The net GHG balance and budget of the permafrost region (2000-2020) from ecosystem flux upscaling

Abstract

The northern permafrost region has been projected to shift from a net sink to a net source of carbon under global warming. However, estimates of the contemporary net greenhouse gas (GHG) balance and budgets of the permafrost region remain highly uncertain. Here we construct the first comprehensive bottom-up budgets of CO2, CH4, and N2O across the terrestrial permafrost region using databases of more than 1000 in-situ flux measurements and a land cover-based ecosystem flux upscaling approach for the period 2000-2020. Estimates indicate that the permafrost region emitted a mean annual flux of 0.36 (-620, 652) Tg CO2-C y-1, 38 (21, 53) Tg CH4-C y-1, and 0.62 (0.03, 1.2) Tg N2O-N y-1 to the atmosphere throughout the period. While the region was a net source of CH4 and N2O, the CO2 budget was near neutral with large uncertainties. Terrestrial ecosystems remained a CO2 sink, but emissions from fire disturbances and inland waters largely offset the sink in vegetated ecosystems. Including lateral fluxes, the permafrost region was a net source of C and N, releasing 136 (-517, 821) Tg C y-1 and 3.2 (1.9, 4.8) Tg N y-1. Large uncertainty ranges in these estimates point to a need for further expansion of monitoring networks, continued data synthesis efforts, and better integration of field observations, remote sensing data, and ecosystem models to constrain the contemporary net GHG budgets of the permafrost region and track their future trajectory.

An increasing Arctic-boreal CO2sink offset by wildfires and source regions

“…For further details on the dataset, see Virkkala et al (2021) 8 and a description of additional data processing and screening in the Supplementary Methods Section 2. Note that our study does not include lateral transport of carbon which was recently summarized to be 93 Tg C yr -1 in a roughly similar region (i.e., 17 % of the net uptake budget calculated in this study) 56 . This dataset is more comprehensive than the ones used in earlier upscaling studies as it represents monthly fluxes from the entire year if available, while Virkkala…”

Permafrost Carbon: Progress on Understanding Stocks and Fluxes Across Northern Terrestrial Ecosystems

“…To our knowledge, terrestrial sites experiencing abrupt thaw that have measured multi-year CO 2 or CH 4 fluxes are limited to wet graminoid ecosystems in Alaska (Schuur et al, 2021), boreal black spruce lowlands in Canada and Alaska (Euskirchen et al, 2017;Helbig et al, 2017), and collapsing palsas from Fennoscandia (Varner et al, 2022). However, the current site network misses thaw slumps, gullies, and active layer detachments (Cassidy et al, 2016) that cover <1% of the areas affected by abrupt thaw; overall abrupt thaw is estimated to affect ∼7% of the permafrost region in total (Ramage et al, 2023). Gradual and abrupt permafrost thaw cause changes in hydrology, often increasing soil moisture and/or lake extent, thus often increasing CH 4 emissions (Helbig et al, 2017;Miner et al, 2022;Varner et al, 2022).…”

“…However, the net ecosystem C accumulation is driven by belowground dynamics in soils and biomass rather than accumulation in above-ground vegetation C stocks (Bradshaw & Warkentin, 2015;Hartley et al, 2012;Shaver et al, 1992). The growing season sink strength has been relatively well synthesized across different moisture gradients and continents (McGuire et al, 2012), biomes (Virkkala et al, 2021), and vegetation types (Ramage et al, 2023). Net growing season C uptake is highest in the boreal permafrost region, particularly in warm evergreen and larch forests and can range between 150 and 240 g C m 2 month 1 during the June-August period (Hiyama et al, 2021); moist to wet graminoid-dominated tundra ecosystems also show strong growing season C uptake between 90 and 150 g C m 2 month 1 (Celis et al, 2017;Kittler et al, 2017;Pirk et al, 2017).…”

“…Estimates of C loss from abrupt thaw may exceed those from active layer deepening but are highly uncertain (Estop-Aragonés et al, 2020;Zolkos et al, 2022). For example, less than ten site-level studies were available to use for a recent in-situ-based greenhouse gas budget estimate that showed that areas affected by abrupt thaw were net emitters of 31 (21, 42) Tg CO 2 -C yr 1 and 31 (20, 42) Tg CH 4 -C yr 1 (Ramage et al, 2023;Turetsky et al, 2020); the large uncertainties represent the potential spatial distribution of abrupt thaw areas that have only been quantified in limited regions (Nitze et al, 2018). To our knowledge, terrestrial sites experiencing abrupt thaw that have measured multi-year CO 2 or CH 4 fluxes are limited to wet graminoid ecosystems in Alaska (Schuur et al, 2021), boreal black spruce lowlands in Canada and Alaska (Euskirchen et al, 2017;Helbig et al, 2017), and collapsing palsas from Fennoscandia (Varner et al, 2022).…”

“…Lateral fluxes of CO 2 , CH 4 , and dissolved organic matter from terrestrial ecosystems to riverine and lacustrine systems can comprise a key part of the C budgets, ranging from 2% to 16% of NEE in areas with intact permafrost or up to 60% of NEE in upland areas experiencing thaw slumping Olefeldt et al, 2012;Zolkos et al, 2022). Earlier reviews have discussed lateral fluxes and controls on aquatic system C cycling in the permafrost region (Ramage et al, 2023;Tank et al, 2020;Vonk et al, 2015). Here, we focus on terrestrial ecosystem C exchange with the atmosphere.…”