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  • 1. Chadburn, Sarah E.
    et al.
    Krinner, Gerhard
    Porada, Philipp
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Bartsch, Annett
    Beer, Christian
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Belelli Marchesini, Luca
    Boike, Julia
    Ekici, Altug
    Elberling, Bo
    Friborg, Thomas
    Hugelius, Gustaf
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Johansson, Margareta
    Kuhry, Peter
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kutzbach, Lars
    Langer, Moritz
    Lund, Magnus
    Parmentier, Frans-Jan W.
    Peng, Shushi
    Van Huissteden, Ko
    Wang, Tao
    Westermann, Sebastian
    Zhu, Dan
    Burke, Eleanor J.
    Carbon stocks and fluxes in the high latitudes: using site-level data to evaluate Earth system models2017In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 14, no 22, p. 5143-5169Article in journal (Refereed)
    Abstract [en]

    It is important that climate models can accurately simulate the terrestrial carbon cycle in the Arctic due to the large and potentially labile carbon stocks found in permafrost-affected environments, which can lead to a positive climate feedback, along with the possibility of future carbon sinks from northward expansion of vegetation under climate warming. Here we evaluate the simulation of tundra carbon stocks and fluxes in three land surface schemes that each form part of major Earth system models (JSBACH, Germany; JULES, UK; ORCHIDEE, France). We use a site-level approach in which comprehensive, high-frequency datasets allow us to disentangle the importance of different processes. The models have improved physical permafrost processes and there is a reasonable correspondence between the simulated and measured physical variables, including soil temperature, soil moisture and snow. We show that if the models simulate the correct leaf area index (LAI), the standard C3 photosynthesis schemes produce the correct order of magnitude of carbon fluxes. Therefore, simulating the correct LAI is one of the first priorities. LAI depends quite strongly on climatic variables alone, as we see by the fact that the dynamic vegetation model can simulate most of the differences in LAI between sites, based almost entirely on climate inputs. However, we also identify an influence from nutrient limitation as the LAI becomes too large at some of the more nutrient-limited sites. We conclude that including moss as well as vascular plants is of primary importance to the carbon budget, as moss contributes a large fraction to the seasonal CO2 flux in nutrient-limited conditions. Moss photosynthetic activity can be strongly influenced by the moisture content of moss, and the carbon uptake can be significantly different from vascular plants with a similar LAI. The soil carbon stocks depend strongly on the rate of input of carbon from the vegetation to the soil, and our analysis suggests that an improved simulation of photosynthesis would also lead to an improved simulation of soil carbon stocks. However, the stocks are also influenced by soil carbon burial (e.g. through cryoturbation) and the rate of heterotrophic respiration, which depends on the soil physical state. More detailed below-ground measurements are needed to fully evaluate biological and physical soil processes. Furthermore, even if these processes are well modelled, the soil carbon profiles cannot resemble peat layers as peat accumulation processes are not represented in the models. Thus, we identify three priority areas for model development: (1) dynamic vegetation including (a) climate and (b) nutrient limitation effects; (2) adding moss as a plant functional type; and an (3) improved vertical profile of soil carbon including peat processes.

  • 2. Jammet, Mathilde
    et al.
    Dengel, Sigrid
    Kettner, Ernesto
    Parmentier, Frans-Jan W.
    Wik, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Friborg, Thomas
    Year-round CH4 and CO2 flux dynamics in two contrasting freshwater ecosystems of the subarctic2017In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 14, no 22, p. 5189-5216Article in journal (Refereed)
    Abstract [en]

    Lakes and wetlands, common ecosystems of the high northern latitudes, exchange large amounts of the climate-forcing gases methane (CH4) and carbon dioxide (CO2) with the atmosphere. The magnitudes of these fluxes and the processes driving them are still uncertain, particularly for subarctic and Arctic lakes where direct measurements of CH4 and CO2 emissions are often of low temporal resolution and are rarely sustained throughout the entire year. Using the eddy covariance method, we measured surface-atmosphere exchange of CH4 and CO2 during 2.5 years in a thawed fen and a shallow lake of a subarctic peatland complex. Gas exchange at the fen exhibited the expected seasonality of a subarctic wetland with maximum CH4 emissions and CO2 uptake in summer, as well as low but continuous emissions of CH4 and CO2 throughout the snow-covered winter. The seasonality of lake fluxes differed, with maximum CO2 and CH4 flux rates recorded at spring thaw. During the ice-free seasons, we could identify surface CH4 emissions as mostly ebullition events with a seasonal trend in the magnitude of the release, while a net CO2 flux indicated photosynthetic activity. We found correlations between surface CH4 emissions and surface sediment temperature, as well as between diel CO2 uptake and diel solar input. During spring, the breakdown of thermal stratification following ice thaw triggered the degassing of both CH4 and CO2. This spring burst was observed in 2 consecutive years for both gases, with a large inter-annual variability in the magnitude of the CH4 degassing. On the annual scale, spring emissions converted the lake from a small CO2 sink to a CO2 source: 80% of total annual carbon emissions from the lake were emitted as CO2. The annual total carbon exchange per unit area was highest at the fen, which was an annual sink of carbon with respect to the atmosphere. Continuous respiration during the winter partly counteracted the fen summer sink by accounting for, as both CH4 and CO2, 33% of annual carbon exchange. Our study shows (1) the importance of overturn periods (spring or fall) for the annual CH4 and CO2 emissions of northern lakes, (2) the significance of lakes as atmospheric carbon sources in subarctic landscapes while fens can be a strong carbon sink, and (3) the potential for ecosystem-scale eddy covariance measurements to improve the understanding of short-term processes driving lake-atmosphere exchange of CH4 and CO2.

  • 3. 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.

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