Sweden

Physical Geography

Climate warming is more pronounced in the Arctic than in other parts of the world. This warming affects the terrestrial cryosphere including permafrost with consequences ranging from societal impacts to changes in hydrology and feedbacks in the climate system such as those imposed by changing greenhouse gas exchanges. Permafrost dynamics in a marginal zone for its very existence is in this context of outmost importance as it represents a very sensitive environment where changes may appear first. The main objective of this project was to understand the current status of such sensitive lowland permafrost in the Torneträsk area, sub-arctic Sweden and to explore its development over the past 1000 yrs in relation to various environmental drivers of change.Monitoring of permafrost temperatures and active layer thickness showed that permafrost was during the last three decades degrading in the Torneträsk catchment. Increasing ground temperatures and active layer thickness were correlated with increases in air temperatures and in some cases with snow depth. A manipulation experiment that simulated future scenarios of increases in winter precipitation showed that permafrost and vegetation were sensitive to changes in snow depth after only three years of treatment.Modelled ground temperatures showed two periods of lowland permafrost degradation during the last Century. Over the last 1000 yrs, the modelled ground temperatures at one site currently with permafrost indicated that permafrost existed throughout this period. However, this contradicts proxy data from the area that suggests that permafrost formation occurred during the Little Ice Age (around AD1300).This study has improved our understanding of current and past dynamics of lowland sub-arctic permafrost in northernmost Sweden. The presence or absence of permafrost in the Torneträsk catchment is determined by many factors, but air temperatures, snow depth, vegetation and soil type are the most important. A major conclusion of the study is that the strength of the relationship between snow and permafrost dynamics varies considerably and is not only determined by the snow depth. The manipulation study indicated that the structure of the vertical snow profile, for example an occurrence of a bottom ice layer, could potentially affect the thermal regime of the soil via lateral runoff of melt water. Another important conclusion was that the lowland permafrost in the Torneträsk catchment is thawing from above but also from underneath, most likely caused by slightly warmer or more freely flowing ground water around and below the frozen body. This opens the possibility for permafrost degradation at the top and bottom surfaces, thereby making it very sensitive to the projected climate change during the 21st Century.