Palsa

Palsas are raised peatland features with a perennially frozen core; they may occur as individual mounds, elongated ridges or broader plateau-like surfaces. Their dimensions vary: published descriptions report mound heights from roughly half a metre to about 10 metres, while peat plateaus can cover much larger areas and stand only modestly above surrounding wetlands. Palsas occur in the discontinuous or sporadic permafrost regions of the Northern Hemisphere, including northern Fennoscandia, Siberia, Alaska and Canada. Palsas and peat plateaus grade into one another, and researchers do not always use the terms identically.

Palsa formation depends on local combinations of climate, peat, snow and terrain rather than temperature alone. One commonly described sequence begins where wind leaves a peatland with relatively thin snow cover in winter, allowing frost to penetrate deeply. If the frozen ground survives the summer, ice accumulates within the peat and raises the surface. The newly elevated, often drier mound may lose snow more readily, helping maintain deeper winter freezing; peat can also insulate the frozen core during summer. This feedback is conditional: sufficient winter cold, suitable snow conditions and peat characteristics are required, and similar regional climates do not guarantee that palsas will form everywhere.

When the frozen core warms and thaws, the mound can subside or collapse. Thaw may produce irregular, water-filled hollows known as thermokarst features, and change the moisture conditions and vegetation of the mire. The resulting landscape is a mosaic of raised dry surfaces, wet hollows and surrounding fen. InSAR satellite measurements combined with airborne mapping in northern Sweden found subsidence attributable to degrading permafrost in 55% of the country's eight largest palsa peatlands over the study's 2017–2021 observation period; that result describes the studied sites and period, not every palsa region.

Palsa microtopography creates distinct habitats and hydrological conditions within peatlands. A 2025 study of thermokarst ponds in northern Norway found a clear ecological gradient from Sphagnum-dominated ponds high on a peat plateau to brown-moss-dominated fen hollows nearer a river, illustrating how thaw-related topography can structure plant communities. Peatlands store carbon, and thaw can change carbon dioxide and methane exchange; the direction and magnitude of net ecosystem effects depend on hydrology, vegetation and subsequent peat accumulation, so thaw does not imply an identical emissions outcome everywhere. Circumpolar modelling projects substantial contraction of suitable environmental space for palsas and peat plateaus under future warming, with the scale depending on the emissions scenario.