Preprints
https://doi.org/10.5194/acp-2021-736
https://doi.org/10.5194/acp-2021-736

  31 Aug 2021

31 Aug 2021

Review status: this preprint is currently under review for the journal ACP.

Towards reconstructing the Arctic atmospheric methane history over the 20th century: measurement and modeling results for the NGRIP firn

Taku Umezawa1, Satoshi Sugawara2, Kenji Kawamura3,4,5, Ikumi Oyabu3, Stephen J. Andrews1,6, Takuya Saito1, Shuji Aoki7, and Takakiyo Nakazawa7 Taku Umezawa et al.
  • 1National Institute for Environmental Studies, Tsukuba, Japan
  • 2Miyagi University of Education, Sendai, Japan
  • 3National Institute of Polar Research, Tokyo, Japan
  • 4Department of Polar Science, The Graduate University of Advanced Studies (SOKENDAI), Tokyo, Japan
  • 5Japan Agency for Marine Science and Technology, Yokosuka, Japan
  • 6Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, UK
  • 7Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Sendai, Japan

Abstract. Systematic measurements of atmospheric methane (CH4) mole fractions at the northern high latitudes only began in the early 1980s, and whilst CH4 measurements from Greenland ice cores covered the period before ~1900, no reliable observational record is available for the intermediate period. In this study, we reconstruct the atmospheric CH4 for that period, when the mole fraction started to increase rapidly. We use a set of trace gas data measured from firn (an intermediate stage between snow and glacial ice formation) air samples collected at the NGRIP (North Greenland Ice Core Project) site in 2001, in combination with a firn air transport model whose performance is validated by using a set of published firn air data at the NEEM (North Greenland Eemian ice Drilling) site. We examine a variety of possible firn diffusivity profiles using a suite of measured trace gases, and reconstruct the CH4 mole fraction by an iterative dating method. We find that, given the currently available firn air data sets from Greenland, reliable reconstruction of the Arctic CH4 mole fraction is possible only back to the mid 1970s. For the earlier period, it is difficult to identify the atmospheric CH4 history that consistently reproduce the depth profiles of CH4 in firn at both NGRIP and NEEM sites. Therefore, the currently proposed Arctic CH4 history should still be considered preliminary and uncertain, and should not be treated as the known history for constraining firn-air transport models.

Taku Umezawa et al.

Status: open (until 15 Oct 2021)

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  • RC1: 'Comment on acp-2021-736', Anonymous Referee #1, 13 Sep 2021 reply

Taku Umezawa et al.

Data sets

Composition of firn air at the North Greenland Ice Core Project (NGRIP) site Kawamura, K., T. Umezawa, S. Sugawara, S. Ishidoya, K. Ishijima, T. Saito, I. Oyabu, S. Murayama, S. Morimoto, S. Aoki, T. Nakazawa https://ads.nipr.ac.jp/data/meta/A20210609-001

Taku Umezawa et al.

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Short summary
Atmospheric concentration of methane (CH4), an important greenhouse gas, in the Arctic has not been accurately reported for the period 1900–1980 either by direct observations or ice core reconstructions. Using trace gas data from firn (compacted snow layers above ice sheet) air samples at Greenland sites and a firm-air transport model, we examine reconstruction of the atmospheric CH4 for that period and show that the Arctic CH4 concentration can be reliably estimated only back to the mid 1970s.
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