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:
- Non-permafrost Wetlands include permafrost free bogs, fens, and
marshes with no near-surface permafrost (see Canadian Wetland
Classification system).
- Boreal Forests are forested ecosystems with non-wetland soils.
Coniferous trees are dominant, but the class also includes deciduous
trees in warmer climates and/or certain landscape positions. Boreal
Forests ecosystem may have permafrost or be permafrost free.
- Permafrost Bogs are ecosystems with near surface permafrost and
thick surface peat layers (>40 cm). This includes palsas,
peat plateaus, and the elevated portions of high- and low-centre
polygonal permafrost bogs. They typically have ombrotrophic conditions
that cause nutrient-poor conditions. The vegetation is dominated by
lichens, Sphagnum mosses, woody shrubs, and sometimes sparse
coniferous forest.
- Dry Tundra include treeless ecosystems (both lowland arctic and
alpine tundra) dominated by graminoid or shrub vegetation. Dry Tundra
ecosystems generally have near-surface permafrost. Dry Tundra is
differentiated from Permafrost Bogs by their thinner organic soil
(<40 cm), and from Tundra Wetlands by their drained soils
(average water table position >5 cm below soil surface).
- Tundra Wetlands are treeless ecosystems with near surface
permafrost and saturated to inundated conditions for large parts of
the year. Tundra Wetlands can both be mineral (<40 cm peat)
or have peat (>40 cm peat). They are distinguished from
Dry Tundra and Permafrost Bogs by being wetter and having more dynamic
hydrology. Tundra Wetlands includes areas that can be classified as
tundra fen wetlands in the Canadian Wetland Classification System.
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.