[A-CC29] Ice cores and paleoenvironmental modeling

[ACC29-11] Towards reconstructing the Arctic atmospheric methane history over the 20th century: measurement and modeling results for the NGRIP firn

*Taku Umezawa¹, Satoshi Sugawara², Kenji Kawamura^3,4,5, Ikumi Oyabu³, Stephen Andrews^1,6, Takuya Saito¹, Shuji Aoki⁷, Takakiyo Nakazawa⁷ (1.National Institute for Environmental Studies, 2.Miyagi University of Education, 3.National Institute of Polar Research, 4.The Graduate University of Advanced Studies (SOKENDAI), 5.Japan Agency for Marine Science and Technology, 6.University of York, 7.Tohoku University)

Keywords:methane, firn

Systematic measurements of atmospheric methane (CH₄) mole fractions at the northern high laittudes 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 (Kawamura et al. 2021), 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, based on the two Arctic firn data sets in the same manner. We find that, given the currently available firn air data sets from Greenland, reliable reconstruction of the Arctic CH₄ mole fraction before the mid 1970s is highly uncertain. Although it is difficult to accurately identify the atmospheric CH₄ history that consistently reproduce the depth profiles of CH₄ in firn at both NGRIP and NEEM sites, both firn data sets are more consistent with the atmospheric CH₄ scenario prepared for the NEEM firn modeling (Buizert et al. 2012) than that for the CMIP6 (Climate Model Intercomparison Project Phase 6) studies (Meinshausen et al. 2017). It is considered that the former is the current best choice for the available synthetic Arctic CH₄ history, but should not be treated as the known history for constraining firn-air transport models until supported by a new data set from an Arctic ice core or a firn site.

Reference:
Umezawa et al. (in review) Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2021-736.
Buizert et al. (2012) Atmos. Chem. Phys., https://doi.org/10.5194/acp-12-4259-2012.
Kawamura et al. (2021) Polar Data J., http://doi.org/10.20575/00000030
Meinshausen et al. (2017) Geosci. Model Dev., https://doi.org/10.5194/gmd-10-2057-2017.