Seasonal and interannual variations in boreal Alaskan methane and carbon dioxide fluxes using atmospheric measurements from the CARVE tower

Abstract:

Northern high-latitude carbon sources and sinks, including those resulting from degrading permafrost, are expected to be sensitive to the rapidly warming climate in the coming decades. Because the daytime lower atmosphere integrates surface fluxes over large (~500-1000 km) scales, atmospheric monitoring of CO₂ and CH₄ mole fractions is a promising method for the detection of change throughout boreal Alaska. Here we use greenhouse gas measurements from a NOAA tower 17 km north of Fairbanks AK, established as part of NASA’s Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE), to investigate regional fluxes of CO₂ and CH₄ for 2012-2014. CARVE was designed to use aircraft and surface observations to better understand and quantify the sensitivity of Alaskan carbon fluxes to climate variability over several years. We use high-resolution meteorological fields from the Polar Weather Research Forecast (WRF) model coupled with the Stochastic Time Inverted Lagrangian Transport (STILT) model, along with the Polar Vegetation Photosynthesis and Respiration Model (PVPRM), to infer fluxes of CO₂ in boreal Alaska using the tower observations, which are sensitive to large areas of central Alaska. We show that simulated mole fractions agree remarkably well with tower observations, indicating that the model represents the meteorology of the region quite well, and that the PVPRM flux magnitudes and spatial distribution are largely correct. CO₂ flux signals at the tower are larger than predicted by the model, however, with significant respiration occurring in the fall that is not captured by the model. Using the same WRF-STILT model, we find that average CH₄ fluxes in boreal Alaska are lower than those estimated by Chang et al. (2014) over all of Alaska, with emissions persisting during some wintertime periods, augmenting those observed during the summer and fall. The presence of significant fall and winter CO₂ and CH₄ fluxes underscores the need for year-round in situ observations to quantify changes in boreal Alaskan annual carbon balance.