3.2 Net GHG emissions from inland waters

Inland aquatic ecosystems were a net source of CO2(230.6 (132.4, 359.8) Tg CO2-C y-1), CH4 (9.4 (4.5, 13.1) Tg CH4-C y-1), and N2O (0.0019 (0.0008, 0.0029) Tg N2O-N y-1). Rivers emitted annually 164.4 (107.3, 222.5) Tg CO2-C y-1, 2.3 (1.6, 2.9) Tg CH4-C y-1 and 0.0006 (0.0004, 0.0008) Tg N2O-N y-1 to the atmosphere. These high riverine fluxes are due to their supersaturation in CO2 as they are receiving and degassing CO2 derived from adjacent soils. To our knowledge, there are no specific annual estimates of riverine GHGs for the permafrost region to compare our estimates, however, when compared to emissions from high latitude, our methane emissions for rivers are within the lower range of published estimates (0.3-7.5 Tg CH4-C y-1) (Thornton et al. 2016).
In comparison to riverine emissions, lakes were a weaker source of CO2 (66.2 (25.1, 137.3) Tg CO2-C y-1) but a stronger source of CH4 (7.1 (2.9, 10.2) Tg CH4 y-1) and N2O (0.0012 (0.0004, 0.002) Tg N2O-N y-1) (Table 1). Our annual lake CH4emission estimate is lower than previous estimates reported by Wik et al. (2016) (12.4 (7.3, 25.7) Tg CH4-C y-1) and Matthews et al. (2020) (13.8-17.7 Tg CH4-C y-1). This is partly related to the difference in lake classifications where in this study lakes were separated by both types and size categories, whereas these previous estimates separated the lakes by type alone- although domain sizes differ slightly. The largest source of lake CH4emissions were from small peatland lakes (~ 30% of lakes emissions, Table A3), which are dominant in the peat-rich regions of the Hudson Bay Lowlands in Canada and the West Siberian Lowland in western Russia (Olefeldt et al. 2021). However, the areas of small lakes estimated by BAWLD are among the most uncertain of the land cover classes (Olelfedt et al. 2021), due to limited spatial data used for lakes and great flux variability among small lakes across the domain (Muster et al. 2019). Our mean lake and river CO2emission estimates for the permafrost region constitute ~12% of reported global annual CO2emissions for lakes (Holgerson et al. 2016) and rivers (Liu et al. 2021). We note that there is a substantial lack of CH4flux data for Boreal-Arctic lakes (Stanley et al. 2016), making our estimates highly uncertain. While there is no estimate of N2O emissions from arctic lakes, Kortelainen et al. (2020) estimated boreal lakes N2O emissions at 0.029 (0.026, 0.032) Tg N2O-N y-1.

3.3 Net GHG emissions from disturbances: fires and abrupt thaw

Fires within the study region affected 1.1 x 106km2 during the period 2000-2016. On average, fires impacted 0.06 million km2 annually, emitting 109.4 (83.5, 135.3) Tg CO2-C yr-1, 1.2 (0.9, 1.5) Tg CH4-C yr-1, and 0.07 (0.06, 0.08) Tg N2O-N yr-1. Ninety percent of the annually burned area was in the boreal biome, contributing to more than 92% of the permafrost region fire GHG emissions (Table 1). Fire CO2 emissions offset a third of the CO2uptake from terrestrial ecosystems, while CH4 and N2O emissions from fires represented 5% and 13% of the CH4 and N2O emitted by terrestrial ecosystems, respectively. Our fire flux estimates mainly reflect direct emissions from combustion. There is also a component of increased growth during post-fire recovery, which we do not explicitly account for. However, it is indirectly accounted for as many of the in situ flux data were collected from previously burned ecosystems (which drives up the mean land cover flux). Our fire carbon emission estimate for boreal ecosystems (CO2 and CH4, 113.2 TgC yr-1) is slightly lower than the one of 142 Tg CO2-C yr-1 previously reported by Veraverbeke et al. (2021). Using GFED4s data, our budget might underestimate fire CO2 emissions as shown in Potter et al. (2022), where GFED4s emissions were 36% lower than the ones obtained using the ABoVE-FED data-driven product.
The total area affected by active and stabilised abrupt thaw between 2000 and 2020 was estimated to be 1.2 x 106km2 (0.43 x 106 in lowlands, 0.01 x 106 in uplands, and 0.72 x 106 in wetlands), accounting for ca. 7% of the permafrost region (Table 1). All together, areas affected by abrupt thaw were net emitters of 31 (21, 42) Tg CO2-C yr-1 and 31 (20, 42) Tg CH4-C yr-1 (Table 1, details in Table A6). CO2 and CH4 emissions from wetland abrupt thaw were the largest. GHG estimates from abrupt thaw were not directly included in the permafrost GHG budget as it was not possible to know how much were already accounted for in the budget from terrestrial upscaling. Yet, the impact of abrupt thaw processes on C cycling in the permafrost region is large, and it is projected that it will contribute nearly as much as gradual thaw to future radiative forcing from permafrost thaw (Turetsky et al. 2020).