Quantifying the Effect of Neglecting Variable Methane LIfetime on Methane Emissions Estimates

Methane is a potential target for short-term radiative forcing mitigation, due to its turnover time of only about 9 years. This steady-state lifetime of methane implies that the main sink, oxidation by the hydroxyl radical (OH), is constant over time. However, variations in carbon monoxide (CO), methane itself and and OH can induce variations in methane by modulating the methane loss rate. As such, the actual methane decay time to perturbations can last 40% longer (13.5 yr vs 9 yr). This effect is often ignored in methane flux inversions as the impact is assumed to be negligible and would greatly increase the computational expense.

Here we show, through inversions of a 2-hemispheres box model with coupled chemistry, that neglecting these secondary reactions can lead up to a ~50%/yr bias in estimations of changes in methane emissions over decadal timescales. In particular, incorporating decreasing CO concentrations, beginning in the 1990's, overcompensate the effect of increasing methane on OH, and neglecting indirect effects of CO on methane can result in errors in the estimate of inter-annual methane emissions of up to 10 Tg/yr. Incorporating variations of OH production and recycling dampens methane emissions variability by compensating for changes in OH concentrations.

We also present a case study on the impacts of El Nino on methane abundances and decompose the effects of methane and CO emissions. Specifically, increased CO emissions during El Nino can have a comparable effect on methane abundances (via reduced OH concentrations) as the direct impact from methane emissions themselves. Methane emissions inversions over decadal timescales therefore depend on the assumptions in OH chemistry, due to effects on the methane lifetime, and we make recommendations on how to account for this complexity in global Methane inversions.