Abstract
The atmospheric methane (CH4) burden is rising sharply, but the causes are still not well understood. One factor of uncertainty is the importance of tropical CH4 emissions into the global mix. Isotopic signatures of major sources remain poorly constrained, despite their usefulness in constraining the global methane budget. Here, a collection of new δ13CCH4 signatures is presented for a range of tropical wetlands and rice fields determined from air samples collected during campaigns from 2016 to 2020. Long-term monitoring of δ13CCH4 in ambient air has been conducted at the Chacaltaya observatory, Bolivia and Southern Botswana. Both long-term records are dominated by biogenic CH4 sources, with isotopic signatures expected from wetland sources. From the longer-term Bolivian record, a seasonal isotopic shift is observed corresponding to wetland extent suggesting that there is input of relatively isotopically light CH4 to the atmosphere during periods of reduced wetland extent. This new data expands the geographical extent and range of measurements of tropical wetland and rice δ13CCH4 sources and hints at significant seasonal variation in tropical wetland δ13CCH4 signatures which may be important to capture in future global and regional models.This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’.
Highlights
The atmospheric methane (CH4) burden began again increasing in 2007, after some years of stability, and the growth rate accelerated in 2014 [1,2]
The importance of tropical wetlands to the global methane budget is well established, but the role that wetlands play in the current increase in global methane mixing ratios is still under debate
Continued work to understand the isotopic composition of wetland emissions is important to improve the accuracy of global models using δ13CCH4 to determine the causes behind the recent global methane atmospheric growth
Summary
The atmospheric methane (CH4) burden began again increasing in 2007, after some years of stability, and the growth rate accelerated in 2014 [1,2]. Several hypotheses have been postulated for the cause of the isotopic shift and can be summarized as one or a combination of the following: (i) a change in the oxidative capacity of the atmosphere [3], (ii) changes in the relative strengths of anthropogenic sources, such as changes to agriculture, waste and fossil fuel emissions with an overall net effect of increasing emissions [4,5]) (iii) an increase in natural sources such as wetlands, potentially as a feedback effect from regional climatic change Background air is measured as δ13CCH4 ∼ −47‰, and global bulk CH4 inputs are estimated at approximately −53‰ [2]. The discrepancy between global input average and global background average is due to fractionation from sinks, with OH destruction of CH4 expected to be responsible for the majority of the 6‰
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