[ASAP] Where the Methane Is—Insights from Novel Airborne LiDAR Measurements Combined with Ground Survey Data

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Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions

Hmiel, Benjamin; Petrenko, V. V.; Dyonisius, M. N.; Buizert, C.; Smith, A. M.; Place, P. F.; Harth, C.; Beaudette, R.; Hua, Q.; Yang, B.; Vimont, I.; Michel, S. E.; Severinghaus, J. P.; Etheridge, D.; Bromley, T.; Schmitt, J.; Fain, X.; Weiss, R. F.; Dlugokencky, E.

Nature (London, United Kingdom) (2020), 578 (7795), 409-412CODEN: NATUAS; ISSN:0028-0836. (Nature Research)

Atm. CH4 is a potent greenhouse gas whose mole fraction has more than doubled since the pre-Industrial Era (Meinshausen, M. et al., 2017). Fossil fuel extn./use are among the largest anthropogenic CH4 emission sources, but the precise magnitude of these contributions is debated (Saunois, M., et al., 2016; Schwietzke, S. et al., 2017). 14C in CH4 (14CH4) can distinguish between fossil (14C-free) CH4 emissions and contemporaneous biogenic sources; however, poorly constrained, direct 14CH4 emissions from nuclear reactors have complicated this approach since the middle of the 20th century (Lassey, K.R., et al., 2007; Zazzeri, G. et al., 2018). Partitioning of total fossil CH4 emissions (currently 172-195 Tg CH4/yr [Saunois, 2017; Schwietzke, 2017]) between anthropogenic and natural geol. sources (e.g., seeps, mud volcanoes) is debated; emission inventories suggested the latter accounted for ∼40-60 Tg CH4/yr (Etiope, G., 2015; Etiope, G. et al., 2019). Geol. emissions were <15.4 Tg CH4/yr at the end of the Pleistocene, ∼11,600 years ago (Petrenko, V.V., et al., 2017), but that period is an imperfect analog for present-day emissions due to the large terrestrial ice sheet cover, lower sea level, and extensive permafrost. This work used pre-Industrial Era ice core 14CH4 measurements to show natural geol. CH4 atm. emissions were ∼1.6 Tg CH4/yr, with a max. of 5.4 Tg CH4/yr (95% confidence limit), an order of magnitude lower than currently used ests. This result indicated anthropogenic fossil CH4 emissions were underestimated by ∼38-58 Tg CH4/yr, i.e., ∼25-40% of recent ests. This record highlighted the human impact on atm. and climate, provided a firm target for global CH4 budget inventories, and will help inform strategies for targeted emission redns. (Hoglund-Isaksson, L, 2012; Howarth, R.W., 2015).

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