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. Field measurements have shown that cold-season methane(CH4) and carbon dioxide (CO2) emissions contribute a substantialportion to the annual net carbon emissions in permafrost regions. However,most earth system land models do not accurately reproduce cold-seasonCH4 and CO2 emissions, especially over the shoulder (i.e., thawingand freezing) seasons. Here we use the Energy Exascale Earth System Model(E3SM) land model version 1 (ELMv1-ECA) to tackle this challenge and fillthe knowledge gap of how cold-season CH4 and CO2 emissionscontribute to the annual totals at Alaska Arctic tundra sites. Specifically,we improved the ELMv1-ECA soil water phase-change scheme, environmentalcontrols on microbial activity, and the methane module. Results demonstrate that both soil temperature and the duration ofzero-curtain periods (i.e., the fall period when soil temperatures lingeraround 0 ∘C) simulated by the updated ELMv1-ECA were greatlyimproved; e.g., the mean absolute error (MAE) in zero-curtain durations at12 cm depth was reduced by 62 % on average. Furthermore, the MAEs ofsimulated cold-season carbon emissions at three tundra sites were improvedby 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 entirecold season (September to May) in the model, compared with around 49.4 %(43 %–58 %) in observations. From 1950 to 2017, both CO2 emissionsduring the zero-curtain period and during the entire cold season showedincreasing trends, for example, of 0.17 and 0.36 gC m−2 yr−1 at Atqasuk. This study highlights the importance ofzero-curtain periods in facilitating cold-season CH4 and CO2 emissions fromtundra ecosystems.

the cryosphere

Improved ELMv1-ECA simulations of zero-curtain periods and cold-season CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; emissions at Alaskan Arctic tundra sites