1 Introduction

The northern permafrost region covers up to 21 million km2 of land in the Northern Hemisphere of which ca. 70% (14 million km2) is entirely underlain by permafrost (Obu et al. 2021) – ground that is at or below 0°C for at least two consecutive years. Unprecedented and amplified increases in air temperature in the Arctic (Rantanen et al. 2022) have strong impacts on the permafrost ground temperatures and extent (Biskaborn et al. 2019; Li et al. 2022), with future climate projections indicating a potential loss of permafrost extent of 4.0 (−1.1+1.0, 1σ confidence interval) million km2 for each °C of global temperature change (Chadburn et al. 2017). Consequences are already visible, as ground temperatures near the depth of zero annual amplitude in the continuous permafrost zone increased by 0.39 ± 0.15 °C between 2007 and 2016, reducing the permafrost extent by 7% between 1969 and 2018 (Biskaborn et al. 2019; Li et al. 2022). Changes in ground temperature exposes substantial quantities of organic carbon (C), resulting in C degradation and atmospheric release of greenhouse gases (GHGs) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from permafrost into the atmosphere (Schuur et al. 2009, Schuur et al. 2015; Treat et al. 2018; Natali et al. 2019; Chen et al. 2021, Voigt et al. 2020).
This release of GHGs to the atmosphere could have a strong impact on the global carbon cycle as the upper three metres of permafrost region soils are estimated to store 1000 ± 200 Pg (1 Pg = 1000 Tg) of soil organic carbon (Hugelius et al. 2014, Mishra et al. 2021) and 55 Pg of soil nitrogen (N) (Palmtag et al. 2022). Deeper deposits store an additional 400-1000 Pg C, making the permafrost region the largest terrestrial carbon and nitrogen pool on Earth (Schuur et al. 2022, Strauss et al. 2021). These soil C and N have been accumulating over thousands of years due to limited microbial decomposition at low temperatures and water-logged conditions, leading to long-term accumulation of organic matter and incorporation into permafrost (Tarnocai et al. 2009). Upon thaw – that can occur gradually or abruptly – permafrost landscapes are changing, impacting their hydrology and biogeochemical cycling (e.g. Christensen et al. 2020), creating a potentially significant feedback to the global climate (Schuur et al. 2008; Schuur et al. 2015; Schuur et al. 2022). The release of GHGs from permafrost has the potential to accelerate global climate warming, known as the “permafrost carbon feedback” (Schuur et al. 2015, Burke et al. 2017, Burke et al. 2022). While longer growing seasons, increased CO2concentrations, and additional nutrient release from thawing permafrost may lead to increased vegetation productivity and partly offset the release of permafrost GHGs (Koven et al., 2015; McGuire et al., 2018; Liu et al., 2022; Schuur and Mack, 2018; López-Blanco et al., 2022), other processes such as disturbances cause rapid shifts to landscape structure (Schuur et al., 2008; Schuur et al., 2011) and might accelerate the release of GHGs into the atmosphere.
Although presumably crucial for the global carbon cycle, the role of the northern permafrost region in the global carbon budget is unknown. Existing estimates of terrestrial GHG exchange from land cover-based or machine learning-based ecosystem vertical flux upscaling identify the northern permafrost terrestrial ecosystem as a net sink of CO2 (-181 Tg CO2-C y-1,Virkkala et al. 2021) and a net source of N2O (0.14 Tg N2O-N y-1, Voigt et al. 2020), although large uncertainties remain. The northern permafrost region GHG budgets remain poorly constrained as our understanding of the GHG balance of this region has been hampered by low data availability (both temporal and spatial) and a heterogeneous landscape that is complex to map accurately. Watts et al (2023) show that in northern high latitude, the Net Ecosystem Carbon Budget (NECB) is reduced by ca. 7% when inland waters (e.g. lakes, ponds, streams, and rivers) – known to be significant emitters of CO2 and CH4 (Cole et al. 2007; Stanley et al. 2016; Thornton et al. 2016; Wik et al. 2016; Stackpoole et al. 2017; Serikova et al. 2018) – are included, and by ca. 30% when emissions from inland waters and fires are considered. However, no study has yet included inland waters and disturbances to constrain the GHG budget of the permafrost region and provide an overall net GHG balance.
Here we fill this gaps and present comprehensive budgets of GHGs (CO2, CH4, and N2O), by key permafrost land cover types over the period 2000-2020 across the northern permafrost region using a single flux upscaling approach for all three GHGs. We include most relevant ecosystem components that include terrestrial ecosystems, inland waters, geological fluxes, lateral fluxes, and fire fluxes.
This permafrost regional budget is part of the REgional Carbon Cycle Assessment and Processes-2 (RECCAP2) project of the Global Carbon Project that aims to collect and integrate regional GHGs budgets for 12 land regions and 5 ocean basins covering all global lands and oceans (Ciais et al. 2022; https://www.globalcarbonproject.org/reccap/). Comparisons of GHG budgets using this upscaling flux approach and budgets based on atmospheric inversion models and terrestrial process-based models are discussed in Hugelius et al. (in prep ).

2 Materials and Methods

2.1 Study area

The spatial extent of permafrost defined in this study includes areas within the northern permafrost region as defined in Obu et al. (2021) and restricted to the Boreal Arctic Wetlands and Lakes Dataset area that had the key land cover classes for our upscaling (BAWLD, Olefeldt et al. 2021a,b) (Fig. 1). As a consequence, the BAWLD-RECCAP2 permafrost region does not include large areas underlain by permafrost in Central Asia and the Tibetan plateau. The BAWLD-RECCAP2 permafrost region considered in this study is 18.5 million km2. All flux estimates were scaled to the BAWLD-RECCAP2 permafrost region (hereafter permafrost region). The study area overlaps several other RECCAP2 regions (Ciais et al. 2022), and no specific effort to harmonise the budgets presented here with the RECCAP2 budgets of those regions is made in this paper.