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

Abstract

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 emissions^2-7. 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 plateau^10-13, 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 (NO_x) emissions towards developing regions and the oceans, spurred by economic growth in tropical countries and increased shipping emissions linked to global trade expansion^14,15. 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.