The ozone-climate penalty over South America and Africa by 2100

Climate change has the potential to increase surface ozone concentrations, known as the ‘ozone-climate penalty’, through changes to atmospheric chemistry, transport and land surface behaviour. In the tropics, the response of surface ozone to a changing climate is relatively understudied, but will have important consequences for air pollution, human and ecosystem health. In this study, we predict the change in surface ozone due to climate change over South America and Africa using data from 3 state-of-the-art Earth system models from CMIP6. To identify the changes driven by climate change alone, we use the difference between the Shared Socioeconomic Pathway 3 7.0 emissions scenario which includes climate change and the same scenario without climate change. The SSP3 7.0 scenario has high emissions of near-term climate forcers and substantial land-use change leading to large temperature changes.

We find that by 2100, there will be an ozone-climate penalty in areas where background ozone is already high, namely urban and biomass burning areas. This includes robust annual mean increases in surface ozone of up to 4 ppb over polluted regions such as the arc of deforestation in the Amazon, with dry season months showing increases of up to 15 ppb. These areas have high NOx emissions from fires, transport or industry. However, models disagree on the role of climate change in remote, low-NOx regions, partly because of uncertainties in NOx concentrations. We also find that the magnitude and location of the ozone-climate penalty in the Congo basin has greater inter-model variation than the Amazon.

We attribute the increase in surface ozone concentration to an increase in the rate of ozone chemical production, which is strongly influenced by the background NOx concentration. As NOx emissions are largely anthropogenic, this suggests that without reduction in emissions, forested areas in urban and agricultural locations are at increasing risk of ozone damage due to climate change. This has implications for the success of secondary forests and other human-modified forests which are mostly located in agricultural areas, deforestation frontiers and forest edges.