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Discussion papers
https://doi.org/10.5194/acp-2019-529
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-529
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 12 Jul 2019

Submitted as: research article | 12 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Investigation of the global methane budget over 1980–2017 using GFDL-AM4.1

Jian He1,2, Vaishali Naik2, Larry W. Horowitz2, Ed Dlugokencky3, and Kirk Thoning3 Jian He et al.
  • 1Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey, USA
  • 2NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA
  • 3NOAA Earth System Research Laboratory, Boulder, Colorado, USA

Abstract. Changes in atmospheric methane abundance have implications for both chemistry and climate as methane is both a strong greenhouse gas and an important precursor for tropospheric ozone. A better understanding of the drivers of trends and variability in methane abundance over the recent past is therefore critical for building confidence in projections of future methane levels. In this work, the representation of methane in the atmospheric chemistry model AM4.1 is improved by optimizing total methane emissions (to an annual mean of 576 ± 32 Tg yr−1) to match surface observations over 1980–2017. The simulations with optimized global emissions are in general able to capture the observed global trend, variability, seasonal cycle, and latitudinal gradient of methane. Simulations with different emission adjustments suggest that increases in methane sources (mainly from energy and waste sectors) balanced by increases in methane sinks (mainly due to increases in OH levels) lead to methane stabilization (with an imbalance of 5 Tg yr−1) during 1999–2006, and that increases in methane sources combined with little change in sinks (despite small decreases in OH levels) during 2007–2012 lead to renewed methane growth (with an imbalance of 14 Tg yr−1 for 2007–2017). Compared to 1999–2006, both methane emissions and sinks are greater (by 31 Tg yr−1 and 22 Tg yr−1, respectively) during 2007–2017. Our results also indicate that the energy sector is more likely a major contributor to the methane renewed growth after 2006 than wetland, as increases in wetland emissions alone are not able to explain the renewed methane growth with constant anthropogenic emissions. In addition, a significant increase in wetland emissions would be required starting in 2006, if anthropogenic emissions declined, for wetland emissions to drive renewed growth in methane, which is a less likely scenario. Simulations with varying OH levels indicate that 1 % change in OH levels could lead to an annual mean of ~ 4 Tg yr−1 difference in the optimized emissions and 0.08 year difference in the estimated tropospheric methane lifetime. Continued increases in methane emissions along with decreases in tropospheric OH concentrations during 2008–2015 prolong methane lifetime and therefore amplify the response of methane concentrations to emission changes. Uncertainties still exist in the partitioning of emissions among individual sources and regions.

Jian He et al.
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Short summary
In this work, methane representation in AM4.1 is improved by optimizing total CH4 emissions to match surface observations. We find increases in CH4 sources balanced by increases in sinks lead to methane stabilization during 1999–2006, and increases in CH4 sources combined with little change in sinks during 2007–2012 lead to renewed methane growth. Increases in CH4 emissions and decreases in OH levels during 2008–2015 prolong methane lifetime and amplify methane response to emission changes.
In this work, methane representation in AM4.1 is improved by optimizing total CH4 emissions to...
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