Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Palsas and peat plateaus are expected to disappear from many regions, including Finnish Lapland. However, detailed long-term monitoring data of the degradation process on palsas are scarce. Here, we present the results of the aerial photography time series analysis (1960–2021) and annual RTK-GNSS and active layer monitoring (2007–2021) at two palsa sites (Peera and Laassaniemi) located in north-west Finland. The emphasis is on detailed change detection for the period covered by Unmanned Aerial System surveys (2016–2021) and connections to climate . At both sites, the decrease in palsa area by -77 % to -90 % since 1960 and height by -16 % to -49 % since 2007 indicate substantial permafrost degradation throughout the study periods . The area loss rates are mainly connected to winter air temperature changes at Peera and winter precipitation changes at Laassaniemi. The active layer thickness (ALT) has varied each year with no significant trend and is related mainly to snow depths and summer air temperatures at Peera. At Laassaniemi, the ALT is weakly related to climate and has been decreasing in the middle part of the palsa during the past eight years despite the continuous decrease in palsa volume. Our findings imply that the ALT in the inner parts of palsas do not necessarily reflect the overall permafrost conditions and underline the importance of surface position monitoring alongside the active layer measurements. The results also showed a negative relationship between the ALT and snow cover onset, indicating the complexity of climate–permafrost feedbacks in palsa mires.
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