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.