Implication of strongly increased atmospheric methane concentrations for chemistry–climate connections

Abstract

Abstract. Methane (CH4) is the second-most important directly emitted greenhouse gas, the atmospheric concentration of which is influenced by human activities. In this study, numerical simulations with the chemistry–climate model (CCM) EMAC are performed, aiming to assess possible consequences of significantly enhanced CH4 concentrations in the Earth's atmosphere for the climate. We analyse experiments with 2×CH4 and 5×CH4 present-day (2010) mixing ratio and its quasi-instantaneous chemical impact on the atmosphere. The massive increase in CH4 strongly influences the tropospheric chemistry by reducing the OH abundance and thereby extending the CH4 lifetime as well as the residence time of other chemical substances. The region above the tropopause is impacted by a substantial rise in stratospheric water vapour (SWV). The stratospheric ozone (O3) column increases overall, but SWV-induced stratospheric cooling also leads to a enhanced ozone depletion in the Antarctic lower stratosphere. Regional patterns of ozone change are affected by modification of stratospheric dynamics, i.e. increased tropical upwelling and stronger meridional transport towards the polar regions. We calculate the net radiative impact (RI) of the 2×CH4 experiment to be 0.69 W m−2, and for the 5×CH4 experiment to be 1.79 W m−2. A substantial part of the RH is contributed by chemically induced O3 and SWV changes, in line with previous radiative forcing estimates. To our knowledge this is the first numerical study using a CCM with respect to 2- and 5-fold CH4 concentrations and it is therefore an overdue analysis as it emphasizes the impact of possible strong future CH4 emissions on atmospheric chemistry and its feedback on climate.

Assessment of pre-industrial to present-day anthropogenic climate forcing in UKESM1

“…Hansen et al, 2005;Shindell et al, 2009;Myhre et al, 2013a), who concluded that the total climate forcing by CH4 is almost double that of the direct forcing due to indirect effects. The UKESM1 estimate is also larger than the +0.69 W m -2 radiative impact of an increase in CH4 concentration of 1800 ppbv above present-day levels quantified by Winterstein et al (2019) (Lohmann et al, 2010). The relative contributions of the direct and indirect forcings to the total CH4 ERF quantified here and an emissions-based perspective of the CH4 forcing can be found in O'Connor et al (2019).…”

Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2020

“…Methane contributes directly to the chemical loss of stratospheric ozone, and also indirectly (like other GHGs) by warming the troposphere and cooling the stratosphere [ 397 , 398 ]. Natural sources of CH 4 are likely to increase in the future because of climate change.…”

Assessment of pre-industrial to present-day anthropogenic climate forcing in UKESM1

“…When comparing the ERF and IRF for black carbon (BC), as an example, RAs lead to the ERF being half that of its IRF (Stjern et al, 2017;Smith et al, 2018). On the other hand, Xia et al (2016) found in their model that cloud and sea ice adjustments driven by stratospheric O 3 recovery included in the ERF definition lead to the ERF being different in both sign and magnitude from the SARF. And when comparing different forcing metrics, Hansen et al (2005) found that the efficacy of a climate forcing (i.e.…”