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

• Main Authors: O. Karjalainen, M. Luoto, J. Aalto, J. Hjort
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-02-01
• Series: The Cryosphere
• Online Access: https://www.the-cryosphere.net/13/693/2019/tc-13-693-2019.pdf
• ISSN: 1994-0416; 1994-0424

<p>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 (<span class="inline-formula">∼1</span>&thinsp;km<span class="inline-formula">²</span>) 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 <span class="inline-formula"><i>R</i>²</span> values being <span class="inline-formula">∼0.94</span> and 0.78. Corresponding predictive performances in terms of root-mean-square error were <span class="inline-formula">∼1.31</span>&thinsp;<span class="inline-formula">^∘</span>C and 87&thinsp;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&thinsp;<span class="inline-formula">^∘</span>C, respectively. Soil properties had marginal effects on MAGT (effect size <span class="inline-formula">=0.4</span>–0.7&thinsp;<span class="inline-formula">^∘</span>C). For ALT, rainfall (effect size <span class="inline-formula">=181</span>&thinsp;cm) and solar radiation (161&thinsp;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 <span class="inline-formula">≤0</span>&thinsp;<span class="inline-formula">^∘</span>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<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><msub><mi/><mrow><mo>≤</mo><mn mathvariant="normal">0</mn><mspace width="0.125em" linebreak="nobreak"/><msup><mi/><mo>∘</mo></msup><mrow class="unit"><mi mathvariant="normal">C</mi></mrow></mrow></msub></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="9pt" class="svg-formula" dspmath="mathimg" md5hash="0321ce79795071595303634f7d118a25"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-13-693-2019-ie00001.svg" width="22pt" height="9pt" src="tc-13-693-2019-ie00001.png"/></svg:svg></span></span> reverted after reaching an optimum at <span class="inline-formula">∼250</span>&thinsp;mm, and that of snowfall started to level off at <span class="inline-formula">∼300</span>–400&thinsp;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&thinsp;km scale should improve future hemispheric-scale studies.</p>