Permafrost cooled in winter by thermal bridging through snow-covered shrub branches

• Published in: Nature Geoscience
• Main Authors: Domine, Florent, Fourteau, Kévin, Picard, Ghislain, Lackner, Georg, Sarrazin, Denis, Poirier, Mathilde
• Other Authors: Takuvik Joint International Laboratory ULAVAL-CNRS, Université Laval Québec (ULaval)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Centre d'Etudes Nordiques (CEN), Université Laval Québec (ULaval), Centre national de recherches météorologiques (CNRM), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Centre National de la Recherche Scientifique (CNRS), Institut des Géosciences de l’Environnement (IGE), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP ), Université Grenoble Alpes (UGA)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2022
• Subjects: [SDU.STU.GL]Sciences of the Universe [physics]/Earth Sciences/Glaciology; [SDE]Environmental Sciences; Arctic; Bylot Island; permafrost; Tundra
• Online Access: https://hal.science/hal-03870786; https://hal.science/hal-03870786/document; https://hal.science/hal-03870786/file/142-DomineNatGeo2022.pdf; https://doi.org/10.1038/s41561-022-00979-2

International audience ermafrost stores about 1,400 Pg of frozen carbon, mostly in the form of decomposing vegetal material 1. Permafrost thaw accelerates the metabolism of soil microbes, increasing the release of the greenhouse gases (GHG) CO 2 and CH 4 (ref. 2). The rate of thawing is affected by little-understood feedbacks 3. Shrub expansion in the Arctic 4 is suspected of accelerating permafrost thaw by increasing snow accumulation 5,6 , which reduces winter cooling by insulating permafrost from the cold winter air. Furthermore, shrubs enhance snowpack insulation in the high Arctic by favouring the formation of depth hoar, a highly insulating snow type, at the expense of more conductive wind slabs prevailing over windswept herb tundra 6,7. Several Arctic field observations and manipulations indicate that shrub expansion leads to permafrost winter warming. One study 6 observed that in shrubs, the snow was 60% more insulating at shrub sites than at tussock tundra sites, resulting in 3 °C warmer soils. Dead shrubs placed on open tundra resulted in topsoil warming by 4-5 °C in January 8. Another study 9 found that March temperatures were about 2.5 °C and 5 °C warmer under dwarf shrubs and tall shrubs, respectively, than under lichen. In these three studies, snow at shrub sites was thicker than in the absence of shrubs. At Bylot Island (73° N), previous studies 7 reported a greater proportion of depth hoar in willow shrubs and measured mean snow thermal conductivities 29% lower in willows than on herb tundra. However, snow was not thicker in willows. Simulations of the permafrost thermal regime under willows and under herb tundra, accounting for snow differences, indicated that minimum winter permafrost temperature should be 7-13 °C warmer under willows. This large shrub-induced warming motivated the installation of instruments at shrub and tundra sites to test model predictions. Three years of monitoring contradict predictions and show that shrubs lead to ground cooling in winter. Here, we propose a new ...