Towards reconstructing the Arctic atmospheric methane history over the 20th century: measurement and modelling results for the North Greenland Ice Core Project firn

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Umezawa, Taku, Sugawara, Satoshi, Kawamura, Kenji, Oyabu, Ikumi, Andrews, Stephen J., Saito, Takuya, Aoki, Shuji, Nakazawa, Takakiyo
• Format: Text
• Language: English
• Published: 2022
• Subjects: Arctic; Greenland; Greenland ice core; Greenland Ice core Project; Greenland ice cores; ice core; NGRIP; North Greenland; North Greenland Ice Core Project
• Online Access: https://doi.org/10.5194/acp-22-6899-2022; https://acp.copernicus.org/articles/22/6899/2022/

Systematic measurements of atmospheric methane (CH 4 ) mole fractions at the northern high latitudes only began in the early 1980s. Although CH 4 measurements from Greenland ice cores consistently covered the period before ∼ 1900, no reliable observational record is available for the intermediate period. We newly report a data set of trace gases from the air trapped in firn (an intermediate stage between snow and glacial ice formation) collected at the NGRIP (North Greenland Ice Core Project) site in 2001. We also use a set of published firn air data at the NEEM (North Greenland Eemian ice Drilling) site. The two Arctic firn air data sets are analysed with a firn air transport model, which translates historical variations to depth profiles of trace gases in firn. We examine a variety of possible firn diffusivity profiles, using a suite of measured trace gases, and reconstruct the CH 4 mole fraction by an iterative dating method. Although the reconstructions of the Arctic CH 4 mole fraction before the mid-1970s still has large uncertainties ( > 30 ppb – parts per billion), we find a relatively narrow range of atmospheric CH 4 history that is consistent with both depth profiles of NGRIP and NEEM. The atmospheric CH 4 history inferred by this study is more consistent with the atmospheric CH 4 scenario prepared for the NEEM firn modelling than that for the CMIP6 (Climate Model Intercomparison Project Phase 6) experiments. Our study shows that the atmospheric CH 4 scenario used for the NEEM firn modelling is considered to be the current best choice for the Arctic CH 4 history, but it should not be used to tune firn air transport models until being verified by further measurements from sources such as the Arctic ice cores. Given the current difficulty in reconstructing the CH 4 history with low uncertainty from the firn air data sets from Greenland, future sampling and measurements of ice cores at a high-accumulation site may be the only way to accurately reconstruct the atmospheric CH 4 trend over the 20th century.