Methane Efflux Measured by Eddy Covariance in Alaskan Upland Tundra Undergoing Permafrost Degradation

Greenhouse gas emissions from thawing permafrost in arctic ecosystems may amplify global warming, yet estimates of the rate of carbon release, and the proportion of carbon released as methane (CH₄) or carbon dioxide (CO₂), have a high degree of uncertainty. There are many areas where no measurements exist, and few year-round or long-term records. Existing year-round eddy covariance measurements of arctic CH₄ fluxes suggest that nongrowing season emissions make up a significant proportion of tundra systems emissions on an annual basis. Here we present continuous CH₄ flux measurements made at Eight Mile Lake, an upland tundra ecosystem undergoing permafrost degradation in Interior Alaska. We found net CH₄ emissions throughout the year (1.2 ∓ 0.011 g C-CH₄ m²/yr) that made up 61% of total radiative forcing from annual C emissions (CO₂ and CH₄; 32.3 g C m²/yr) when taking into account the greenhouse warming potential of CH₄ relative to CO₂. Nongrowing season emissions accounted for 50% of the annual CH₄ budget, characterized by large pulse emissions. These were related to abrupt increases in air and shallow soil temperatures rather than consistent emissions during the zero curtain—a period of the fall/early winter season when subsurface soil temperatures remain near the 0 °C freezing point. Weekly growing season CH₄ emissions in 2016 and 2017 were significantly related with thaw depth, and the magnitude of CH₄ emissions between these seasons was proportional to the rate of active layer thaw throughout the season.