Law Dome 14CH4 measurements confirm revised fossil methane emissions estimates

Abstract

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). This is inconsistent with bottom-up geological CH₄ emissions estimates (40-60 Tg CH₄ /yr) and implies an 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 suggests opportunities for methane emissions reductions. © The Authors