Assessing the distribution of thermokarst landscapes

This study distinguishes between wetland, lake and hillslope thermokarst landscapes (Fig. 1). This distinction between different types of thermokarst landscapes is broad and it is recognized that a high degree of landscape diversity is retained within each type. Each thermokarst landscape type is defined by its association with a set of characteristic thermokarst landforms5 (see below). Characteristic thermokarst landforms of each thermokarst landscape preferentially co-occur spatially due to similarities in their dependencies on landscape characteristics for initiation and development. Nearly two dozen distinct thermokarst landforms have been identified in the permafrost affected northern boreal forest and tundra4,5. Our use of the term thermokarst landforms includes landforms traditionally termed thermo-erosional landforms. These differ from other thermokarst landforms in their dominance of lateral rather than vertical soil movement during landform development. We define the spatial extents of thermokarst landscapes to include both the areas of current thermokarst landforms and the areas susceptible to future thermokarst development. We further consider the three thermokarst landscape types to potentially overlap spatially, thus assuming that some landscape positions can be susceptible to the development of thermokarst landforms characteristic of more than one thermokarst landscape type. As an example, some landscape positions within tundra lowlands can potentially be susceptible to develop thermokarst landforms characteristic of any thermokarst landscape type.

Figure 1: Examples of thermokarst landscapes. Photos of (a) wetland thermokarst landscape in the Northwest Territories of Canada, with characteristic thermokarst bogs (Photo: M. Helbig), (b) lake thermokarst landscape in northern Sweden with characteristic shallow thermokarst lakes (Photo: A.B.K. Sannel) and (c) hillslope thermokarst landscape on the Taymyr Peninsula, Russia, with characteristic thermal erosion gullies (Photo: G. Hugelius). Full size image

Areal extents of thermokarst landscapes in this study are estimated through a conceptual modelling framework that weighs the perceived relative influence of landscape characteristics, including ground ice content25, sedimentary overburden thickness25, permafrost zonation25, terrestrial ecoregion26, topographical roughness27 and the presence of permafrost peat soils (histels)28 (Table 1). This study encompasses the boreal and tundra ecoregions26 within the northern circumpolar permafrost zones25, covering 12.4% of the world land area (Table 2). The spatial intersection of layers containing information on landscape characteristics used in the modelling framework yields >130,000 polygons, which we henceforth refer to as regions within the overall study area. Weights of landscape characteristics for determining regional coverage of thermokarst landscapes were decided through an expert elicitation, which included input from all co-authors as well as from members of the Permafrost Carbon Network29. This process was iterative, with consensus achieved through the sharing and discussion of experts’ arguments for increasing or decreasing weights. The main arguments for the final weights are described in the paragraphs below. In the final model, regional coverage of thermokarst landscapes is binned into five classes; ‘Very High’, ‘High’, ‘Moderate’, ‘Low’ and ‘None’. Each coverage class corresponds to a fractional coverage of a region (Table 1). However, since we consider the three thermokarst landscapes to have the potential to spatially overlap within a region, the fractional regional coverage of individual thermokarst landscapes is adjusted to reflect this when more than one thermokarst landscape is present in a region (see ‘Methods’ section for full explanation of how areal extent of thermokarst landscapes within a region is estimated from coverage classes). The resulting maps show the modelled regional coverage classes of wetland, lake and hillslope thermokarst landscapes, respectively (Fig. 2a–c), and the dominant or co-dominant thermokarst landscape types within each region (Fig. 3). Overall, we estimate that thermokarst landscapes cover 3.6 × 106 km2, or ∼20% of the overall study area, with each of the three thermokarst landscapes contributing 5–8% each (Table 2).

Table 1 Framework for estimating regional coverage of thermokarst landscapes. Full size table

Table 2 Estimates of areas and soil organic carbon storage within the overall study region and in each thermokarst landscape type. Full size table

Figure 2: Distribution and regional coverage of thermokarst landscapes in the northern boreal and tundra circumpolar permafrost region. Differentiation is made for (a) wetland (green shading), (b) lake (blue shading) and (c) hillslope thermokarst landscapes (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%). Hillslope thermokarst landscapes are assumed to not reach ‘Very High’ regional coverage. Yellow star symbols indicate study sites, described in literature, of thermokarst landforms characteristic of each thermokarst landscape (Supplementary Table 4-6). Background map of topography is based on GTOPO30 data (USGS, EROS, ESRI), accessed through ArcGIS 9.3.1. Full size image

Figure 3: Dominant or co-dominant thermokarst landscapes within the northern boreal and tundra circumpolar permafrost region. The thermokarst landscapes with the greatest estimated regional coverage are shown. Co-dominance of two or all thermokarst landscapes is shown if estimated coverage classes are equal. Green, Blue and Red shadings indicate individual dominance of wetland, lake and hillslope thermokarst landscapes, respectively. Co-dominance is indicated by cyan (wetland and lake thermokarst landscapes), yellow (wetland and hillslope thermokarst landscapes), magenta (lake and hillslope thermokarst landscapes) and grey (equal coverage of all thermokarst landscapes). Dominant coverage is classified as ‘Very High’ (60–100% regional coverage), ‘High’ (30–60%), ‘Moderate’ (10–30%), ‘Low’ (1–10%) and ‘None’ (0–1%). Coverage of hillslope thermokarst landscapes are assumed to not reach ‘Very High’ regional coverage. Co-dominance combinations that have no representation in the map and are hashed in the legend. Background map of topography is based on GTOPO30 data (USGS, EROS, ESRI), accessed through ArcGIS 9.3.1. Full size image

Wetland thermokarst landscapes

Typical thermokarst landforms in wetland thermokarst landscapes include thermokarst bogs, fens and shore fens, with development dependent on hydrological landscape position. Development causes transition from boreal forest or tundra dry shrub ecosystems into sedge or Sphagnum moss wetland ecosystems with near-surface water table position4,5,15. Thermokarst landforms are typically 0.5–10 ha but can reach sizes up to 100 ha. Development leads to 1–3 m land settlement but limited lateral soil movement.

The two landscape characteristics considered most important for estimating regional coverage of wetland thermokarst landscapes are histel soil coverage and topographic ruggedness (Table 1). We consider histels to be largely susceptible to the development of wetland thermokarst landforms due to their high-ground ice content30. Because histels have high-ground ice content, we did not further use landscape information on ground ice content by itself for estimating regional coverage (Table 1). Wetland thermokarst landscapes can dominate flat landscapes with extensive histels but are assumed to be largely confined to topographic lows in regions with more topographic ruggedness, including valley bottoms and adjacent to ponds and lakes4,5,15,30,31,32,33.

Secondary influences on regional coverage include permafrost zonation and sedimentary overburden thickness (Table 1). All else equal, we consider wetland thermokarst landscapes to have lower regional coverage in regions with thin sedimentary overburden and in colder permafrost zones. Thin sedimentary overburden is considered to limit the potential for vertical land subsidence and thus the development of characteristic thermokarst landforms. In colder permafrost zones, histels often occur in polygonal peatlands characterized by relatively thin organic soils1 and abundant ice wedges. In such polygonal peatlands it is more likely that thermokarst leads to the development of thermokarst troughs and pits develop5, which we consider characteristic of lake thermokarst landscapes (see below). In the non-continuous permafrost zones, our model allows wetland thermokarst landscape coverage to be greater than the permafrost coverage. This follows our definition of thermokarst landscapes, which includes both permafrost areas susceptible to future thermokarst development and non-permafrost areas of current thermokarst landforms23,31.

The resulting maps show ‘Very High’ wetland thermokarst landscape coverage in well-known and extensive boreal peatland regions such as the West Siberian Lowlands, the Hudson Bay Lowlands and the Mackenzie River valley, but also indicate additional widespread ‘Low’ coverage in much of boreal Canada and Russia (Fig. 2a). Wetland thermokarst landscapes in different settings will have distinct characteristics, for example, with thermokarst landforms developing from treed peat plateaus in boreal western Canada and Alaska31 or from non-treed palsas in tundra regions of Scandinavia and northwestern Russia33. In warmer permafrost zones it is also assumed that current thermokarst landforms dominate thermokarst landscapes, while areas susceptible to future thermokarst development are more prevalent in colder permafrost zones15.

Lake thermokarst landscapes

Lake thermokarst landscapes are characterized by lake initiation, expansion, drainage and drainage basin development. Typical thermokarst landforms include deep, shallow and glacial thermokarst lakes, thermokarst lake basins, alas basins and thaw sinks4,5. Lake thermokarst landscapes are also considered associated with collapse pingos, and thermokarst troughs and pits, which all can have aquatic phases that may develop into thermokarst lakes5. Land settlement varies between 1 and 20 m, with potentially substantial lateral soil movement into inundated, anaerobic, conditions through wave action and colluvial processes. Resulting landforms vary greatly in size, with the largest landforms covering more than 5,000 ha.

Regional coverage of lake thermokarst landscapes is considered strongly influenced by topography, ground ice content, sedimentary overburden and permafrost zone34 (Table 1). Flat landscapes with thick sedimentary overburden and high-ground ice content are landscape characteristics considered important for abundant development of characteristic landforms4,5,21. Conceptually, we consider lake thermokarst landscapes to include the open-water areas of thermokarst landforms. Topography strongly limit the maximum attainable regional coverage of lake thermokarst landscapes in our model, and ‘Moderate’ coverage is the maximum coverage possible in regions that do not have flat topography. Regional coverage of lake thermokarst landscapes is considered limited in warmer permafrost zones due to the lower permafrost coverage, but also due to the better drainage when thermokarst landforms develop in locations without underlying deeper permafrost layers34,35. Permafrost thaw in the boreal ecoregion is further considered less likely to lead to characteristic thermokarst landforms due to thicker surface organic mats that can reduce the mechanical removal of soil material36. Thermokarst lakes are however known to potentially occur in landscapes with thick organic soils if ground ice content is high21,32, and thus we do not consider histel coverage by itself to reduce the regional coverage of lake thermokarst landscapes (Table 1).

‘Very High’ regional coverage of lake thermokarst landscapes is shown for several lowland tundra regions, including the Yukon delta, the Alaska north slope and the coastal regions along the Kara Sea, Laptev Sea and East Siberian Sea in Russia (Fig. 2b). These regions are known to have abundant thermokarst lakes34. ‘Very High’ regional coverage further includes the boreal lowland along the Lena River and its tributaries, which is known to have a high abundance of alas landforms14. Lake thermokarst landscapes are further shown to overlap substantially with wetland thermokarst landscapes in major boreal peatland regions (Fig. 2a,b), albeit with lake thermokarst landscape generally having ‘Low’ to ‘Moderate’ coverage, while wetland thermokarst landscapes have ‘High’ to ‘Very High’ coverage.

Hillslope thermokarst landscapes

Typical thermokarst landforms in hillslope thermokarst landscapes include active layer detachment slides, retrogressive thaw slumps, thermal erosion gullies, beaded streams and thermokarst water tracks4,5. Development can cause both substantial land subsidence and lateral soil transport through fluvial or colluvial processes. These thermokarst landforms are generally smaller than landforms of the other thermokarst landscapes, but can in some cases reach up to 10 ha. Hillslope thermokarst landforms are also to a greater degree limited in their development to landscape positions on moderate slopes or along watercourses, lake shores and coasts4,5,22. For that reason, we exclude hillslope thermokarst landscapes from attaining ‘Very High’ regional coverage (Table 1). While hillslope thermokarst landscapes are considered most likely in undulating and hilly topography, ‘Moderate’ regional coverage is still considered possible in flat regions due to the association of characteristic landforms with watercourses and shores5.

Primary factors, in addition to topography, considered to increase regional coverage of hillslope thermokarst landscapes are higher ground ice content, and colder permafrost zones5,36 (Table 1). Hillslope thermokarst development in warmer permafrost zones is considered limited since landscape positions preferential for characteristic thermokarst landforms is likely to already lack permafrost. Secondary influences considered for the regional coverage of hillslope thermokarst landscapes relate to the susceptibility of the landscape to erosion. Regions with thin sedimentary overburden or extensive histels, are thus less likely to be suitable for many of the characteristic thermo-erosional landforms typical of hillslope thermokarst landscapes5. Regions in the boreal ecozone are less likely to have extensive ice wedges that often act as points for hillslope thermokarst landform initiation36 (Table 1).

The resulting map shows ‘Moderate’ and ‘High’ regional coverage of hillslope thermokarst landscapes in many tundra regions, including the Alaska Seward Peninsula, the Alaska North Slope, the Mackenzie River Valley, the Canadian Arctic Archipelago and coastal regions along the Kara Sea, Laptev Sea and East Siberian Sea in Russia (Fig. 2c). This indicates that hillslope thermokarst landscapes often have their greatest concentrations in regions overlapping with ‘High’ or ‘Very High’ coverage of lake thermokarst landscapes (Fig. 2b). However, ‘Low’ and ‘Moderate’ regional coverage of hillslope thermokarst landscapes is widespread in vast continental regions with more pronounced topography in western North America, the Central Siberian Plateau and in the Russian Far East.

Distribution of thermokarst landscapes

While the three thermokarst landscapes have similar estimated total areas (Table 2), their distributions differ with regards to both their spatial concentration and their relations to current climate conditions. Hillslope thermokarst landscapes are spatially least concentrated, with a majority (62%) of its total area found in regions with ‘Moderate’ or ‘Low’ coverage, compared with 32% and a mere 13% for wetland and lake thermokarst landscapes, respectively. Lake thermokarst landscapes are spatially the most concentrated, with 76% of its total area found in regions with ‘Very High’ coverage. Thermokarst landscapes also differ in their relation to current climate conditions37, with hillslope thermokarst landscapes located in regions with colder and drier climate than lake thermokarst landscapes, which in turn are generally located in regions with colder and drier climate than wetland thermokarst landscapes (Fig. 4).

Figure 4: Distribution of thermokarst landscapes under current climate conditions. Data uses the area and climate for each region in the map of thermokarst landscape distribution. Mean annual (a) air temperature and (b) precipitation estimates for the period 1990–2010 is based on a fused CRU-NCEP re-analysis data set31. Grey boxes indicate the overall study region, while green, blue and red indicate wetland, lake and hillslope thermokarst landscapes, respectively. The boxes represent 25th and 75th percentiles (that is, indicating that 50% of the areas of the overall study region and of the three thermokarst landscape types fall within the box estimates). In addition, the bar indicates the 50th percentile, and whiskers showing 5th and 95th percentiles. Full size image

Evaluation of mapped thermokarst landscapes

Two independent approaches were used to evaluate the mapped regional coverages of thermokarst landscapes. First, we compiled a database of 225 locations of thermokarst landforms characteristic of wetland, lake and hillslope thermokarst landscapes (63, 92 and 69 landforms of wetland, lake, and hillslope thermokarst landscapes, respectively) described in 161 published studies (Supplementary Tables 4–6). The locations of these sites were compared with the corresponding mapped regional coverage of thermokarst landscapes (Figs 2a–c and 5). Roughly equal numbers of study sites (35–55 sites) were found within each coverage class. However, study site spatial concentrations were highest in regions with ‘Very High’ coverage, reaching a concentration of 21.0 × 10−6 sites km−2, compared with 15.6, 8.0, 4.6 and 1.1 × 10−6 sites km−2 for ‘High’, ‘Moderate’, ‘Low’ and ‘None’ coverage, respectively. This pattern of decreasing study site concentrations in lesser coverage classes remained, with minor exceptions, when assessed for each thermokarst landscape separately (Supplementary Fig. 1). While study sites are not chosen randomly due to issues of accessibility in the north, increasing site concentrations in regions with greater mapped coverage of thermokarst landscapes is what would be expected from accurate maps.

Figure 5: Locations of studied thermokarst landforms and mapped dominant thermokarst landscapes in Alaska and western Canada. Legend for thermokarst landscapes is same as in Fig. 3. Green circles, blue squares and red triangles represent studied thermokarst landforms characteristic of wetland, lake and hillslope thermokarst landscapes, respectively (Supplementary Tables 4–6). Background map of topography is based on GTOPO30 data (USGS, EROS, ESRI), accessed through ArcGIS 9.3.1. Full size image

The second approach for map evaluation is based on a comparison between the mapped regional coverages of thermokarst landscapes and an expert assessment of regional coverages based on satellite image interpretation and general site knowledge at 435 sites. Site locations for this expert assessment were chosen using a stratified random sampling approach, including 150 sites each for wetland and lake thermokarst landscape assessment, and 135 sites for hillslope thermokarst landscapes (Supplementary Fig. 2). Within each thermokarst landscape type, 50 sites were selected within regions of ‘None’ coverage, and ∼25 sites within regions of each of the other coverage classes. Five experts per thermokarst landscape type independently assessed the coverage of thermokarst landscapes within a 20 km diameter circular polygon centred at each site location, using imagery in Google Earth (Google Inc., Mountainview, CA, USA, www.google.com/earth). All experts have previously published peer-reviewed articles on topics of relevance to the specific thermokarst landscape they assessed. Each expert was given instructions that included the definitions of the thermokarst landscapes, and of the five coverage classes used in the mapping framework. In their assessment of coverage at each site, experts were asked to take into account both their interpretation of satellite imagery available in Google Earth and their personal knowledge of local thermokarst conditions based, for example, on nearby field work. Experts were also asked to assess their confidence in their estimates of coverage at each site as high (satellite imagery highly suitable for accurate coverage estimate, and/or detailed personal knowledge of local thermokarst conditions), moderate (satellite imagery passable for accurate coverage estimate, and/or some personal knowledge of local thermokarst conditions) or low (satellite imagery unsuitable for accurate coverage estimate, and no personal knowledge of local thermokarst conditions). A consensus expert assessment of coverage for each site was determined based on the median coverage estimate of the five individual assessments.

The agreement between the expert assessment and the mapped coverages of thermokarst landscapes was evaluated through analysis of error matrices (Supplementary Tables 1–3; Fig. 3), including determination of a weighted Kappa coefficient38. The Kappa coefficient represents a measure of agreement between expert assessment and mapped coverage which takes into account the agreement that is expected due to chance alone. A Kappa coefficient of 1 indicates perfect agreement and a value ≤0 indicates no agreement39. A weighted Kappa coefficient further takes into account that not all disagreements are equally serious, as is the case of the ordered coverage classes in this study40. We chose to report a quadratic weighted Kappa coefficient, indicating that disagreements between expert assessment and maps are considered to be successively more egregious with greater divergence. Our map evaluation shows that experts who reported higher average confidence in their estimates of coverage also had greater agreement with the maps (Fig. 6). Both expert confidence and agreement further varied by thermokarst landscape type: the lowest confidence and agreement was found for hillslope thermokarst landscapes and the highest for lake thermokarst landscapes (Fig. 6). We note that expert level of confidence increases with the typical size of thermokarst landforms characteristic for each thermokarst landscape type, suggesting that the evaluation method is less suitable for assessing hillslope thermokarst landscape coverage due to issues with identifying typical landforms. Despite experts expressing overall limited confidence in their estimates of ground conditions, weighted Kappa coefficients for the consensus expert assessments of each thermokarst landscape type were 0.59±0.13 (±2 s.e.), 0.70±0.08 and 0.42±0.14, for wetland, lake and hillslope thermokarst landscapes, respectively (Fig. 6), indicating moderate to substantial agreement with the maps39,40.

Figure 6: Relationship between expert confidence and the agreement between expert assessment and mapped coverages of thermokarst landscapes. Agreement between expert assessment and mapped coverages are estimated using a Kappa coefficient with quadratic weighting (κ w ). Interpretation of κ w has been proposed33, where 0.01–0.20 indicate slight agreement, 0.21–0.40 fair agreement, 0.41–0.60 moderate agreement, 0.61–0.80 substantial agreement and 0.81–1 almost perfect agreement. Small symbols represent average individual expert confidence and assessment agreement with mapped coverages, while the larger symbol for each thermokarst landscape type is based on the consensus expert assessment. Expert assessments of wetland, lake and hillslope thermokarst landscape coverages are indicated by green, blue and red symbols, respectively. Error bars indicate±2 s.e. of κ w , and is only shown for the consensus expert assessments. Full size image

Carbon storage in thermokarst landscapes

Thermokarst landforms are often associated with landscapes that have high concentrations of below-ground organic carbon content15,16,17,18. To estimate the fraction of the below-ground organic carbon within the overall study region stored in thermokarst landscapes, we overlaid the resulting maps of thermokarst landscapes with estimates of regional 0–3 m SOC content28. The spatially interpolated data of regional overall SOC content further include information on constituent SOC concentrations (kg C m−2) for both permafrost soils (histels, orthels and turbels), non-permafrost soils (histosols, and a combined class of other mineral soil types), as well as on spatial coverage of non-soils (for example, rock lands) with negligible SOC concentrations. To estimate 0–3 m regional SOC storage of thermokarst landscapes, we first assumed that regional wetland thermokarst landscape SOC concentration could be approximated by using the regionally area-weighted SOC concentrations of histels and histosols (see ‘Methods’ section). Using information on SOC concentrations from both permafrost and non-permafrost organic soils acknowledges that wetland thermokarst landscapes are a mosaic of non-permafrost thermokarst landforms and permafrost areas susceptible to future thermokarst. For lake and hillslope thermokarst landscapes, we then assumed that SOC concentrations were approximated by the area-weighted SOC concentrations of histels, orthels and turbels (that is, of permafrost soils). Lastly, regional SOC content of non-thermokarst landscapes was estimated as a residual by subtracting SOC content of thermokarst landscapes from the overall SOC content.

We estimate that thermokarst landscapes accordingly store ∼330 Pg SOC in the upper 3 m, constituting ∼30% of the total 0–3 m SOC storage within the overall study region (Table 2). Wetland thermokarst landscapes are estimated to have the highest 0–3 m SOC storage at ∼165 Pg SOC, about half of the total thermokarst landscape SOC storage, as a result of having the highest average SOC concentration at ∼115 kg C m−2. Both lake and hillslope thermokarst landscapes also had higher 0–3 m SOC concentrations at ∼80 and ∼75 kg C m−2, respectively, than non-thermokarst landscapes at ∼50 kg C m−2 (Table 2). While estimates of 0–3 m SOC storage in thermokarst sensitive landscapes include large uncertainties, stemming from both spatial extrapolation of SOC content from site specific pedon data28 and from our assumptions in determining coverage and SOC concentrations, our results support the notion that SOC within the overall study region is highly concentrated within thermokarst landscapes.

It is very likely that thermokarst landscapes would be found to store an even greater fraction of the total study region below-ground organic carbon storage if thermokarst lake sediments and SOC storage below 3 m depth were accounted for. Boreal peatlands within wetland thermokarst landscapes can reach depths well below 3 m41. Of great significance also are Yedoma landscapes and Arctic Ocean deltas, both dominated by thick frozen deposits that often are >20 m deep and thus have very large total organic carbon storage, despite low to moderate carbon content in the sediments42. Together they store an estimated ∼380 Pg C below 3 m in areas that spatially largely overlap with regions that have ‘Very High’ coverage of lake thermokarst landscapes1,18,42. Including these Yedoma, lacustrine and deltaic sediments and their deeper carbon stores suggest that thermokarst landscapes store approximately half of the total study region below-ground organic carbon, despite covering only 20% of the area.