Figure 3: Scheme of annual atmospheric GHGs exchange (CO2, CH4, and N2O) for the five terrestrial land cover classes (Boreal Forests, Non-permafrost Wetlands, Dry Tundra, Tundra Wetlands and Permafrost Bogs); inland water classes (Rivers and Lakes). Annual lateral fluxes from coastal erosion and riverine fluxes are also reported in Tg C yr-1 and Tg N yr-1. Symbols for fluxes indicate high (x>Q3), medium (Q1<x<Q3), and low (<Q1) fluxes, in comparison the quartile (Q). Note that the magnitudes across three different GHG fluxes within each land cover class cannot be compared with each other.

3.4 Total GHGs, C, and N budgets

Summing up all budget components, the permafrost region was a source of GHGs throughout the period 2000-2020 (Table 1). Emissions of CO2 were weak with 0.36 (-619.7, 651.5) Tg CO2-C yr-1 due to the large CO2 uptake from terrestrial ecosystems. Emissions from aquatic ecosystems were the largest source of CO2annualy. CH4 and N2O emissions were 37.7 (21.3, 52.8) Tg CH4-C yr-1 and 0.62 (0.03, 1.19) Tg N2O-N yr-1, respectively with terrestrial ecosystems as largest contributors (68 and 89%, respectively). Lateral fluxes were 94 (79, 111) Tg C yr-1 and 2.6 Tg N yr-1 (Table 1), riverine flux contributing 83 and 38%, respectively.
Taking into account all the above mentioned budget components, the total C (including atmospheric CO2, CH4, and lateral fluxes) budget for the permafrost region between 2000- 2020 were estimated to 136.4 (-516.7, 820.5) Tg C y-1. Close to 70% of the C released from the permafrost region was through lateral fluxes with 57% being released through coastal erosion. Atmospheric CO2 contributed ca. 1% to the total C released from the region while atmospheric CH4 contributed 31%. The total N budget for the permafrost region was 3.2 (1.9, 4.8) Tg N y-1. Most of (81%) the N released was through lateral fluxes with coastal erosion releasing 50% of the total N from the region. Atmospheric N2O from inland waters was negligible while atmospheric N2O from terrestrial ecosystems represented 17% of the total N released in the permafrost region. Atmospheric N2O losses due to fires represented 2% of the N in the permafrost region.

3.5 Main sources of uncertainty and research directions

Limitations in the number of observations

A major challenge in the representation of GHG exchange in high-latitude and remote environments relates to limitations in spatial representation, length and quality of observational time series (Pallandt et al. 2022, Virkkala et al. 2018). The synthesis datasets used here to estimate GHGs fluxes are the most comprehensive ones currently available and have been significantly growing during the past decade. However, more observations covering the full annual cycles are still needed to improve the representativeness of heterogeneous and underrepresented landscapes and climatic conditions. Specifically, more observations from the dry tundra land cover class are needed to verify its GHG sink-source status and from ecosystems experiencing disturbances such as abrupt thaw. CH4 flux measurements are limited in boreal forests, and N2O flux measurements are scarce for all terrestrial and aquatic ecosystems. Across all the GHG fluxes, measurements in environments with low fluxes are also important to avoid biasing our understanding to hotspot regions. Limitations related to the number of flux measurements could be overcome by increasing in situ and laboratory manipulation studies. This would improve process-based understanding of fluxes and their response to changes in temperature, moisture, permafrost thaw and other disturbances. Improvements in the reporting of measurements and metadata should be prioritised for a better integration of available data, especially to address reporting of net-zero or negative fluxes. Difficulties in measuring small exchange rates can be overcome by using new technologies based on portable, high-precision laser instruments (e.g., Juncher Jørgensen et al. 2015, D’Imperio et al. 2017, Juutinen et al. 2022). Very recently, such portable high-precision instruments are becoming available also for N2O, opening possibilities for more numerous and accurate N2O flux estimates, including capturing of N2O uptake.
N2O flux measurements from inland waters are still scarce and ice-out estimates are often missing for CH4fluxes. Moreover, seasonally inundated water bodies are not well represented although they might contribute substantially to the release of GHGs in short periods of time.
Estimates of high latitude lateral fluxes of C and N are fairly well constrained in comparison to land-atmosphere GHG fluxes. However, available estimates are provided for the major six largest arctic rivers that represent 50% of the total area covered by rivers (Speetjens et al. 2023). Although smaller catchments are highly abundant, estimates of GHG fluxes are not well constrained for the permafrost region. Improving this understanding will allow lateral flux integration of these smaller catchments in the main estimates of lateral fluxes from inland waters.

Limitations related to the land cover classification

Differences in GHG fluxes among land cover classes are large. Therefore, it is crucial to get their representation correctly to improve land cover-based GHG flux upscaling. To date, there is no accurate land cover classification of permafrost landscapes (both dry and wet) at a circumpolar scale. We used the BAWLD land cover classification (Olefeldt et al. 2021) in which land cover classes were defined to enable upscaling of CH4 fluxes at large spatial scales. While very relevant to facilitate large-scale mapping of CH4fluxes it lacks sufficient classes to allow separation among groups of dryer ecosystems that might have large variability in CO2 or N2O fluxes. This is the case for the dry tundra and boreal forest classes that comprise a mosaic of ecosystems with different vegetation types. This results in a large uncertainty range in the class flux estimate of the dry tundra (see Table 1, Table A5), making the interpretation of the flux estimates difficult.
Emissions from small water bodies (<0.1 km2) globally represent important inland water CO2 and CH4 fluxes (Holgerson and Raymond, 2016) and even more at high latitudes. Although accounted for in this study, emissions from small water bodies are quite uncertain as they are difficult to map at a large scale due to their high temporal and spatial variability. Small ponds and lakes can be temporary and their size can vary depending on the amount of precipitation after snowmelt; they expand much in wet years and after snowmelt and can often disappear in dry years or late in summer. Improving the spatial and temporal resolution of the products used to map inland waters would benefit the representation of small water bodies, which would resolve a critical source of uncertainty in calculating GHGs exchange.

Limited understanding on the impact of disturbances on the GHG budget

As ecosystems go through disturbance cycles, there are both losses and gains of C and N to ecosystems. It is unclear how well post-disturbance dynamics, e.g. post-fire regrowth, is captured in our ecosystem flux upscaling. Updated budgets need to consider new datasets of fire emissions to cover the period post-2016 as well as post-fire recovery processes. Our emissions from fires consider direct GHG emissions but not the indirect and longer-term soil emissions resulting from fire-induced ground thaw. Although carbon losses might be offset by shifts in species composition (Randerson et al. 2006; Ueyama et al. 2019; Mack et al. 2021), fires can also initiate further permafrost thaw and degradation (Genet et al. 2013; Jafarov et al. 2013; Gibson et al. 2018). As such, fires can trigger shifts in the landscapes, impacting biogeochemical cycling (Randerson et al. 2006; Bouskill et al. 2022; Hermesdorf et al. 2022; Köster et al. 2018b; Ullah et al. 2009; Abbott and Jones, 2015; Voigt et al. 2017; Marushchak et al. 2021; Wilkerson et al. 2019). Improving our understanding of landscape transitions due to fire will help constrain the contribution of disturbances to the GHG budget.
The spatial extent and GHG emissions from abrupt thaw disturbances remain poorly constrained due to a lack of available data (Turetsky et al. 2020). Flux measurements from abrupt thaw are still scarce and thus their reported flux estimate should be interpreted carefully. Improving the numbers of in situ measurements from abrupt thaw disturbances and consistent reporting should be a key to understanding the impact of abrupt thaw on permafrost GHG budgets. Transition rates (from active to stabilised abrupt thaw feature) need to be further understood and systematic mapping of abrupt thaw areas remain to be improved to better constrain emissions from abrupt thaw. N2O emissions from abrupt thaw were not included in this study due to the small number of observations reported in the literature and little understanding on the impact of abrupt thaw on emissions N2O. It was shown that such disturbances frequently cause N2O emission hotspots (Voigt et al. 2020) with two recent studies using a terrestrial ecosystem model simulate enhanced gaseous N losses from thawing permafrost (Lacroix et al. 2022; Yuan et al. 2023). However, another study shows that atmospheric uptake of N2O in peat plateaus and thermokarst bogs increased with soil temperature and soil moisture following disturbances (Schulze et al. 2023). Local hydrology will determine whether the site will turn into a source of N2O after thaw, as high emissions can occur at intermediate moisture conditions in N rich soils (Marushchak et al. 2021) but a transition to wetland would promote denitrification with N2 as the final product and prevent N2O release (Voigt et al. 2017; Butterbach-Bahl and Dannenmann 2011) or even cause or enhance net N2O uptake (Schulze et al. 2023).
As our understanding of processes leading to GHG release through abrupt thaw is constantly improving, future permafrost GHG budgets will be able to better integrate both atmospheric and lateral fluxes from abrupt thaw. So far, the abrupt thaw model (Turestky et al. 2020) does not consider lateral fluxes from abrupt thaw. While we might capture these losses through our lateral fluxes budget, future budgets should allow measuring the fraction of what is lost due to abrupt thaw. Other disturbances including anthropogenic disturbances (e.g. clear cutting and logging) have not been estimated in this study. Future budgets could aim at constraining the impact of these disturbances on the permafrost GHG budget.

5 Conclusions

Using a land cover-based ecosystem flux upscaling approach (including fluxes from terrestrial ecosystems, inland water, disturbances and geological fluxes), the permafrost region was identified as an annual source of GHGs between 2000-2020. The region emitted 0.36 (-620, 652) Tg CO2-C y-1 (mean and 95% confidence interval range used hereafter), 42 (24, 58) Tg CH4-C y-1, and 0.62 (0.03, 1.2) Tg N2O-N y-1 to the atmosphere. The region was thus a net source of CH4 and N2O. For CO2, although the 20-year mean is a net source, the uncertainty range remains large, extending from a large sink to an even larger source of CO2 and, therefore, challenging the calculation of the net flux sign. We suggest that terrestrial ecosystems were likely an ecosystem CO2 sink, but emissions from disturbances and inland waters offset this flux, making the full CO2 budget largely indistinguishable from zero (neutral). The total C (including atmospheric CO2, CH4, and lateral fluxes) and N budget for the permafrost region were estimated to 136 (-517, 821) Tg C y-1 and 3.2 (1.9, 4.8) Tg N y-1.