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).