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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Jan 2018

Research article | 08 Jan 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Detecting changes in Arctic methane emissions: limitations of the inter-polar difference of atmospheric mole fractions

Oscar B. Dimdore-Miles1, Paul I. Palmer1, and Lori P. Bruhwiler2 Oscar B. Dimdore-Miles et al.
  • 1School of GeoSciences, University of Edinburgh, Edinburgh, UK
  • 2National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Boulder, Colorado, USA

Abstract. We consider the utility of the annual inter-polar difference (IPD) as a metric for changes in Arctic emission of methane (CH4). The IPD has been previously defined as the difference between weighted annual means of CH4 mole fraction data collected at polar stations (−53°>latitude>53°). This subtraction approach (IPDΔ) implicitly assumes that extra-polar CH4 emissions arrive within the same calendar year at both poles. Using an analytic approach we show that a comprehensive description of the IPD includes terms corresponding to the atmospheric transport of air masses from lower latitudes to the polar regions. We show the importance of these transport flux terms in understanding the IPD using idealized numerical experiments with the TM5 global 3-D atmospheric chemistry transport model run from 1980 to 2010. A northern mid-latitude pulse in January 1990, which increases prior emission distributions, arrives at the Arctic with a higher mixing ratio and ≃12 months earlier than at the Antarctic. The perturbation at the poles subsequently decays with an e-folding lifetime of ≃4 years. A similarly timed pulse emitted from the tropics arrives with a higher value at the Antarctic ≃11 months earlier than at the Arctic. This perturbation decays with an e-folding lifetime of ≃7 years. These simulations demonstrate that the assumption of symmetric transport of extra-polar emissions to the poles is not realistic, resulting in considerable IPDΔ variations due to variations in emissions and atmospheric transport. We assess how well the annual IPD can detect a constant annual growth rate of Arctic emissions for three scenarios, 0.5%, 1%, and 2%, superimposed on signals from lower latitudes, including random noise. We find that it can take up to 16 years to detect the smallest prescribed trend in Arctic emissions at the 95% confidence level. Scenarios with higher, but likely unrealistic, growth in Arctic emissions are detected in less than a decade. We argue that a more reliable measurement-driven IPD metric would include data collected from all latitudes, emphasizing the importance of maintaining a global monitoring network to observe decadal changes in atmospheric greenhouse gases.

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Oscar B. Dimdore-Miles et al.
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Oscar B. Dimdore-Miles et al.
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Publications Copernicus
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Short summary
The Arctic is experiencing warming trends higher than the global mean. Arctic ecosystems are a large store of carbon. As the soil organic carbon thaws and decomposes some fraction of this store will eventually be released to the atmosphere as methane. We show that a previously-used measurement-based metric to identify changes in Arctic methane emissions does not reliably quantify these changes because it neglects the effect of atmospheric transport. A better metric will combine data and models.
The Arctic is experiencing warming trends higher than the global mean. Arctic ecosystems are a...