Abstract. Changes in atmospheric methane abundance have
implications for both chemistry and climate as methane is both a strong
greenhouse gas and an important precursor for tropospheric ozone. A better
understanding of the drivers of trends and variability in methane abundance
over the recent past is therefore critical for building confidence in
projections of future methane levels. In this work, the representation of
methane in the atmospheric chemistry model AM4.1 is improved by optimizing
total methane emissions (to an annual mean of 580±34 Tg yr−1) to
match surface observations over 1980–2017. The simulations with optimized
global emissions are in general able to capture the observed trend,
variability, seasonal cycle, and latitudinal gradient of methane.
Simulations with different emission adjustments suggest that increases in
methane emissions (mainly from agriculture, energy, and waste sectors)
balanced by increases in methane sinks (mainly due to increases in OH
levels) lead to methane stabilization (with an imbalance of 5 Tg yr−1)
during 1999–2006 and that increases in methane emissions (mainly from
agriculture, energy, and waste sectors) combined with little change in sinks
(despite small decreases in OH levels) during 2007–2012 lead to renewed
growth in methane (with an imbalance of 14 Tg yr−1 for 2007–2017).
Compared to 1999–2006, both methane emissions and sinks are greater (by 31 and 22 Tg yr−1, respectively) during 2007–2017. Our tagged
tracer analysis indicates that anthropogenic sources (such as agriculture,
energy, and waste sectors) are more likely major contributors to the renewed
growth in methane after 2006. A sharp increase in wetland emissions (a
likely scenario) with a concomitant sharp decrease in anthropogenic emissions
(a less likely scenario), would be required starting in 2006 to drive the
methane growth by wetland tracer. Simulations with varying OH levels
indicate that a 1 % change in OH levels could lead to an annual mean
difference of ∼4 Tg yr−1 in the optimized emissions and a
0.08-year difference in the estimated tropospheric methane lifetime.
Continued increases in methane emissions along with decreases in
tropospheric OH concentrations during 2008–2015 prolong methane's lifetime
and therefore amplify the response of methane concentrations to emission
changes. Uncertainties still exist in the partitioning of emissions among
individual sources and regions.
A Structured Approach for the Mitigation of Natural Methane Emissions—Lessons Learned from Anthropogenic Emissions
