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  • 1. López-Blanco, Efrén
    et al.
    Exbrayat, Jean-Francois
    Lund, Magnus
    Christensen, Torben R.
    Tamstorf, Mikkel P.
    Slevin, Darren
    Hugelius, Gustaf
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Bloom, Anthony A.
    Williams, Mathew
    Evaluation of terrestrial pan-Arctic carbon cycling using a data-assimilation system2019In: Earth System Dynamics, ISSN 2190-4979, E-ISSN 2190-4987, Vol. 10, no 2, p. 233-255Article in journal (Refereed)
    Abstract [en]

    There is a significant knowledge gap in the current state of the terrestrial carbon (C) budget. Recent studies have highlighted a poor understanding particularly of C pool transit times and of whether productivity or biomass dominate these biases. The Arctic, accounting for approximately 50 % of the global soil organic C stocks, has an important role in the global C cycle. Here, we use the CARbon DAta MOdel (CARDAMOM) data-assimilation system to produce pan-Arctic terrestrial C cycle analyses for 2000-2015. This approach avoids using traditional plant functional type or steady-state assumptions. We integrate a range of data (soil organic C, leaf area index, biomass, and climate) to determine the most likely state of the high-latitude C cycle at a 1 degrees x 1 degrees resolution and also to provide general guidance about the controlling biases in transit times. On average, CARDAMOM estimates regional mean rates of photosynthesis of 565 g C m(-2) yr (-1) (90 % confidence interval between the 5th and 95th percentiles: 428, 741), autotrophic respiration of 270 g Cm-2 yr(-1) (182, 397) and heterotrophic respiration of 219 g Cm-2 yr(-1) (31, 1458), suggesting a pan-Arctic sink of -67 (-287, 1160) g Cm-2 yr(-1), weaker in tundra and stronger in taiga. However, our confidence intervals remain large (and so the region could be a source of C), reflecting uncertainty assigned to the regional data products. We show a clear spatial and temporal agreement between CARDAMOM analyses and different sources of assimilated and independent data at both pan-Arctic and local scales but also identify consistent biases between CARDAMOM and validation data. The assimilation process requires clearer error quantification for leaf area index (LAI) and biomass products to resolve these biases. Mapping of vegetation C stocks and change over time and soil C ages linked to soil C stocks is required for better analytical constraint. Comparing CARDAMOM analyses to global vegetation models (GVMs) for the same period, we conclude that transit times of vegetation C are inconsistently simulated in GVMs due to a combination of uncertainties from productivity and biomass calculations. Our findings highlight that GVMs need to focus on constraining both current vegetation C stocks and net primary production to improve a process-based understanding of C cycle dynamics in the Arctic.

  • 2. Natali, Susan M.
    et al.
    Watts, Jennifer D.
    Rogers, Brendan M.
    Potter, Stefano
    Ludwig, Sarah M.
    Selbmann, Anne-Katrin
    Sullivan, Patrick F.
    Abbott, Benjamin W.
    Arndt, Kyle A.
    Birch, Leah
    Björkman, Mats P.
    Bloom, A. Anthony
    Celis, Gerardo
    Christensen, Torben R.
    Christiansen, Casper T.
    Commane, Roisin
    Cooper, Elisabeth J.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Czimczik, Claudia
    Davydov, Sergey
    Du, Jinyang
    Egan, Jocelyn E.
    Elberling, Bo
    Euskirchen, Eugenie S.
    Friborg, Thomas
    Genet, Hélène
    Göckede, Mathias
    Goodrich, Jordan P.
    Grogan, Paul
    Helbig, Manuel
    Jafarov, Elchin E.
    Jastrow, Julie D.
    Kalhori, Aram A. M.
    Kim, Yongwon
    Kimball, John S.
    Kutzbach, Lars
    Lara, Mark J.
    Larsen, Klaus S.
    Lee, Bang-Yong
    Liu, Zhihua
    Loranty, Michael M.
    Lund, Magnus
    Lupascu, Massimo
    Madani, Nima
    Malhotra, Avni
    Matamala, Roser
    McFarland, Jack
    McGuire, A. David
    Michelsen, Anders
    Minions, Christina
    Oechel, Walter C.
    Olefeldt, David
    Parmentier, Frans-Jan W.
    Pirk, Norbert
    Poulter, Ben
    Quinton, William
    Rezanezhad, Fereidoun
    Risk, David
    Sachs, Torsten
    Schaefer, Kevin
    Schmidt, Niels M.
    Schuur, Edward A. G.
    Semenchuk, Philipp R.
    Shaver, Gaius
    Sonnentag, Oliver
    Starr, Gregory
    Treat, Claire C.
    Waldrop, Mark P.
    Wang, Yihui
    Welker, Jeffrey
    Wille, Christian
    Xu, Xiaofeng
    Zhang, Zhen
    Zhuang, Qianlai
    Zona, Donatella
    Large loss of CO2 in winter observed across the northern permafrost region2019In: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 9, no 11, p. 852-857Article in journal (Refereed)
    Abstract [en]

    Recent warming in the Arctic, which has been amplified during the winter(1-3), greatly enhances microbial decomposition of soil organic matter and subsequent release of carbon dioxide (CO2)(4). However, the amount of CO2 released in winter is not known and has not been well represented by ecosystem models or empirically based estimates(5,6). Here we synthesize regional in situ observations of CO2 flux from Arctic and boreal soils to assess current and future winter carbon losses from the northern permafrost domain. We estimate a contemporary loss of 1,662 TgC per year from the permafrost region during the winter season (October-April). This loss is greater than the average growing season carbon uptake for this region estimated from process models (-1,032 TgC per year). Extending model predictions to warmer conditions up to 2100 indicates that winter CO2 emissions will increase 17% under a moderate mitigation scenario-Representative Concentration Pathway 4.5-and 41% under business-as-usual emissions scenario-Representative Concentration Pathway 8.5. Our results provide a baseline for winter CO2 emissions from northern terrestrial regions and indicate that enhanced soil CO2 loss due to winter warming may offset growing season carbon uptake under future climatic conditions.

  • 3. Voigt, Carolina
    et al.
    Marushchak, Maija E.
    Mastepanov, Mikhail
    Lamprecht, Richard E.
    Christensen, Torben R.
    Dorodnikov, Maxim
    Jackowicz-Korczynski, Marcin
    Lindgren, Amelie
    Stockholm University, Faculty of Science, Department of Physical Geography. Lund University, Sweden.
    Lohila, Annalea
    Nykänen, Hannu
    Oinonen, Markku
    Oksanen, Timo
    Palonen, Vesa
    Treat, Claire C.
    Martikainen, Pertti J.
    Biasi, Christina
    Ecosystem carbon response of an Arctic peatland to simulated permafrost thaw2019In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 25, no 5, p. 1746-1764Article in journal (Refereed)
    Abstract [en]

    Permafrost peatlands are biogeochemical hot spots in the Arctic as they store vast amounts of carbon. Permafrost thaw could release part of these long-term immobile carbon stocks as the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4) to the atmosphere, but how much, at which time-span and as which gaseous carbon species is still highly uncertain. Here we assess the effect of permafrost thaw on GHG dynamics under different moisture and vegetation scenarios in a permafrost peatland. A novel experimental approach using intact plant-soil systems (mesocosms) allowed us to simulate permafrost thaw under near-natural conditions. We monitored GHG flux dynamics via high-resolution flow-through gas measurements, combined with detailed monitoring of soil GHG concentration dynamics, yielding insights into GHG production and consumption potential of individual soil layers. Thawing the upper 10-15 cm of permafrost under dry conditions increased CO2 emissions to the atmosphere (without vegetation: 0.74 +/- 0.49 vs. 0.84 +/- 0.60 g CO2-C m(-2) day(-1); with vegetation: 1.20 +/- 0.50 vs. 1.32 +/- 0.60 g CO2-C m(-2) day(-1), mean +/- SD, pre- and post-thaw, respectively). Radiocarbon dating (C-14) of respired CO2, supported by an independent curve-fitting approach, showed a clear contribution (9%-27%) of old carbon to this enhanced post-thaw CO2 flux. Elevated concentrations of CO2, CH4, and dissolved organic carbon at depth indicated not just pulse emissions during the thawing process, but sustained decomposition and GHG production from thawed permafrost. Oxidation of CH4 in the peat column, however, prevented CH4 release to the atmosphere. Importantly, we show here that, under dry conditions, peatlands strengthen the permafrost-carbon feedback by adding to the atmospheric CO2 burden post-thaw. However, as long as the water table remains low, our results reveal a strong CH4 sink capacity in these types of Arctic ecosystems pre- and post-thaw, with the potential to compensate part of the permafrost CO2 losses over longer timescales.

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