Ice core and firn air $^{14}$CH$_{4}$ measurements from preindustrial to present suggest that anthropogenic fossil CH$_{4}$ emissions are underestimated.

Concentrations of atmospheric methane (CH4), a potent greenhouse gas, have more thandoubled since preindustrial times yet its contemporary budget is incompletely understood,with substantial discrepancies between global emission inventories and atmosphericobservations (Kirschke et al., 2013; Saunois et al., 2016). Radiomethane (14CH4) candistinguish between fossil emissions from geologic reservoirs (radiocarbon free) andcontemporaneous biogenic sources, although poorly constrained direct 14CH4 emissionsfrom nuclear reactors complicate this interpretation in the modern era (Lassey et al.,2007; Zazzeri et al 2018). It has been debated how fossil emissions (172-195 TgCH4/yr, (Saunois et al., 2016; Schwietzke et al., 2016)) are partitioned betweenanthropogenic sources (such as fossil fuel extraction and consumption) and naturalsources (such as geologic seeps); emission inventories suggest the latter accountsfor ∼50-60 Tg CH4/yr (Etiope, 2015; Etiope et al., 2008). Geologic emissionswere recently shown to be much smaller at the end of the Pleistocene ∼11,600years ago (Petrenko et al. 2017); However, this period is an imperfect analog for thepresent day due to the much larger terrestrial ice sheet cover, lowered sea level,and more extensive permafrost. We use preindustrial ice core measurements of14CH4 to show that natural fossil CH4 emissions to the atmosphere are ∼1.7 TgCH4/yr, with a maximum of 6.1 Tg CH4/yr (95% confidence limit), an order ofmagnitude smaller than estimates from global inventories. This result suggeststhat contemporary anthropogenic fossil emissions are likely underestimated by acorresponding amount (∼48-58 Tg CH4/yr, or ∼25-33% of current estimates). [ABSTRACT FROM AUTHOR]