A 21st-century shift from fossil-fuel to biogenic methane emissions indicated by ¹³ CH ₄

Abstract

Between 1999 and 2006, a plateau interrupted the otherwise continuous increase of atmospheric methane concentration [CH4] since preindustrial times. Causes could be sink variability or a temporary reduction in industrial or climate-sensitive sources. We reconstructed the global history of [CH4] and its stable carbon isotopes from ice cores, archived air, and a global network of monitoring stations. A box-model analysis suggests that diminishing thermogenic emissions, probably from the fossil-fuel industry, and/or variations in the hydroxyl CH4 sink caused the [CH4] plateau. Thermogenic emissions did not resume to cause the renewed [CH4] rise after 2006, which contradicts emission inventories. Post-2006 source increases are predominantly biogenic, outside the Arctic, and arguably more consistent with agriculture than wetlands. If so, mitigating CH4 emissions must be balanced with the need for food production.

The global methane budget 2000–2012

“…Particularly, the budget assessment of these regions should strongly benefit from the ongoing effort to develop a network of in situ atmospheric measurement stations. Finally, additional tracers (methane isotopes, ethane, CO) have potential to bring more constraint on the global methane cycle if their information content relative to methane emission trends is consistent with each other, which is not fully the case at present (Schaefer et al, 2016;Hausmann et al, 2016). Building on the improvement of the points above, our aim is to update this synthesis as a living review paper on a regular basis (∼ every 2 years).…”

“…Scenarios of increasing fossil and/or microbial sources have been pro-posed to explain this increase Bergamaschi et al, 2013;Nisbet et al, 2014). Whereas the decreasing trend in δ 13 C in CH 4 suggests a significant, if not dominant, contribution from increasing emissions by microbial CH 4 sources (Schaefer et al, 2016;Nisbet et al, 2014), concurrent ethane and methane column measurements suggest a significant role (likely at least 39 %) for oil and gas production (Hausmann et al, 2016), which could be consistent when assuming a concomitant decrease in biomass burning emissions (heavy source for 13 C), as suggested by the GFED database (Giglio et al, 2013). Yet accounting for the uncertainties in the isotopic signatures of the sources and their trends may suggest different portionings of the global methane sources between fossil fuel and biogenic methane emissions (Schwietzke et al, 2016).…”

“…Carbon monoxide (FortemsCheiney et al, 2011) can provide constraints for biomass burning for instance. However, additional tracers can also bring contradictory trends in emissions such as the ones suggested since 2007 by 13 C (Schaefer et al, 2016) and ethane (Hausmann et al, 2016). Such discrepancies have to be understood and solved to be able to properly use additional tracers to constrain methane emissions.…”

“…The partition of regional emissions by processes remains very uncertain today, waiting for the development or consolidation of measurements of more specific tracers, such as methane isotopes or ethane, dedicated to constrain the different methane sources or groups of sources (e.g. Simpson et al, 2012;Schaefer et al, 2016;Hausmann et al, 2016).…”

The coming thaw

“…However, the rate of conversion of this ancient carbon to methane has not measurably increased over the sixty-year study period. This finding is backed up by satellite studies of methane emissions as well as methane inventories, none of which suggest substantial increases in methane emissions from the Arctic tundra [5][6][7] .…”