Abstract
AbstractAtmospheric methane grew very rapidly in 2014 (12.7 ± 0.5 ppb/year), 2015 (10.1 ± 0.7 ppb/year), 2016 (7.0 ± 0.7 ppb/year), and 2017 (7.7 ± 0.7 ppb/year), at rates not observed since the 1980s. The increase in the methane burden began in 2007, with the mean global mole fraction in remote surface background air rising from about 1,775 ppb in 2006 to 1,850 ppb in 2017. Simultaneously the13C/12C isotopic ratio (expressed as δ13CCH4) has shifted, now trending negative for more than a decade. The causes of methane's recent mole fraction increase are therefore either a change in the relative proportions (and totals) of emissions from biogenic and thermogenic and pyrogenic sources, especially in the tropics and subtropics, or a decline in the atmospheric sink of methane, or both. Unfortunately, with limited measurement data sets, it is not currently possible to be more definitive. The climate warming impact of the observed methane increase over the past decade, if continued at >5 ppb/year in the coming decades, is sufficient to challenge the Paris Agreement, which requires sharp cuts in the atmospheric methane burden. However, anthropogenic methane emissions are relatively very large and thus offer attractive targets for rapid reduction, which are essential if the Paris Agreement aims are to be attained.
Highlights
Methane is the second most important anthropogenic greenhouse gas
All methane measurements are reported on the WMO X2004A scale (Dlugokencky et al, 2005; www.esrl.noaa.gov/gmd/ccl/ch4_scale.html) in mole fraction units, nanomole per mole of dry air, or parts per billion, abbreviated as “ppb.” Uncertainties range from approximately ±3 ppb in the early 1980s to approximately ±1 ppb in 2017 as measurement precisions have improved
Further very strong increases in 2015 (10.1 ± 0.7 ppb), 2016 (7.0 ± 0.7 ppb), and 2017 (7.7 ± 0.7 ppb) added to the methane burden already measured in 2014
Summary
Methane is the second most important anthropogenic greenhouse gas R. Allen et al, 2018; Etminan et al, 2016; G. Myhre et al, 2013). In the 1990s, the atmospheric methane burden trended toward equilibrium, which it reached by the end of the twentieth century (Dlugokencky et al, 2011), with little or no growth in its atmospheric burden in the early years of this century. In 1984, the first year with detailed records, the global annual average atmospheric mole fraction of methane in the remote
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