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. The thermal state of permafrost affects Earth surface systems andhuman activity in the Arctic and has implications for global climate.Improved understanding of the local-scale variability in the global groundthermal regime is required to account for its sensitivity to changingclimatic and geoecological conditions. Here, we statistically relatedobservations of mean annual ground temperature (MAGT) and active-layerthickness (ALT) to high-resolution ( ∼1  km 2 ) geospatialdata of climatic and local environmental conditions across the NorthernHemisphere. The aim was to characterize the relative importance of keyenvironmental factors and the magnitude and shape of their effects on MAGTand ALT. The multivariate models fitted well to both response variables withaverage R2 values being ∼0.94 and 0.78.Corresponding predictive performances in terms of root-mean-square errorwere ∼1.31 ∘ C and 87 cm. Freezing (FDD) andthawing (TDD) degree days were key factors for MAGT inside and outside thepermafrost 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 ofvariable importance further underlined the dominance of climate for MAGT andhighlighted the role of solar radiation for ALT. Most response shapes forMAGT ≤0 ∘ C and ALT were non-linear andindicated thresholds for covariation. Most importantly, permafrosttemperatures had a more complex relationship with air temperatures thannon-frozen ground. Moreover, the observed warming effect of rainfall onMAGT ≤ 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 largeglobal variation (i.e. climate) suppressed the effects of local-scalefactors (i.e. soil properties and vegetation) owing to the extensive studyarea and limited representation of soil organic matter. Our new insightsinto the factors affecting the ground thermal regime at a 1 km scale shouldimprove future hemispheric-scale studies.

the cryosphere

New insights into the environmental factors controlling the ground thermal regime across the Northern Hemisphere: a comparison between permafrost and non-permafrost areas