Articles | Volume 8, issue 6
Biogeosciences, 8, 1643–1665, 2011
Biogeosciences, 8, 1643–1665, 2011

Research article 23 Jun 2011

Research article | 23 Jun 2011

Constraining global methane emissions and uptake by ecosystems

R. Spahni1,2, R. Wania2,3, L. Neef4,5, M. van Weele4, I. Pison6, P. Bousquet6, C. Frankenberg7,*, P. N. Foster2, F. Joos1, I. C. Prentice2,8,9, and P. van Velthoven4 R. Spahni et al.
  • 1Climate and Environmental Physics, Physics Institute, and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
  • 2QUEST, Department of Earth Sciences, University of Bristol, Bristol, UK
  • 3School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada
  • 4KNMI, Royal Netherlands Meteorological Institute, De Bilt, The Netherlands
  • 5Earth System modeling, Helmholtz Centre Potsdam, Potsdam, Germany
  • 6Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Gif-sur-Yvette, France
  • 7Netherlands Institute for Space Research, Utrecht, The Netherlands
  • 8Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia
  • 9Grantham Institute and Division of Biology, Imperial College, Ascot SL5 7PY, UK
  • *now at: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA

Abstract. Natural methane (CH4) emissions from wet ecosystems are an important part of today's global CH4 budget. Climate affects the exchange of CH4 between ecosystems and the atmosphere by influencing CH4 production, oxidation, and transport in the soil. The net CH4 exchange depends on ecosystem hydrology, soil and vegetation characteristics. Here, the LPJ-WHyMe global dynamical vegetation model is used to simulate global net CH4 emissions for different ecosystems: northern peatlands (45°–90° N), naturally inundated wetlands (60° S–45° N), rice agriculture and wet mineral soils. Mineral soils are a potential CH4 sink, but can also be a source with the direction of the net exchange depending on soil moisture content. The geographical and seasonal distributions are evaluated against multi-dimensional atmospheric inversions for 2003–2005, using two independent four-dimensional variational assimilation systems. The atmospheric inversions are constrained by the atmospheric CH4 observations of the SCIAMACHY satellite instrument and global surface networks. Compared to LPJ-WHyMe the inversions result in a~significant reduction in the emissions from northern peatlands and suggest that LPJ-WHyMe maximum annual emissions peak about one month late. The inversions do not put strong constraints on the division of sources between inundated wetlands and wet mineral soils in the tropics. Based on the inversion results we diagnose model parameters in LPJ-WHyMe and simulate the surface exchange of CH4 over the period 1990–2008. Over the whole period we infer an increase of global ecosystem CH4 emissions of +1.11 Tg CH4 yr−1, not considering potential additional changes in wetland extent. The increase in simulated CH4 emissions is attributed to enhanced soil respiration resulting from the observed rise in land temperature and in atmospheric carbon dioxide that were used as input. The long-term decline of the atmospheric CH4 growth rate from 1990 to 2006 cannot be fully explained with the simulated ecosystem emissions. However, these emissions show an increasing trend of +3.62 Tg CH4 yr−1 over 2005–2008 which can partly explain the renewed increase in atmospheric CH4 concentration during recent years.

Final-revised paper