Articles | Volume 16, issue 12
Atmos. Chem. Phys., 16, 7943–7956, 2016
Atmos. Chem. Phys., 16, 7943–7956, 2016

Research article 30 Jun 2016

Research article | 30 Jun 2016

Role of OH variability in the stalling of the global atmospheric CH4 growth rate from 1999 to 2006

Joe McNorton1,2, Martyn P. Chipperfield1,2, Manuel Gloor3, Chris Wilson1,2, Wuhu Feng1,4, Garry D. Hayman5, Matt Rigby6, Paul B. Krummel7, Simon O'Doherty6, Ronald G. Prinn8, Ray F. Weiss9, Dickon Young6, Ed Dlugokencky10, and Steve A. Montzka10 Joe McNorton et al.
  • 1School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
  • 2National Centre for Earth Observation, University of Leeds, Leeds, LS2 9JT, UK
  • 3School of Geography, University of Leeds, Leeds, LS2 9JT, UK
  • 4National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK
  • 5Centre for Ecology and Hydrology, Wallingford, UK
  • 6School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK
  • 7CSIRO Oceans and Atmosphere Flagship, Aspendale, Victoria, Australia
  • 8Center for Global Change Science, Massachusetts Instititute of Technology, Cambridge, MA 02139, USA
  • 9Scripps Institution of Oceanography, University of California, San Diego, CA 92093, USA
  • 10National Oceanic and Atmospheric Administration, Boulder, CO, USA

Abstract. The growth in atmospheric methane (CH4) concentrations over the past 2 decades has shown large variability on a timescale of several years. Prior to 1999 the globally averaged CH4 concentration was increasing at a rate of 6.0 ppb yr−1, but during a stagnation period from 1999 to 2006 this growth rate slowed to 0.6 ppb yr−1. From 2007 to 2009 the growth rate again increased to 4.9 ppb yr−1. These changes in growth rate are usually ascribed to variations in CH4 emissions. We have used a 3-D global chemical transport model, driven by meteorological reanalyses and variations in global mean hydroxyl (OH) concentrations derived from CH3CCl3 observations from two independent networks, to investigate these CH4 growth variations. The model shows that between 1999 and 2006 changes in the CH4 atmospheric loss contributed significantly to the suppression in global CH4 concentrations relative to the pre-1999 trend. The largest factor in this is relatively small variations in global mean OH on a timescale of a few years, with minor contributions of atmospheric transport of CH4 to its sink region and of atmospheric temperature. Although changes in emissions may be important during the stagnation period, these results imply a smaller variation is required to explain the observed CH4 trends. The contribution of OH variations to the renewed CH4 growth after 2007 cannot be determined with data currently available.

Short summary
Methane (CH4) is an important greenhouse gas. The growth of atmospheric CH4 stalled from 1999 to 2006, with current explanations focussed mainly on changing surface fluxes. We combine models with observations and meteorological data to assess the atmospheric contribution to CH4 changes. We find that variations in mean atmospheric hydroxyl concentration can explain part of the stall in growth. Our study highlights the role of multi-annual variability in atmospheric chemistry in global CH4 trends.
Final-revised paper