Abstract
Abstract. The thermal state of permafrost affects Earth surface systems and human activity in the Arctic and has implications for global climate. Improved understanding of the local-scale variability in the global ground thermal regime is required to account for its sensitivity to changing climatic and geoecological conditions. Here, we statistically related observations of mean annual ground temperature (MAGT) and active-layer thickness (ALT) to high-resolution (∼1 km2) geospatial data of climatic and local environmental conditions across the Northern Hemisphere. The aim was to characterize the relative importance of key environmental factors and the magnitude and shape of their effects on MAGT and ALT. The multivariate models fitted well to both response variables with average R2 values being ∼0.94 and 0.78. Corresponding predictive performances in terms of root-mean-square error were ∼1.31 ∘C and 87 cm. Freezing (FDD) and thawing (TDD) degree days were key factors for MAGT inside and outside the permafrost domain with average effect sizes of 6.7 and 13.6 ∘C, respectively. Soil properties had marginal effects on MAGT (effect size =0.4–0.7 ∘C). For ALT, rainfall (effect size =181 cm) and solar radiation (161 cm) were most influential. Analysis of variable importance further underlined the dominance of climate for MAGT and highlighted the role of solar radiation for ALT. Most response shapes for MAGT ≤0 ∘C and ALT were non-linear and indicated thresholds for covariation. Most importantly, permafrost temperatures had a more complex relationship with air temperatures than non-frozen ground. Moreover, the observed warming effect of rainfall on MAGT≤0∘C reverted after reaching an optimum at ∼250 mm, and that of snowfall started to level off at ∼300–400 mm. It is suggested that the factors of large global variation (i.e. climate) suppressed the effects of local-scale factors (i.e. soil properties and vegetation) owing to the extensive study area and limited representation of soil organic matter. Our new insights into the factors affecting the ground thermal regime at a 1 km scale should improve future hemispheric-scale studies.
Highlights
In the face of a changing Arctic, it is crucial to understand the mechanisms that drive the current geocryological dynamics of the region
The observations were standardized by requiring that mean annual ground temperature (MAGT) was recorded at or near the depth of zero annual amplitude (ZAA) where annual temperature variation was less than 0.1 ◦C and that active-layer thickness (ALT) (n = 298) values represented the maximum thaw depth of a given year based on mechanical probing or derived from ground temperature measurements or thaw tubes (Brown et al, 2000; Aalto et al, 2018a)
In contrast to MAGT, ALT was not significantly correlated with TDD but had stronger associations with soil properties (Fig. 2c)
Summary
In the face of a changing Arctic, it is crucial to understand the mechanisms that drive the current geocryological dynamics of the region. Thaw of permafrost is expected to significantly attribute to hydrological and geoecological alterations in landscapes (Jorgenson et al, 2013; Liljedahl et al, 2016). Greenhouse gas emissions from thawing permafrost soils have the potential to affect the global climate system Permafrost temperature and the depth of the overlying seasonally thawed layer, i.e. active layer, are key components of the ground thermal regime that govern various geomorphological and ecological processes (Frauenfeld et al, 2007; Aalto et al, 2017), as well as human activity, in permafrost regions (Callaghan et al, 2011; Vincent et al, 2017; Hjort et al, 2018). Extensive regions undergo seasonal freezing, which in itself affects many aspects of natural and human activities Extensive regions undergo seasonal freezing, which in itself affects many aspects of natural and human activities (e.g. Shiklomanov, 2012; Westermann et al, 2015).
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