Using atmospheric observations to quantify annual biogenic carbon dioxide fluxes on the Alaska North Slope

Abstract

Abstract. The continued warming of the Arctic could release vast stores of carbon into the atmosphere from high-latitude ecosystems, especially from thawing permafrost. Increasing uptake of carbon dioxide (CO2) by vegetation during longer growing seasons may partially offset such release of carbon. However, evidence of significant net annual release of carbon from site-level observations and model simulations across tundra ecosystems has been inconclusive. To address this knowledge gap, we combined top-down observations of atmospheric CO2 concentrations from aircraft and a tall tower, which integrate ecosystem exchange over large regions, with bottom-up observed CO2 fluxes from tundra environments and found that the Alaska North Slope is not a consistent net source or net sink of CO2 to the atmosphere (ranging from –6 to +6 TgC yr–1 for 2012–2017). Our analysis suggests that significant biogenic CO2 fluxes from unfrozen terrestrial soils, and likely inland waters, during the early cold season (September–December) are major factors in determining the net annual carbon balance of the North Slope, implying strong sensitivity to the rapidly warming freeze-up period. At the regional level, we find no evidence for previously reported large late cold season (January–April) CO2 emissions to the atmosphere during the study period. Despite the importance of the cold season CO2 emissions to the annual total, the interannual variability of the net CO2 flux is driven by the variability in growing season fluxes. During the growing season, the regional net CO2 flux is also highly sensitive to the distribution of tundra vegetation types throughout the North Slope. This study shows that quantification and characterization of year-round CO2 fluxes from the heterogeneous terrestrial and aquatic ecosystems in the Arctic using both site-level and atmospheric observations is important to accurately project the earth system response to future warming.

Carbon uptake in Eurasian boreal forests dominates the high‐latitude net ecosystem carbon budget

“…Based on our comparisons with the EC tower records, TCFM‐Arctic may be missing up to 78% (CO 2 ) and 40% (CH 4 ) of the episodic emissions from tundra environments during the shoulder seasons, which is considerable especially since field studies in northern Alaska (Arndt et al, 2020; Raz‐Yaseef et al, 2017) have found, at some locations, the amount of built‐up CO 2 released from soil during the spring snowmelt period can offset up to 41%–46% of summer CO 2 uptake. Additionally, regional studies (Byrne, Liu, et al, 2022; Commane et al, 2017; Schiferl et al, 2022) have documented a shift toward more respiration in autumn, that might increasingly offset summer GPP. This emphasizes the need for models to effectively account for the mechanisms driving shoulder season emissions.…”

“…In contrast to the boreal zone, a majority of the models evaluated here (including TCFM-Arctic) indicated the tundra domain as being, on average, neutral or a small source for NEE. However, adjusting the TCFM-Arctic tundra NEE budget to account for a potentially large underestimation of episodic CO 2 emissions during spring and autumn shoulder seasons (Arndt et al, 2020;Byrne, Baker, et al, 2022;Byrne, Liu, et al, 2022;Commane et al, 2017;Liu et al, 2022;Schiferl et al, 2022) and source (when soils are warm and less wet) (e.g., Euskirchen et al, 2014;Laine et al, 2019;Olefeldt et al, 2017;Rinne et al, 2020;Schulze et al, 1999) depending on water table depth and soil wetness.…”

“…Various efforts have been taken to quantify high‐latitude carbon budgets through field studies (e.g., Belshe et al, 2013; Euskirchen et al, 2017; Fox et al, 2008; Hashemi et al, 2021; Helbig et al, 2017; Ueyama et al, 2014), the statistical upscaling of in situ flux observations (Jung et al, 2020; Peltola et al, 2019; Virkkala et al, 2021), Earth system modeling (Birch et al, 2021; McGuire et al, 2012; Wang et al, 2019; White et al, 2001), and the combination of atmospheric observations and modeling (Ciais et al, 2010; Hartery et al, 2018; Schiferl et al, 2022; Sweeney et al, 2020; Tan et al, 2016; Welp et al, 2016). In situ field studies provide the most direct approach for understanding and monitoring ecosystem carbon status.…”

“…Some assessments of tundra indicate that arctic landscapes have been relatively near-neutral, varying between carbon sinks and sources (Belshe et al, 2013;Li et al, 2021;Virkkala et al, 2021). Other studies indicate trends toward net carbon source activity, especially in more recent years (Christensen et al, 2017;Commane et al, 2017;Natali et al, 2019;Schiferl et al, 2022;Watts et al, 2021). Additionally, boreal wetlands and many tundra environments are net emitters of methane (CH 4 ; Kuhn et al, 2021;Strom & Christensen, 2007;Turetsky et al, 2014), which has a global warming potential 28-36 times higher than CO 2 over a 100-year period (Balcombe et al, 2018;Forster et al, 2021) and likely impacts the net ecosystem carbon budget (NECB; CO 2 + CH 4 ).…”

Mapping Surface Organic Soil Properties in Arctic Tundra Using C-Band SAR Data

“…Our study did not consider temporal changes in SOC properties. However, out study period was relatively short, during which limited SOC changes are expected to occur in the undisturbed tundra [55]. On the other hand, we assembled three different in-situ SOC datasets, and the associated differences in the soil sampling process may add large uncertainties to the assembled surface (0-10 cm) SOC dataset.…”