AGU - Very Strong Methane Growth In The 4 Years 2014-2017: Implications For The Paris Agreement
Abstract
Atmospheric 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 the 13C/12C isotopic ratio (expressed as δ13CCH4) has shifted, 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.
Plain Language Summary
The rise in atmospheric methane (CH4), which began in 2007, accelerated in the past 4 years. The growth has been worldwide, especially in the tropics and northern midlatitudes. With the rise has come a shift in the carbon isotope ratio of the methane. The causes of the rise are not fully understood, and may include increased emissions and perhaps a decline in the destruction of methane in the air. Methane's increase since 2007 was not expected in future greenhouse gas scenarios compliant with the targets of the Paris Agreement, and if the increase continues at the same rates it may become very difficult to meet the Paris goals. There is now urgent need to reduce methane emissions, especially from the fossil fuel industry.
1 Introduction
Methane is the second most important anthropogenic greenhouse gas (M. 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 marine boundary layer was 1,645 ppb. In 2006, just before the recent growth phase began, it was about 1,775 ppb. This grew rapidly to an annual global mean of 1,850 ppb in 2017, a total rise of about 75 ppb in the 2007–2017 period (Figure 1). The increase is continuing. The growth in the mole fraction of methane in Arctic air was strong in late 2007, but since then in many years the strongest growth has been in the tropics and subtropics of the Northern and Southern Hemispheres (Figure 2). Overall, since 2007, methane growth has been sustained globally. Preindustrial methane was about 720 ppb around 1750 C.E. (Etheridge et al., 1998); thus, the decade of recent growth is equivalent to more than 6% of total growth to date since industrialization began.
EDIT
Although methane's apparent equilibration in the early years of this century was perhaps only a temporary pause in the human‐induced secular increase in atmospheric methane (Bousquet et al., 2006), the renewed strong methane growth that began in 2007 (Nisbet et al., 2014, 2016) was so unexpected that it was not considered in pathway models preparatory to the Paris Agreement (M. Collins et al., 2013; Meinshausen et al., 2011; Moss et al., 2008; Rogelj et al., 2012). The current growth has now lasted over a decade. If growth continues at similar rates through subsequent decades, evidence presented here demonstrates that the extra climate warming impact of the methane can significantly negate or even reverse progress in climate mitigation from reducing CO2 emissions. This will challenge efforts to meet the target of the 2015 UN Paris Agreement on Climate Change, to limit climate warming to 2 °C.
EDIT
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GB006009