The Climate, Air Quality and Health co-benefits and trade-offs from different future mitigation scenarios involving Near-Term Climate Forcers in UKESM1

Near-term climate forcers (NTCF) are a group of chemically and radiatively active constituents that have a relatively short lifetime in the atmosphere (<20 years). They can exert effects on the climate, important for the future rate of climate warming, and in elevated concentrations at the lowest most levels of the atmosphere can lead to poor air quality and detrimental impacts on human health. Two important NTCFs that are considered in this study are tropospheric O₃ and fine particulate matter (with a diameter less than 2.5 microns – PM_2.5). Future climate mitigation scenarios that seek to limit future temperature increases, and include reductions in air pollutant emissions, need to consider the impact on climate, air quality and human health from changes in NTCFs. Here we use results from UKESM1 (an Earth system model with interactive chemistry and aerosols) in different future sensitivity scenarios that consider air pollutant emission mitigation, future climate change and land-use change, conducted as part of the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP). We assess the impact on climate (in terms of effective radiative forcing), air pollutants (in terms of change in ambient surface concentrations) and human health (in terms of long-term adult mortality from exposure to ambient air pollutants) by comparing the results from these sensitivity scenarios to the future reference scenario ssp370, a scenario that involves low mitigation of climate and air pollutants.

Scenarios that involve combined strong mitigation of aerosols and O₃ precursors, including large reductions in global CH₄ concentrations, produce the largest benefits to climate (an ERF of -1.2 Wm^-2), air quality (10-25% reduction in O₃ and PM_2.5 concentrations) and human health (>25% reduction in the rate of long-term premature mortality). Benefits to health are largest across Asia for these scenarios (a 44% reduction in the mortality rate). If global CH₄ concentrations are not reduced or aerosol precursors emissions are reduced in isolation, then there is a detrimental impact to future climate but there are still improvements to future air quality and a reduction in the long-term air pollutant health burden. If climate and air quality mitigation measures are not enacted on top of ssp370 then there is a penalty to global climate, a detrimental impact on air pollutant concentrations and an increase in the long-term air pollutant health burden across certain regions (e.g. by 20% over Africa). Considering only the impacts from climate change show increases in air pollutant concentrations over some continental regions and also an increase in the long-term rate of premature mortality by more than 10% over Europe and parts of Asia, offsetting some of the benefits achieved from emission mitigation measures. Quantifying co-benefits and trade-offs between climate, air quality, and human health together in this way, enables policy makers to understand the outcomes of different mitigation strategies and to identify pathways with maximum benefits across all three axes.