The Climate and Air Quality response under different future ssp370 pathways in UKESM1

Short lived climate forcers (SLCFs) are important atmospheric components as they can influence climate, through interactions with the Earth’s radiative balance, but also impact regional air quality. Two important SLCFs are tropospheric O₃ and fine particulate matter (with a diameter less than 2.5 microns – PM_2.5). Future policy measures aim to provide co-benefits to both climate and air quality via mitigation of SLCFs. However, it is still uncertain how future reductions in SLCFs will impact both climate and air quality. Sensitivity experiments conducted as part of the 6^th Coupled Model Intercomparison Project (CMIP6) provide an opportunity to assess the climate and air quality impacts of different mitigation scenarios.

Here we use results from UKESM1 (an Earth system model with interactive chemistry and aerosols) for the future climate and emission scenario ssp370SST, an atmosphere only simulation that assumes low mitigation of climate and air pollutants. We then compare results from this scenario to different sensitivity experiments to assess the impact on climate, through effective radiative forcing (ERF), and air quality, by changes in surface concentrations of O₃ and PM_2.5. The sensitivity experiments consider reductions to CH₄ concentrations and emissions of O₃ and aerosol precursors, individually and combined. Additional sensitivity experiments also consider the individual impact from land-use change, climate change and all emissions.

Compared to ssp370SST, scenarios that strongly mitigate both aerosol and O₃ precursors, including CH₄, produce the largest benefits in 2100 to global air quality, a 10-25% reduction in global annual mean O₃ and PM_2.5 concentrations, and climate, a change in ERF of up to -1.2 Wm^-2. If CH₄ concentrations are not reduced but other aerosols and O₃ precursors are, then there are still benefits to air quality in 2100, relative to ssp370SST, but the change in the ERF becomes positive (up to +0.4 Wm^-2), mainly due to aerosols reductions. The impact of solely reducing CH₄ concentrations results in a large change in ERF (-1.4 Wm^-2) and large reductions in surface O₃ (-15%) in 2100 but has negligible impacts on surface PM_2.5. Reducing only aerosol precursors decreases PM_2.5 concentrations but results in a positive change in O₃ concentrations and ERF in 2100. Reducing only tropospheric O₃ precursors decreases surface O₃ concentrations but has minimal impact on PM_2.5 concentrations and the ERF in 2100. Implementing different land-use policies has a small impact on the ERF in 2100 but slightly increases both global surface O₃ and PM_2.5.

The results from this single model study show the importance of considering the different impacts of SLCFs on future air quality and climate metrics.