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

Abstract

Abstract. The thermal state of permafrost affects Earth surface systems and\nhuman activity in the Arctic and has implications for global climate.\nImproved understanding of the local-scale variability in the global ground\nthermal regime is required to account for its sensitivity to changing\nclimatic and geoecological conditions. Here, we statistically related\nobservations of mean annual ground temperature (MAGT) and active-layer\nthickness (ALT) to high-resolution ( ∼1 \u2009km 2 ) geospatial\ndata of climatic and local environmental conditions across the Northern\nHemisphere. The aim was to characterize the relative importance of key\nenvironmental factors and the magnitude and shape of their effects on MAGT\nand ALT. The multivariate models fitted well to both response variables with\naverage R2 values being ∼0.94 and 0.78.\nCorresponding predictive performances in terms of root-mean-square error\nwere ∼1.31 ∘ C and 87\u2009cm. Freezing (FDD) and\nthawing (TDD) degree days were key factors for MAGT inside and outside the\npermafrost domain with average effect sizes of 6.7 and 13.6\u2009 ∘ C, respectively. Soil properties had marginal effects on MAGT\n(effect size\xa0 =0.4 –0.7\u2009 ∘ C). For ALT, rainfall (effect size\xa0 =181 \u2009cm) and solar radiation (161\u2009cm) were most influential. Analysis of\nvariable importance further underlined the dominance of climate for MAGT and\nhighlighted the role of solar radiation for ALT. Most response shapes for\nMAGT ≤0 ∘ C and ALT were non-linear and\nindicated thresholds for covariation. Most importantly, permafrost\ntemperatures had a more complex relationship with air temperatures than\nnon-frozen ground. Moreover, the observed warming effect of rainfall on\nMAGT ≤ 0 ∘ C reverted after reaching an optimum at\n ∼250 \u2009mm, and that of snowfall started to level off at\n ∼300 –400\u2009mm. It is suggested that the factors of large\nglobal variation (i.e. climate) suppressed the effects of local-scale\nfactors (i.e. soil properties and vegetation) owing to the extensive study\narea and limited representation of soil organic matter. Our new insights\ninto the factors affecting the ground thermal regime at a 1\u2009km scale should\nimprove future hemispheric-scale studies.