Apportionment of the Pre-Industrial to Present-Day Climate Forcing by Methane using UKESM1: The role of the cloud radiative effect

Abstract

The pre-industrial (Year 1850) to present-day (Year 2014) increase in methane from 808 to 1831 ppb leads to an effective radiative forcing (ERF) of 0.97±0.04 Wm -2 in the United Kingdom's Earth System Model, UKESM1. The direct methane contribution is 0.54+-0.04 Wm -2 . It is better represented in UKESM1 than in its predecessor due to the inclusion of shortwave absorption, updates to the longwave spectral properties, and no interference from dust. An indirect ozone ERF of 0.13-0.20 Wm -2 is largely due to the radiative effect of the tropospheric ozone increase outweighing that of the stratospheric ozone decrease. An indirect water vapor ERF of 0.07+-0.05/0.02+-0.04 Wm -2 is consistent with previous estimates based on the stratospherically-adjusted radiative forcing metric. The methane increase also leads to a cloud radiative effect of 0.12+-0.02 Wm -2 from aerosol-cloud interactions and thermodynamic adjustments. The aerosol-mediated contribution (0.28-0.30 Wm -2 ) arises because methane-driven oxidant changes alter the rate of new particle formation (-8 %), causing a change in the aerosol size distribution towards fewer larger particles. There is a resulting decrease in cloud droplet number concentration and an increase in cloud droplet effective radius. There are additional shortwave and longwave contributions of 0.23 and -0.35 Wm -2 to the cloud forcing which are dynamically-driven. They arise from radiative heating and stabilization of the upper troposphere, resulting in a reduction in global cloud cover and convection. These results highlight the importance of chemistry-aerosol-cloud interactions and dynamical adjustments in climate forcing and can explain some of the diversity in multi-model estimates of methane forcing.

Drivers of persistent changes in the global methane cycle under aggressive mitigation action

“…It causes substantially more warming than CO2 per unit mass with a global warming potential (GWP) exceeding that of CO2 by a factor of 29.8±11 for methane from fossil fuel sources (5) over a 100-year period (GWP100). Indirect methane radiative forcing accounts for nearly 50% of its total forcing and includes tropospheric ozone formation (6), stratospheric water vapour formation from methane oxidation, aerosol-mediated cloud forcing due to changes in cloud activation, and dynamically-driven cloud forcing from methane (7,8). The role of methane in achieving climate goals is currently a topic of great interest (9,10,11).…”

“…Other near-term climate forcers such as nitrogen oxides, NOx, and carbon monoxide, CO, strongly impact the atmospheric methane lifetime and burden. Methane impact on climate also includes important indirect effects via its role in tropospheric ozone formation, in stratospheric water vapour formation, in aerosol-mediated cloud forcing due to changes in cloud activation, and dynamically-driven cloud forcing (6,7,8).…”