Law Dome 14CH4 measurements confirm revised fossil methane emissions estimates

Methane is a powerful greenhouse gas and has significant roles in the chemistry of the atmosphere. Its global concentration has risen by 240% since 1750 AD. Atmospheric 14CH4 is an independent and potentially unambiguous tracer of fossil CH4 emissions from anthropogenic and natural geologic sources, however 14C from nuclear weapons tests and 14CH4 from nuclear power plants complicate its interpretation after the late 1950s. Measurements before then rely on air extracted from polar ice and firn. Hmiel et al. (Nature, 2020) measured 14CH4 in air extracted from firn and ice in Greenland and Antarctica and found that the natural global fossil CH4 source is very small (<6 Tg CH4 yr-1). This is inconsistent with bottom-up geological CH4 emissions estimates (40-60 Tg CH4 yr-1) and implies a significant upward revision of anthropogenic fossil source emissions, emphasising the need for further measurements. We present new 14CH4 measurements of air extracted from the high accumulation site DE08-OH on the Law Dome ice sheet in 2018/19, including firn air to 81 m depth and large ice samples combined from parallel ice cores to 240 m. Measurements of trace gases confirm that the samples were uncontaminated and only minor corrections are required for sample processing. The correction for cosmogenic in-situ production of 14CH4 is very small at DE08-OH due to its high accumulation rate and relatively low elevation. The new 14CH4 results compare closely with the previous measurements from the other sites. An atmospheric 14CH4 history is reconstructed from inverse modelling of the combined ice and firn data. The pre-industrial 14CH4 level is almost identical to that expected from contemporaneous biogenic sources, confirming very minor natural fossil CH4 emissions. 14CH4 decreases to a minimum in about 1940 as anthropogenic fossil methane is emitted followed by an increase during the nuclear era from 1950 to present. The record since the 1950s would allow the evolution of the anthropogenic fossil source to be quantified when improved nuclear 14CH4 emissions estimates become available. The larger emissions from anthropogenic fossil sources implied by this result highlight opportunities for methane emissions reductions.

Keywords

Antarctica, Methane, Greenhouse gases, Atmospheric chemistry, Carbon, Nuclear power plants, Fossils, Ice, Nuclear explosions

Citation

Etheridge, D. M., Petrenko, V. V., Smith, A. M., Neff, P. D., Hmiel, B., Trudinger, C. M., Crosier, E., Thornton, D., Langenfelds, R. L., Jong, L. M, Harth, C. M., Mitrevski, B., Buizert, C., Yang, B., Weiss, R. F., & Severinghaus, J. (2021). Law Dome 14CH4 measurements confirm revised fossil methane emissions estimates. Paper presented to the AGU Fall Meeting 2021, New Orleans, Louisiana and Online, 13-17 December 2021. In AGU Fall Meeting Abstracts (Vol. 2021, C41B-03). Retrieved from: https://agu.confex.com/agu/fm21/meetingapp.cgi/Paper/877672