The Climate and Air Quality response under different future ssp370 pathways in UKESM1
Authors: Turnock Steven , Davli Mohit , Keeble James , Robertson Eddy , O'Connor Fiona 
Published: March 2021

&lt;p&gt;Short lived climate forcers (SLCFs) are important atmospheric components as they can influence climate, through interactions with the Earth&amp;#8217;s radiative balance, but also impact regional air quality. Two important SLCFs are tropospheric O&lt;sub&gt;3&lt;/sub&gt; and fine particulate matter (with a diameter less than 2.5 microns &amp;#8211; PM&lt;sub&gt;2.5&lt;/sub&gt;). 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&lt;sup&gt;th&lt;/sup&gt; Coupled Model Intercomparison Project (CMIP6) provide an opportunity to assess the climate and air quality impacts of different mitigation scenarios.&lt;/p&gt;&lt;p&gt;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 O&lt;sub&gt;3&lt;/sub&gt; and PM&lt;sub&gt;2.5&lt;/sub&gt;. The sensitivity experiments consider reductions to CH&lt;sub&gt;4&lt;/sub&gt; concentrations and emissions of O&lt;sub&gt;3&lt;/sub&gt; and aerosol precursors, individually and combined. Additional sensitivity experiments also consider the individual impact from land-use change, climate change and all emissions.&lt;/p&gt;&lt;p&gt;Compared to ssp370SST, scenarios that strongly mitigate both aerosol and O&lt;sub&gt;3&lt;/sub&gt; precursors, including CH&lt;sub&gt;4&lt;/sub&gt;, produce the largest benefits in 2100 to global air quality, a 10-25% reduction in global annual mean O&lt;sub&gt;3&lt;/sub&gt; and PM&lt;sub&gt;2.5&lt;/sub&gt; concentrations, and climate, a change in ERF of up to -1.2 Wm&lt;sup&gt;-2&lt;/sup&gt;. If CH&lt;sub&gt;4&lt;/sub&gt; concentrations are not reduced but other aerosols and O&lt;sub&gt;3&lt;/sub&gt; 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&lt;sup&gt;-2&lt;/sup&gt;), mainly due to aerosols reductions. The impact of solely reducing CH&lt;sub&gt;4&lt;/sub&gt; concentrations results in a large change in ERF (-1.4 Wm&lt;sup&gt;-2&lt;/sup&gt;) and large reductions in surface O&lt;sub&gt;3&lt;/sub&gt; (-15%) in 2100 but has negligible impacts on surface PM&lt;sub&gt;2.5&lt;/sub&gt;. Reducing only aerosol precursors decreases PM&lt;sub&gt;2.5&lt;/sub&gt; concentrations but results in a positive change in O&lt;sub&gt;3&lt;/sub&gt; concentrations and ERF in 2100. Reducing only tropospheric O&lt;sub&gt;3&lt;/sub&gt; precursors decreases surface O&lt;sub&gt;3&lt;/sub&gt; concentrations but has minimal impact on PM&lt;sub&gt;2.5&lt;/sub&gt; 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 O&lt;sub&gt;3&lt;/sub&gt; and PM&lt;sub&gt;2.5&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;The results from this single model study show the importance of considering the different impacts of SLCFs on future air quality and climate metrics.&lt;/p&gt;