Stability Conditions of Peat Plateaus and Palsas in Northern Norway

Abstract

Peat plateaus and palsas are characteristic morphologies of sporadic permafrost, and the transition from permafrost to permafrost‐free ground typically occurs on spatial scales of meters. They are particularly vulnerable to climate change and are currently degrading in Fennoscandia. Here we present a spatially distributed data set of ground surface temperatures for two peat plateau sites in northern Norway for the year 2015–2016. Based on these data and thermal modeling, we investigate how the snow depth and water balance modulate the climate signal in the ground. We find that mean annual ground surface temperatures are centered around 2 to 2.5 °C for stable permafrost locations and 3.5 to 4.5 °C for permafrost‐free locations. The surface freezing degree days are characterized by a noticeable threshold around 200 °C.day, with most permafrost‐free locations ranging below this value and most stable permafrost ones above it. Freezing degree day values are well correlated to the March snow cover, although some variability is observed and attributed to the ground moisture level. Indeed, a zero curtain effect is observed on temperature time series for saturated soils during winter, while drained peat plateaus show early freezing surface temperatures. Complementarily, modeling experiments allow identifying a drainage effect that can modify 1‐m ground temperatures by up to 2 °C between drained and water accumulating simulations for the same snow cover. This effect can set favorable or unfavorable conditions for permafrost stability under the same climate forcing.

Ground temperature and snow depth variability within a subarctic peat plateau landscape

“…On the elevated, central parts of the peat plateau (T2, T3) MAGT is ~−0.3°C at 2 m depth. Late‐season thaw depths of 52–56 cm imply that the permafrost is relatively stable . In both the central elevated parts and along the peat plateau edge, mean December–April snow depth is shallow (<20 cm).…”

“…In both the central elevated parts and along the peat plateau edge, mean December–April snow depth is shallow (<20 cm). A thin snow cover, <30 cm, generally preserves the permafrost in elevated palsas and peat plateaus as it facilitates extensive heat flux from the peat in winter . However, along the peat plateau edge late‐season thaw depth is greater (>69 cm) compared to in the more central parts, despite an equally shallow snow cover.…”

“…A thicker snow cover also generates more meltwater in late spring, increasing the soil moisture in low‐lying landscape units. Because saturated peat has nearly ten times higher thermal conductivity compared to dry peat, a high water content can warm the soil . If ground ice in the upper permafrost melts during the summer the ground surface is lowered as a result of volume loss, and over time depressions can become saturated with water trapped by the surrounding impermeable permafrost.…”

“…Because of topographical differences, winds can redistribute snow within these landscapes. Elevated palsas and peat plateaus typically have a thin snow cover, <30 cm, which allows extensive heat loss from the ground in winter, promoting permafrost development and persistence . In low‐lying landscape units, a thick snow cover can reduce heat flux from the soil, because snow has very low thermal conductivity .…”

“…Several studies have discussed temporal changes in ground temperature and active layer depth in palsa mires, and the impacts of vegetation and snow depth on ground thermal properties and palsa formation . However, few studies have focused on spatial variation in ground thermal regimes across landscape units in peat plateau ecosystems.…”