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Abstract. The continued warming of the Arctic could release vast stores of carbon into the atmosphere from high-latitude ecosystems, especially from thawingpermafrost. 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 concentration enhancements from aircraft and a tall tower, which integrate ecosystem exchange over large regions, with bottom-up observed CO2 fluxes from tundraenvironments and found that the Alaska North Slope is not a consistent net source nor net sink of CO2 to the atmosphere (ranging from −6 to+6 Tg C 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 of the 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 in 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 are important to accurately project the Earth system response to future warming.
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Atmospheric measurements of the terrestrial O 2 : CO 2 exchange ratio of a midlatitude forest
Abstract. Measurements of atmospheric O2have been used to quantify large-scalefluxes of carbon between the oceans, atmosphere and landsince 1992 (Keeling and Shertz, 1992). With time,datasets have grownand estimates of fluxes have become more precise, buta key uncertainty in these calculations is the exchange ratioof O2and CO2 associated with the net land carbon sink(αB). We present measurements of atmosphericO2 and CO2 collected overa 6-year period from a mixed deciduous forest in centralMassachusetts, USA (42.537∘ N, 72.171∘ W).Using a differential fuel-cell-basedinstrument for O2 and a nondispersive infrared analyzer forCO2, we analyzed airstreams collected within and ∼5 m above the forest canopy. Averaged over the entireperiod of record, we find these two species covary with a slope of -1.081±0.007 mol of O2 per mole ofCO2 (themean and standard error of 6 h periods).If we limit the data to values collected on summer days within thecanopy, the slope is -1.03±0.01. These are the conditions in whichbiotic influences are most likely to dominate.This result is significantlydifferent from the value of −1.1 widely used in O2-basedcalculations of the global carbon budget, suggesting the need for a deeper understanding of the exchange ratios of the various fluxes and pools comprising the net sink.
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- Award ID(s):
- 1832210
- PAR ID:
- 10127757
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 19
- Issue:
- 13
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 8687 to 8701
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-19-8687-2019
