean Figure 5. Temperatures recorded 20cm below the surface Figure 3. Snow fence surrounding the experimental palsa in the experimentally snow covered palsa and in the control site. 16 August, 1998. palsa from 1 October, 2000 to 30 April, 2001. Figure 4. Experimental site at Vaisjeäggi palsa mire.
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