Melt period methane emissions in northern high latitude wetlands are governed by the length of the period and presence of permafrost

Abstract. Northern high latitude wetlands are significant sources of methane, with emissions driven by seasonal soil freezing and thawing. To better understand the seasonality of northern high latitude methane emissions, we defined the melt period occurring in spring time using the remote sensing Soil Moisture and Ocean Salinity Freeze/Thaw data from 2011–2021. To estimate methane emissions in the northern high latitudes, we used the atmospheric inverse model CarbonTracker Europe-CH4. The melt period was defined for three permafrost zones and for a seasonally frozen non-permafrost region using two approaches: region-based, which considered climatological conditions of permafrost regions, and grid-based, which defines the melt period at a finer 1°×1° scale. The length and timing of the melt period varied significantly depending on the approach. The melt period generally occurred between March and June and was influenced by air temperature, with a negative correlation between the length and the mean temperature of the melt period. The longest melt period was in the non-permafrost zone and the shortest varied between the two methods. The melt period emissions were on average 1.83 Tg with the region-based approach and 0.45 Tg with the grid-based approach, the non-permafrost zone having the largest share of the emissions. They were largely dependent on the season’s length. Year-to-year variation was modest, within 15 % (region-based) and 23 % (grid-based) of average emissions, and there was also no trend during the study period. Our dual-method approach allows for robust comparison with both large-scale regional studies and localized site-level research.