Thermokarst distribution in northern circumpolar region

Geospatial analysis reveals 20% of northern permafrost region susceptible to abrupt permafrost thawing

Northern permafrost soils represent the largest terrestrial organic carbon pool (1,330–1,580 gigatonnes) on Earth.  Abrupt thaw accelerates release of deeply froze, ancient carbon, increasing permafrost soil carbon emissions by ~125–190% compared to gradual thaw alone.   A study of boreal and tundra ecoregions within the northern circumpolar permafrost zones reveals that landscapes susceptible to abrupt thawing cover 20% of the northern permafrost region and store up to half its soil organic carbon. The release of permafrost carbon as greenhouse gases is likely to amplify climate warming beyond projections of the Intergovernmental Panel on Climate Change (IPCC)’s Fifth Assessment Report. 

Background

Thermokarst is the process whereby the thawing of ice-rich permafrost ground causes land subsidence, resulting in the development of distinctive landforms.  Thermokarst landforms are often associated with landscapes that have high concentrations of below-ground organic carbon content. Thermokarst landscapes are classified as wetland, lake and hillslope depending on the thermokarst landforms that occur.   Gradual increases in average annual temperatures over decades have already been linked to an thermokarst lake expansion and and to accelerated expansion of thermokarst bogs.

Wetland thermokarst landscapes typically include bogs, fens and shore fens. Lake thermokarst landscapes are characterized by lake development and expansion and associated drainage.  Typical hillslope thermokarst landscapes include slides, thaw slumps, and erosion gullies.

Observations

Thermokarst distribution in northern circumpolar
Thermokarst landscapes (a) wetland (green shading), (b) lake (blue shading) and (c) hillslope (red shading). Coverage is classified as ‘Very High’ (60–100% regional coverage), ‘High’ (30–60%), ‘Moderate’ (10–30%), ‘Low’ (1–10%) and ‘None’ (0–1%).

Available spatial information on landscape characteristics within the northern boreal and tundra permafrost region was used to assess the distribution of thermokarst landscapes.  The study distinguishes between wetland, lake and hillslope thermokarst landscapes. Each thermokarst landscape type is defined by its association with a set of characteristic thermokarst landforms. Nearly two dozen distinct thermokarst landforms have been identified in the permafrost affected northern boreal forest and tundra. The three thermokarst landscape types may overlap spatially.

Areal extents of thermokarst landscapes were estimated through a spatial analysis that weighs the relative influence of landscape characteristics, including ground ice content, sedimentary overburden thickness, permafrost zonation, terrestrial ecoregion, topographical roughness and the presence of permafrost peat soils. The spatial intersection of these layers defined more than 130,000 polygons or regions within the overall study area. Weights of landscape characteristics for determining regional coverage of thermokarst landscapes were decided through expert assessment. Each polygon is classified into five classes; ‘Very High’, ‘High’, ‘Moderate’, ‘Low’ and ‘None’.

To estimate the fraction of the below-ground organic carbon within the overall study region stored in thermokarst landscapes, the maps of thermokarst landscapes were overlaid on maps of estimates of regional 0–3?meter soil carbon content.

Results

Based on the spatial analysis of the distribution of thermokarst landscapes including landscapes currently exhibiting thermokarst landforms and areas susceptible to future thermokarst development, thermokarst landscapes are estimated to cover 20% of the northern boreal and tundra permafrost region.  Furthermore it is estimated that approximately half of the below-ground organic carbon within this region is stored in thermokarst landscapes.  This old carbon dating from the Pleistocene is susceptible to abrupt thawing with the potential to release significant amounts of carbon dioxide and methane into the atmosphere.  Abrupt thawing is currently not included in IPCC projections and could increase greenhouse warming beyond IPCC estimates.

Circumpolar distribution and carbon storage of thermokarst landscapes, D. Olefeldt, S. Goswami, G. Grosse, D. Hayes, G. Hugelius, P. Kuhry, A. D. McGuire, V. E. Romanovsky, A.B.K. Sannel, E.A.G. Schuur & M. R. Turetsky, Nature Communications volume 7, Article number: 13043 (2016).