Atmospheric CH4 plateau sustained by NOx emission rise and southward shift.

Methane exhibits decadal variability in atmospheric growth rates1. During the 1990s, atmospheric methane growth slowed relative to the 1980s, largely attributed to the stabilization or decline in anthropogenic emissions2-7. By contrast, economic expansion in the early 2000s (ref. 8) led to an increase in anthropogenic methane emissions9. The anticipated rise in atmospheric methane concentrations, however, was not detected by global monitoring networks1. Instead, almost no atmospheric methane growth was observed between 1999 and 2006 (ref. 1). Several hypotheses have been proposed to explain this methane plateau10-13, but the underlying causes remain uncertain3. Here we combine model simulations with methane and isotopic observations and attribute the plateau to a 1.4 ± 0.4% hydroxyl radical (OH) increase during 2000-2006 relative to 1999, mostly occurring in the tropics. We find that the tropical OH enhancement was primarily driven by a global rise and spatial redistribution of nitrogen oxides (NOx) emissions towards developing regions and the oceans, spurred by economic growth in tropical countries and increased shipping emissions linked to global trade expansion14,15. These changes led to an amplified global methane loss of 2.2 ± 0.7% during 2000-2006 and 3.7 ± 1.3% in 2006 relative to 1999, with the 2000-2006 enhancement equivalent to 136% [90%, 184%] of the contemporaneous anthropogenic methane emission growth, thereby sustaining the observed methane plateau.