The Boreal–Arctic Wetland and Lake Dataset (BAWLD)
Authors: Olefeldt David , Hovemyr Mikael , Kuhn McKenzie A. , Bastviken David , Bohn Theodore J. , Connolly John , Crill Patrick , Euskirchen Eugénie S. , Finkelstein Sarah A. , Genet Hélène , Grosse Guido , Harris Lorna I. , Heffernan Liam , Helbig Manuel , Hugelius Gustaf , Hutchins Ryan , Juutinen Sari , Lara Mark J. , Malhotra Avni , Manies Kristen , McGuire A. David , Natali Susan M. , O'Donnell Jonathan A. , Parmentier Frans-Jan W. , Räsänen Aleksi , Schädel Christina , Sonnentag Oliver , Strack Maria , Tank Suzanne E. , Treat Claire , Varner Ruth K. , Virtanen Tarmo , Warren Rebecca K. , Watts Jennifer D. 
Published: November 2021

Abstract. Methane emissions from boreal and arctic wetlands, lakes, and rivers are
expected to increase in response to warming and associated permafrost thaw.
However, the lack of appropriate land cover datasets for scaling
field-measured methane emissions to circumpolar scales has contributed to a
large uncertainty for our understanding of present-day and future methane
emissions. Here we present the Boreal–Arctic Wetland and Lake Dataset
(BAWLD), a land cover dataset based on an expert assessment, extrapolated
using random forest modelling from available spatial datasets of climate,
topography, soils, permafrost conditions, vegetation, wetlands, and surface
water extents and dynamics. In BAWLD, we estimate the fractional coverage of
five wetland, seven lake, and three river classes within 0.5 × 0.5∘ grid cells that cover the northern boreal and tundra biomes
(17 % of the global land surface). Land cover classes were defined using
criteria that ensured distinct methane emissions among classes, as indicated
by a co-developed comprehensive dataset of methane flux observations. In
BAWLD, wetlands occupied 3.2 × 106 km2 (14 % of domain)
with a 95 % confidence interval between 2.8 and 3.8 × 106 km2. Bog, fen, and permafrost bog were the most abundant wetland
classes, covering ∼ 28 % each of the total wetland area,
while the highest-methane-emitting marsh and tundra wetland classes occupied
5 % and 12 %, respectively. Lakes, defined to include all lentic open-water
ecosystems regardless of size, covered 1.4 × 106 km2
(6 % of domain). Low-methane-emitting large lakes (>10 km2) and glacial lakes jointly represented 78 % of the total lake
area, while high-emitting peatland and yedoma lakes covered 18 % and 4 %,
respectively. Small (<0.1 km2) glacial, peatland, and yedoma
lakes combined covered 17 % of the total lake area but contributed
disproportionally to the overall spatial uncertainty in lake area with a
95 % confidence interval between 0.15 and 0.38 × 106 km2. Rivers and streams were estimated to cover 0.12  × 106 km2 (0.5 % of domain), of which 8 % was associated with
high-methane-emitting headwaters that drain organic-rich landscapes.
Distinct combinations of spatially co-occurring wetland and lake classes
were identified across the BAWLD domain, allowing for the mapping of
“wetscapes” that have characteristic methane emission magnitudes and
sensitivities to climate change at regional scales. With BAWLD, we provide a
dataset which avoids double-accounting of wetland, lake, and river extents
and which includes confidence intervals for each land cover class. As such,
BAWLD will be suitable for many hydrological and biogeochemical modelling
and upscaling efforts for the northern boreal and arctic region, in
particular those aimed at improving assessments of current and future
methane emissions. Data are freely available at
https://doi.org/10.18739/A2C824F9X (Olefeldt et al., 2021).