Figure 1. Map of northern permafrost extent (data from Obu et al. 2021) overlain with the spatial extent of the permafrost domain included (BAWLD-RECCAP2 regions). The spatial extent of the permafrost region defined in this study as an overlap of permafrost extent and the Boreal Arctic Wetlands and Lakes Dataset (BAWLD, Olefeldt et al. 2021a,b). Figure A1 in the supplement shows the additional areas that recorded mean annual air temperature (MAAT) below 0°C between 1990 and 2000 (full extent of ISIMIP3 permafrost model intercomparison), but which were excluded from this budget estimate because they are outside the BAWLD extent.

2.2 GHG budgets from ecosystem flux upscaling

Data-driven ecosystem flux upscaling of GHG budgets for a reference time period of 2000-2020 was calculated by summing up flux budgets from terrestrial ecosystems, inland waters, lateral fluxes, fire emissions, and geological fluxes. To calculate the total net regional GHG flux (Fx ), we used the following equation:
\begin{equation} F_{x}=\sum_{j=1}^{j=n}{A_{j}\ \times\ F_{\text{jx}}}\nonumber \\ \end{equation}
where Fx is annual permafrost region gas flux for the GHG species of interest x, Aj is the area of each land cover class j (Fig. 2, Table A1), and Fjx is the land cover average GHG flux density for species x (Table A1).
We used existing synthesis databases and upscaled gridded data products published in the past five years to estimate annual and growing season mean fluxes per land cover type. All budget numbers are presented as the weight of C and N (i.e. CO2-C, CH4-C and N2O-N yr-1), not as the weight of GHG molecules. Budgets are reported as mean fluxes with 95% confidence intervals (CI) in Tg C or N.

2.3 GHG fluxes from terrestrial land cover types

The land cover classification used for the analysis was adapted from the Boreal-Arctic Wetland and Lake Dataset (BAWLD) land cover (Olefeldt et al. 2021a,b). The BAWLD land cover classes are distinguished based on moisture regime, nutrient/pH regime, organic-soil depth, hydrodynamics, and the presence or absence of permafrost (Olefeldt et al. 2021a). To match the observational GHG flux datasets, we simplified the nine terrestrial land cover classes in BAWLD into five: Boreal Forests; Non-permafrost Wetlands; Dry Tundra; Tundra Wetlands; and Permafrost Bogs (Fig. 2). Classes were defined as:
This choice of land cover classes was done after assessing the type of sites in three flux databases of CO2, CH4, and N2O used for the upscaling (see description below), ensuring that there was sufficient data for each class and that the merging was the most parsimonious grouping that allowed us to estimate each GHG balance for each class. Due to a lack of flux data, rocklands and glaciers were not included in the classification. The area of each land cover class (Aj) in km2 across the permafrost region is shown in Figure 2 and detailed in Table A1.