Wetlands are the single largest natural source of atmospheric methane (CH4), a greenhouse gas, and occur extensively in the northern hemisphere. Large discrepancies remain between bottom-up and top-down estimates of northern CH4 emissions. To explore whether these discrepancies are due to poor representation of non-growing season CH4 emissions, we synthesized non-growing season and annual CH4 flux measurements from temperate, boreal, and tundra wetlands and uplands. Median non-growing season wetland emissions ranged from 0.9 g m-2 in bogs to 5.2 g m-2 in marshes and were dependent on moisture, vegetation, and permafrost. Annual wetland emissions ranged from 0.9 g m-2 y-1 in tundra bogs to 78 g m-2 y-1 in temperate marshes. Uplands varied from CH4 sinks to CH4 sources with a median annual flux of 0.0 ± 0.2 g m-2 y-1. The measured fraction of annual CH4 emissions during the non-growing season (observed: 13 to 47%) was significantly larger than was predicted by two process-based model ensembles, especially between 40-60º N (modeled: 4 to 17%). Constraining the model ensembles with the measured non-growing fraction increased total non-growing season and annual CH4 emissions. Using this constraint, the modeled non-growing season wetland CH4 flux from >40° north was 6.1 ± 1.5 Tg y-1, three times greater than the non-growing season emissions of the unconstrained model ensemble. The annual wetland CH4 flux was 37 ± 7 Tg y-1 from the data-constrained model ensemble, 25% larger than the unconstrained ensemble. Considering non-growing season processes is critical for accurately estimating CH4 emissions from high latitude ecosystems, and necessary for constraining the role of wetland emissions in a warming climate. This dataset contains the synthesis of measured flux data from the study.
