| Summary: | Arctic ecosystems are warming at a rate twice that of the global mean with winters warming faster than the rest of the year. With short, relatively productive summers and freezing cold winters, the Arctic has been a long-term carbon sink, but that is changing. Until recently, measurements scarcely existed outside of the summer growing season and winters have been assumed negligible. But as data coverage increases, estimates show the cold season may release 1662 Tg of carbon per year, greater than summer uptake of 1,032 Tg of carbon based on process models. Here, I analyze dynamics that make up year-round carbon budget using a multi-faceted approach including eddy covariance and satellite imagery. I show that soil is staying unfrozen for longer periods in the fall (2.6 ± 0.5 days y-1 from 2001 to 2017), correlated to methane enhancements on the Alaska North Slope (0.79 ± 0.18 ppb CH4 day-1 unfrozen soil). This long- term temperature trend and elevated methane could be related to vegetation changes as well given the carbon dynamics are further controlled by vegetation communities. To address this, I analyzed fine scale trends of vegetation productivity showing an increase in NDVI over a similar timescale from 2002-2016 (τ = 0.65, p = 0.01, NDVI increase of 0.01 yr-1), which was correlated to earlier thaws (R2 = 0.77, F = 21.5, p < 0.001). As the freeze thaw cycle affected fall emissions and summer growth, I sought to understand how this process may impact the spring carbon balance. Therefore, I analyzed physical soil conditions in the freeze thaw cycle and found air pockets forming, allowing for rapid oxidation and warming by spring snowmelt. These conditions coincide with soil respiration that can offset up to 41% of summer carbon dioxide uptake. Overall, I show increased growth in the summer and increased emissions during the cold season and emphasize the importance of better understanding year-round dynamics to more accurately predict Arctic feedbacks and our planet’s future. University of California ...
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