Articles | Volume 13, issue 17
Biogeosciences, 13, 5043–5056, 2016
Biogeosciences, 13, 5043–5056, 2016

Research article 13 Sep 2016

Research article | 13 Sep 2016

A multi-scale comparison of modeled and observed seasonal methane emissions in northern wetlands

Xiyan Xu1, William J. Riley1, Charles D. Koven1, Dave P. Billesbach2, Rachel Y.-W. Chang3,4, Róisín Commane3, Eugénie S. Euskirchen5, Sean Hartery4, Yoshinobu Harazono6,7, Hiroki Iwata6,8, Kyle C. McDonald9,10, Charles E. Miller10, Walter C. Oechel11,12, Benjamin Poulter13, Naama Raz-Yaseef1, Colm Sweeney14,15, Margaret Torn1,16, Steven C. Wofsy3, Zhen Zhang13,17, and Donatella Zona11,18 Xiyan Xu et al.
  • 1Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
  • 2Biological System Engineering Department, University of Nebraska, Lincoln, Nebraska, USA
  • 3School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA
  • 4Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada
  • 5Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska, USA
  • 6International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, Alaska, USA
  • 7Graduate School of Life and Environmental Sciences, Osaka Prefecture University, Sakai, Osaka, Japan
  • 8Department of Environmental Sciences, Faculty of Science, Shinshu University, Matsumoto, Nagano, Japan
  • 9Department of Earth and Atmospheric Sciences, CUNY Environmental Crossroads Initiative and NOAA-CREST Institute, The City College of New York, City University of New York, New York, USA
  • 10Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
  • 11Global Change Research Group, Department of Biology, San Diego State University, San Diego, California, USA
  • 12Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes, MK7 6AA, UK
  • 13Department of Ecology, Montana State University, Bozeman, MT 59717, USA
  • 14Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80304, USA
  • 15NOAA Earth System Research Laboratory, Global Monitoring Division, Boulder, Colorado, USA
  • 16Energy and Resources Group, University of California-Berkeley, Berkeley, California, USA
  • 17Swiss Federal Research Institute WSL, Birmensdorf 8059, Switzerland
  • 18Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S102TN, UK

Abstract. Wetlands are the largest global natural methane (CH4) source, and emissions between 50 and 70° N latitude contribute 10–30 % to this source. Predictive capability of land models for northern wetland CH4 emissions is still low due to limited site measurements, strong spatial and temporal variability in emissions, and complex hydrological and biogeochemical dynamics. To explore this issue, we compare wetland CH4 emission predictions from the Community Land Model 4.5 (CLM4.5-BGC) with site- to regional-scale observations. A comparison of the CH4 fluxes with eddy flux data highlighted needed changes to the model's estimate of aerenchyma area, which we implemented and tested. The model modification substantially reduced biases in CH4 emissions when compared with CarbonTracker CH4 predictions. CLM4.5 CH4 emission predictions agree well with growing season (May–September) CarbonTracker Alaskan regional-level CH4 predictions and site-level observations. However, CLM4.5 underestimated CH4 emissions in the cold season (October–April). The monthly atmospheric CH4 mole fraction enhancements due to wetland emissions are also assessed using the Weather Research and Forecasting-Stochastic Time-Inverted Lagrangian Transport (WRF-STILT) model coupled with daily emissions from CLM4.5 and compared with aircraft CH4 mole fraction measurements from the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) campaign. Both the tower and aircraft analyses confirm the underestimate of cold-season CH4 emissions by CLM4.5. The greatest uncertainties in predicting the seasonal CH4 cycle are from the wetland extent, cold-season CH4 production and CH4 transport processes. We recommend more cold-season experimental studies in high-latitude systems, which could improve the understanding and parameterization of ecosystem structure and function during this period. Predicted CH4 emissions remain uncertain, but we show here that benchmarking against observations across spatial scales can inform model structural and parameter improvements.

Short summary
Wetlands are the largest global natural methane source. Peat-rich bogs and fens lying between 50°N and 70°N contribute 10–30% to this source. The predictive capability of the seasonal methane cycle can directly affect the estimation of global methane budget. We present multiscale methane seasonal emission by observations and modeling and find that the uncertainties in predicting the seasonal methane emissions are from the wetland extent, cold-season CH4 production and CH4 transport processes.
Final-revised paper