Effects of Strongly Enhanced Atmospheric Methane Concentrations in a Chemistry-Climate Model: Rapid Adjustments and Slow Feedbacks

Abstract:

Methane (CH₄) is the second most important anthropogenic greenhouse gas. Besides its direct radiative impact it induces chemical feedbacks relevant for climate and air quality. We assess the effects of doubled and fivefold present-day (year 2010) CH₄ lower boundary mixing ratios on the basis of sensitivity simulations with the chemistry-climate model EMAC. We investigate rapid adjustments and slow feedbacks, and their radiative impacts, by applying prescribed oceanic conditions and a mixed layer ocean (MLO) model, respectively. In the present study we compare the changes in the MLO sensitivity simulations to the experiments with prescribed oceanic conditions (Winterstein et al., 2019).

In the troposphere the increased CH₄ mixing ratios cause enhanced depletion of the hydroxyl radical (OH), which further results in a prolonged CH₄ lifetime. Slow feedbacks, however, dampen the lifetime increase in comparison to the rapid adjustments. OH precursors (water vapour and ozone (O₃)) are more strongly increased in the MLO experiments and offset the reduction of OH.
The chemical feedbacks in the stratosphere include increases of stratospheric water vapour (SWV) mixing ratios on the order of 50 % for doubled CH₄, and 250 % for fivefold CH₄. In the middle and upper stratosphere the increase of SWV is weaker in the experiments that include slow feedbacks. This is linked to a weaker increase of stratospheric OH and a likewise weaker strengthening of the CH₄ oxidation. In the lower stratosphere slow feedbacks lead to a more pronounced increase of SWV compared to rapid adjustments enlarging its overall radiative impact.
The MLO simulations show further evidence of the strengthening of the Brewer-Dobson circulation.

The rapid radiative adjustments from O₃ and SWV contribute significantly to the CH₄ effective radiative forcing, whereas the radiative impact of the respective slow feedbacks is rather moderate. In accordance with this, the climate sensitivity from CH₄ changes in this chemistry-climate model set-up is not significantly different from the climate sensitivity in CO₂-driven simulations, provided that the rapid adjustments from O₃ and SWV changes are included in the CH₄ effective radiative forcing.