Consistent acceleration of the global methane growth rate over two decades: Comparison with Representative Concentration Pathways and implications for future modeling
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The Representative Concentration Pathways (RCPs) are a set of four harmonized greenhouse gas emission and concentration trajectories, representative of scenarios from the literature, that would result in radiative forcings of 2.6, 4.5, 6.0, and 8.5 Wm − 2 by the year 2100. The RCPs show a particularly large range of methane (CH 4 ) trajectories early in the first quarter of the century, with vastly different implications for near-future climate warming. Using observation-constrained global atmospheric methane estimates from NASA’s Goddard Earth Observing System (GEOS) model, we estimate that atmospheric methane has been consistently accelerating from 2001 through 2024, driven by microbial emissions; only RCP8.5 (associated with a radiative forcing of 8.5 Wm − 2 ) models an acceleration during this time. A continuation of this acceleration would produce average concentrations between 2360 ppb and 2600 ppb (95% confidence) by midcentury, closest to RCP8.5. Thus, scenarios that include methane trajectories between the sharp acceleration of RCP8.5 and the much smaller and more stable growth of RCPs 4.5 and 6 should be resolved. Continued atmospheric monitoring and further investigation of the microbial processes that may be causing this increase, including both agriculture and wetland climate feedbacks, are necessary to produce realistic predictions and mitigation scenarios.