Present distributions of cryogenic LSPs

Our forecasts of the current LSPs show high agreement with the observations thus suggesting robust model transferability to similar environments (Fig. 2). The analysis of the current periglacial realm closely corresponds to earlier definitions1, 20, 30, marking mean annual air temperature (MAAT) of +2 °C as a rough upper limit for cryogenic ground processes (Fig. 3). At present, cryoturbation, gelifluction and nivation are active across a broad range of climate conditions, while permafrost mounding is most concentrated with MAAT of ~ −2 °C and low to moderate annual precipitation sum (400–600 mm) (Fig. 3). The probability of active LSPs increases towards cold air temperatures (Fig. 4). Permafrost mounding, nivation and gelifluction are highly sensitive to TDD (advancing permafrost and snow melt, and soil wetting, respectively), whereas cryoturbation is more constrained by FDD which is strongly linked to frost intensity. In turn, gelifluction is controlled by both FDD and TDD affecting frost penetration and spring melt, respectively20. In addition to climatic factors, the periglacial climate realm is strongly mediated by local topographical heterogeneity and soil characteristics17 (Fig. 4). For example, all studied LSPs are strongly linked to slope angle representing different responses (positive for gelifluction and nivation and negative for cryoturbation and permafrost mounding) to factors such as mass movement potential and drainage, while nivation is linked to low radiation input on poleward-facing aspects. Our high-resolution modelling suggests that concurrently nearly half of the study region has a suitable climate for at least one of the LSP (Fig. 5a), reinforcing their importance as characterizing the geomorphology of cold-climate regions.

Fig. 2 The modelled baseline occurrences of the four LSPs based on majority vote ensemble. The n in the title denotes the number of observed presences (black dots), while all the observation sites (n = 2,917) are presented in Fig. 1c. The modelling performance is measured as the area under the curve of a receiver operating characteristic plot (AUC) and the true skill statistics (TSS). The evaluation statistics show the mean (±s.d.) over four modelling techniques and 100 cross-validation runs conducted for each LSP (a cryoturbation, b gelifluction, c nivation and d permafrost mounding) using a random sampling procedure Full size image

Fig. 3 The dwindling periglacial climate. The density scatterplots represent the modelled occurrence of the LSP (blue shades; combined spatial extent of individual LSPs in the large plot, a) compared to baseline (climate of 1981–2010) mean annual air temperature and mean annual precipitation in the study area. The black dots indicate individual modelled LSP occurrences (b cryoturbation, c gelifluction, d nivation and e permafrost mounding) based on empirical data with total of 2,917 observations. The polygons depict the convex hulls (i.e., the minimum bounding boxes) of the two climate variables in the study area, and at four time periods and/or climate change scenarios, indicating the shift in climatic conditions in respect to the current periglacial climate realm of the study area Full size image

Fig. 4 The relationships between the investigated cryogenic land surface processes and environmental predictors. Partial dependency plots estimated by generalized boosting method (GBM) depict the effect of a predictor (a thawing degree days, b freezing degree days, c water precipitation, d snow precipitation, e solar radiation, f topographic wetness index, g slope angle, h peat cover, and i rock cover) on LSP (indicated with coloured lines: dark brown=cryoturbation, light brown=gelifluction, turquoise=nivation, dark green=permafrost mounding) after other predictors have been fixed to their mean values Full size image

Fig. 5 The predicted loss of conditions allowing cryogenic land surface processes. a, b The suitable overlapping conditions for the land surface processes (LSP) under baseline (i.e., current climate of 1981–2010) and future climate (2040–2069 RCP 4.5), respectively, while c shows the area covered by suitable climate for individual LSP at each time period and climate change scenario. The current extent of glaciers were masked out from the analysis Full size image

Future periglacial climate realm

In Fig. 3 we contrast the future climates in the study area with the current periglacial realm and show the consistently shrinking area of overlapping envelopes (e.g., overlap with the baseline ca. 36% 2070–2099 RCP 4.5 and 11% 2070–2099 RCP 8.5). Our results show how even the operation of an optimistic emission pathway will initiate substantial alterations in the extent of the periglacial climate realm (Fig. 5b, c). For example, the suitable area for cryoturbation in our study area is predicted to shrink 84% (under RCP 2.6 by 2040–2069) compared to baseline. These changes in LSP distributions are driven by profound near-term changes in both increasing winter and summer air temperatures and precipitation in the study area (Supplementary Fig. 1). For example, the average (±s.d.) TDD in the study area was projected to increase from 1,117 °C (±323 °C; baseline) to 1,417 °C (±189 °C; 2040–2069 RCP 2.6) and to 1,645 °C (±417 °C; 2040–2069 RCP 8.5). Similarly, the amount of water precipitation is predicted to increase from 285 mm (±57 mm; baseline) to 350 mm (±35 mm; 2040–2069 RCP 2.6) and further to 387 mm (±82 mm; 2040–2069 RCP 8.5). Therefore, these predicted changes in temperature and precipitation regimes will cause the future periglacial realm to reduce markedly in size and the contemporary spatial extent of the periglacial realm will experience a climate that will be substantially warmer and wetter than present conditions (Supplementary Fig. 2).

The LSP loss will be enhanced in high-latitude low-relief continental areas, and in the Northern Hemisphere their projected future climate space is likely to contract dramatically as the Arctic Ocean will effectively limit their ‘‘range expansion’ northward31. Our modelling shows that with the highest greenhouse gas concentration scenario (RCP 8.5) no suitable climate exists for the development of palsas (permafrost mounding) by the end of this century, while any periglacial conditions are predicted to remain ~6% of the study area (Fig. 5c and Supplementary Table 1). Although our modelling does not account for lag times (i.e., the time difference from altered climate forcing to LSP response), the geographical changes are likely to be rapid because topmost soil layers are closely coupled with lower atmosphere conditions14. Despite a cover of insulating peat, many of the permanently frozen mires in the region formed during past cold climates are showing evidences of accelerated thawing16, 32, 33.

Elevational shift

In addition to a rapid decay of LSP, we predict significant elevational shift in periglacial conditions over the whole study domain (Fig. 6; p ≤ 0.001, t-test). The mean (±s.d.) elevation of the periglacial climate is projected to increase from 509 m above sea level (a.s.l.) (±199; baseline) to 650 m a.s.l. (±247; 2070–2099 RCP 2.6), to 686 m a.s.l. (±248; 2070–2099 RCP 4.5) and to 755 m a.s.l. (±252; 2070–2099 RCP 8.5). Consequently, our results suggest that the periglacial climate will be limited to high-elevation areas, after accounting for topographical and soil constraints. This means that suitable conditions for LSP at low elevations are the first to reduce, but potential shifts of LSPs to the highest mountains may be limited by steep topography and rocky or lack of frost-susceptible soil, although nivation processes may continue. These predicted elevational changes in the periglacial climate realm are likely to alter frost-driven denudation processes (e.g., slope processes and cryoplanation1) and modify decadal- to millennia-scale landscape development11. These climate changes will overwhelm frost as a geomorphic agent at low elevations, this being replaced by fluvial and aeolian activity. One consequence is that temperature- and precipitation-driven changes in perennial ground ice are likely to increase the risk of rapid slope displacements and thaw subsidence with pronounced societal impact in areas with infrastructure development34.

Fig. 6 Shifting elevational distribution of the northern Europe’s periglacial climate realm. The kernel density plots (bandwidth=20) show the elevational distributions of the modelled LSP (a combined spatial extent, b cryoturbation, c gelifluction, d nivation and e permafrost mounding) under baseline 1981–2010 (dashed black line), 2070–2099 RCP 2.6, 2070–2099 RCP 4.5 and 2070–2099 RCP 8.5 climate conditions. For permafrost mounding (e), the kernel density estimation under scenario 2070–2099 RCP 8.5 was unfeasible due the low number of modelled occurrences. All distributions differed significantly (t-test, p ≤ 0.001) Full size image

The reduction in the periglacial climate realm will trigger surface–atmosphere feedbacks with global relevance7. In periglacial environments as much as 90% of the total ecosystem carbon resides in frozen organic and mineral soils, and ground frost regimes are expected to form a major control on gas exchange processes (e.g., CO 2 and CH 4 ) between soil and atmosphere under climate change8, 15, 16. Changes in LSPs are likely to modify ground surface reflective properties by decreasing albedo through vegetation re-establishment13 with potentially significant implications for regional climate. Thawing permafrost in peat mounds will change local hydrology and enhance the formation of thermokarst lakes and ponds further decreasing albedo in a positive feedback loop14. Therefore, we stress the need for further (both spatially and temporally comprehensive) investigations of LSP–climate interactions.

Our approach for estimating the current and evolution of the future periglacial climate realm is based on statistical LSP–environment relationships35. While process-based Earth surface models with dynamical atmospheric components have been developed and applied over high-latitude regions, such process-orientated models require an intense parametrization, are computationally expensive and only provide coarse-scale projections. Moreover, process-based models do not provide spatially explicit prediction of LSP, rather they predict ground thermal regimes from which the inference of namely seasonal frost patterns can be challenging. We argue that scale is a critical issue in defining the periglacial climate realm, since (1) it should resemble the scale of observed LSP features (typically ranging from 1 m (a small polygon feature) to over 100 m (a permafrost peat plateau)) and (2) substantial local variation in LSPs driven by microclimate, topography and soil characteristics will be missed by coarse-scale modelling. This means that the extent of the current and projected future extent of the periglacial realm will be significantly underestimated. However, computational constraints means that modelling at such small spatial scales restricts the size of the region that can be examined, although we argue that our results (covering an area of 78,000 km2) are widely applicable to topographically and climatically similar Northern Hemisphere landscapes.

Our results are significant as they provide the first very fine-scale assessment of the current periglacial climate realm over a broad cold-region domain. Moreover, our findings suggest a near-complete decay of periglacial climate from a climatically sensitive high-latitude area and a significant elevational shift of cryogenic ground processes. Finally, these changes are likely to cause substantial landscape-scale changes in ground surface conditions, ecosystem functioning and biogeochemical cycles especially in high-latitude continental interiors. Our analysis, conducted over a wide range of future emission trajectories, indicates that regardless of the climate change mitigation policies the decay of periglacial system is likely to be rapid towards the end of this century.