2021 Volume 99 Issue 2 Pages 309-337
Methane (CH4) is an important greenhouse gas and plays a significant role in tropospheric and stratospheric chemistry. Despite the relevance of methane (CH4) in human-induced climate change and air pollution chemistry, there is no scientific consensus on the causes of changes in its growth rates and variability over the past three decades. We use a well-validated chemistry–transport model for simulating CH4 concentration and estimation of regional CH4 emissions by inverse modeling during 1988–2016. The control simulations are conducted using seasonally varying hydroxyl (OH) concentrations and assumed no interannual variability. Using inverse modeling of atmospheric observations, emission inventories, a wetland model, and a δ13C-CH4 box model, we show that reductions in emissions from Europe and Russia since 1988, particularly from oil–gas exploitation and enteric fermentation, led to decreased CH4 growth rates in the 1990s. This period was followed by a quasi-stationary state of CH4 in the atmosphere during the early 2000s. CH4 resumed growth from 2007, which we attribute to increases in emissions from coal mining mainly in China and the intensification of ruminant farming in tropical regions. A sensitivity simulation using interannually varying OH shows that regional emission estimates by inversion are unaffected for the mid- and high latitude areas. We show that meridional shift in CH4 emissions toward the lower latitudes and the increase in CH4 loss by hydroxyl (OH) over the tropics finely balance out, keeping the CH4 gradients between the southern hemispheric tropical and polar sites relatively unchanged during 1988–2016. The latitudinal emissions shift is confirmed using the global distributions of the total column CH4 observations via satellite remote sensing. During our analysis period, there is no evidence of emission enhancement due to climate warming, including the boreal regions. These findings highlight key sectors for effective emission reduction strategies toward climate change mitigation.