The atmospheric cycling of radiomethane and the "fossil fraction" of the methane source

Abstract

Abstract. The cycling of 14 CH 4 ("radiomethane") through the atmosphere has been strongly perturbed in the industrial era by the release of 14 C-free methane from geologic reservoirs ("fossil methane" emissions), and in the nuclear era, especially since ca 1970, by the direct release of nucleogenic radiomethane from nuclear power facilities. Contemporary measurements of atmospheric radiomethane have been used to estimate the proportion of fossil methane in the global methane source (the "fossil fraction"), but such estimates carry high uncertainty due to the ill-determined nuclearpower source. Guided by a mass-balance formulation in a companion paper, we apply a contemporary time series of atmospheric radiomethane to quantify both the fossil fraction and the strength of the nuclear power source. We deduce that 30.0±2.3% (1 s.d.) of the global methane source for 1986-2000 has fossil origin, a fraction which may include some 14 C-depleted refractory carbon such as from aged peat deposits. Since this estimate depends upon the validity of assumptions underlying a linear regression model, it should be seen as providing a plausible re-estimate rather than a definitive revision. Such a fossil fraction would be much larger (by 50%) than is commonly accepted, with implications for inventory compilation. The co-estimated strength of the global nuclear-power source of radiomethane is consistent with values inferred independently from local nuclear facilities.

The global methane budget 2000–2012

“…However, summing up all fossil-CH 4 -related sources (including the anthropogenic emissions) leads to a total of 173 Tg CH 4 yr −1 , which is about 31 % [25-35 %] of global methane emissions. This result is consistent with 14 C atmospheric isotopic analyses inferring a 30 % contribution of fossil-CH 4 to global emissions (Lassey et al, 2007b;Etiope et al, 2008). All nongeological and non-wetland land source categories (wild animals, wildfires, termites, permafrost) have been evaluated at a lower level than in Kirschke et al (2013) and contribute only 23 Tg CH 4 yr −1 to global emissions.…”

“…Our estimate is consistent with a top-down global verification, based on observations of radiocarbon-free (fossil) methane in the atmosphere (Etiope et al, 2008;Lassey et al, 2007b), with a range of 33-75 Tg CH 4 yr −1 .…”

Current systematic carbon-cycle observations and the need for implementing a policy-relevant carbon observing system

“…First, the ability to reliably disentangle and attribute fossil fuel (vs. natural) sources of CO 2 by limited quantities of 14 C-CO 2 (the even rarer 14 C-CH 4 can be used insofar only for a global constraint of fossil CH 4 emissions (Lassey et al, 2007), and 14 C-CH 4 is contaminated by nuclear power plants emissions (Levin et al, 1992)) atmospheric measurements and the lack of systematic application of combustion tracers (e.g. CO and NO 2 ) to separate fossil fuel CO 2 .…”

Optimal estimation of the present‐day global methane budget

“…This range is possible within the allowed uncertainty range of the δ 13 signatures of the sources; that is, scenario B07 tends to estimate these to be on the high, and scenario GEO on the low end, of the uncertainty range. From observations of atmospheric radiocarbon, Quay et al [1999] estimate that fossil emissions, which are devoid of radiocarbon, make up 18 ± 9% of total present‐day emissions, while more recent work by Lassey et al [2007a] argues that a fossil fraction of 30 ± 2.3% is more likely. Thus, while fossil fractions of about 20–29% can evidently be accommodated within the global δ 13 budget, the results of Lassey et al [2007a] would suggest that the BQT and GEO scenarios are more likely, though it would mean that the present‐day emissions from coal mining and biofuel are about 34Tg/a stronger than estimated by B07, and also much larger than assumed in the recent EDGAR 4 inventory [ EDGAR , 2010].…”

“…Additional constraints exist in the distinct “signatures” of carbon and hydrogen isotopes that characterize different CH 4 sources. The observed ratio of D/H in atmospheric CH 4 has been used by Whiticar and Schaefer [2009], Sowers [2010], and Mischler et al [2010] to estimate the relative strength of thermogenic clathrate emissions, while Lassey et al [2007a] examined the temporal record of 14 CH 4 to constrain the relative fraction of fossil CH 4 sources. In this study we focus on the inversion of the observed ratio R = 13 CH 4 / 12 CH 4 , typically quantified as δ 13 = R / R std − 1 (where R std represents the Vienna PeeDee Belemnite standard [ Craig , 1957; Lassey et al , 2007b]).…”