Law Dome 14CH4 measurements confirm revised fossil methane emissions estimates
Dr David Etheridge1,2, Dr Vasilii Petrenko3, Dr Andrew Smith4, Dr Peter Neff3,5, Dr Benjamin Hmiel6, Dr Cathy Trudinger1, Dr Edward Crosier3, David Thornton1,2, Dr Ray Langenfelds1, Dr Lenneke Jong2,7, Dr Christina Harth8, Dr Blagoj Mitrevski1, Dr Christo Buizert9, Dr Bin Yang4, Professor Ray Weiss10, Professor Jeffrey Severinghaus8
1Climate Science Centre, CSIRO Oceans and Atmosphere, Aspendale, Australia, 2Australian Antarctic Program Partnership, Institute for Marine and Antarctic Studies, University of Tasmania , Hobart, Australia, 3University of Rochester, Department of Earth and Environmental Sciences, Rochester, USA, 4Australian Nuclear Science and Technology Organisation (ANSTO), Kirrawee, Australia, 5University of Minnesota Twin Cities, Department of Soil, Water, and Climate, Minneapolis, USA, 6Environmental Defense Fund, New York, USA, 7Australian Antarctic Division, Kingston, Australia, 8University of California San Diego, Scripps Institution of Oceanography, La Jolla, USA, 9Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, USA, 10Univ California San Diego, La Jolla, USA
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.
David Etheridge is a principal research scientist with the Climate Science Centre, CSIRO Oceans and Atmosphere and the Australian Antarctic Program Partnership, University of Tasmania. His research involves the long term changes in greenhouse and ozone-depleting gases. It is used in all IPCC Assessment Reports and World Meteorological Organisation Scientific Assessments of Ozone Depletion. He has led 8 field programs on the polar ice sheets, researching atmospheric composition change, climate change, and ice sheet dynamics. He is also investigating the potential impact on the atmosphere of energy technologies, such as natural gas, hydrogen and the geological storage of CO2.