Abstract. Field measurements have shown that cold-season methane
(CH4) and carbon dioxide (CO2) emissions contribute a substantial
portion to the annual net carbon emissions in permafrost regions. However,
most earth system land models do not accurately reproduce cold-season
CH4 and CO2 emissions, especially over the shoulder (i.e., thawing
and freezing) seasons. Here we use the Energy Exascale Earth System Model
(E3SM) land model version 1 (ELMv1-ECA) to tackle this challenge and fill
the knowledge gap of how cold-season CH4 and CO2 emissions
contribute to the annual totals at Alaska Arctic tundra sites. Specifically,
we improved the ELMv1-ECA soil water phase-change scheme, environmental
controls on microbial activity, and the methane module. Results demonstrate that both soil temperature and the duration of
zero-curtain periods (i.e., the fall period when soil temperatures linger
around 0 ∘C) simulated by the updated ELMv1-ECA were greatly
improved; e.g., the mean absolute error (MAE) in zero-curtain durations at
12 cm depth was reduced by 62 % on average. Furthermore, the MAEs of
simulated cold-season carbon emissions at three tundra sites were improved
by 72 % and 70 % on average for CH4 and CO2, respectively.
Overall, CH4 emitted during the early cold season (September and October),
which often includes most of the zero-curtain period in Arctic tundra,
accounted for more than 50 % of the total emissions throughout the entire
cold season (September to May) in the model, compared with around 49.4 %
(43 %–58 %) in observations. From 1950 to 2017, both CO2 emissions
during the zero-curtain period and during the entire cold season showed
increasing trends, for example, of 0.17 and 0.36 gC m−2 yr−1 at Atqasuk. This study highlights the importance of
zero-curtain periods in facilitating cold-season CH4 and CO2 emissions from
tundra ecosystems.