Fundamental trade-off between climate and air quality from sulfur reductions in marine fuels

Abstract

International shipping is a major source of anthropogenic sulfur emissions, affecting both climate and air quality. The high fuel sulfur content (FSC) in marine fuels leads to high sulfur dioxide (SO2) emissions and subsequent sulfate aerosol particle formation in the atmosphere, particularly over major shipping lanes and along coastal regions where cloud susceptibility is high. Sulfate aerosol particles act as cloud condensation nuclei (CCN) and can alter the microphysical and radiative properties of clouds, enhancing cloud albedo and exerting a net cooling effect on the climate. At the same time, sulfate aerosol particles deteriorate air quality and pose risks to human health. To mitigate the adverse health effects, the International Maritime Organization (IMO) implemented the IMO2020 regulations, which reduced the maximum allowed FSC in marine fuels from 3.5% to 0.5% as of January 2020. However, the resulting reduction in sulfate aerosol particle burden also diminishes the aerosol-induced cooling effect, potentially unmasking part of the previously suppressed anthropogenic warming. In this thesis, aerosol-climate model simulations suggest that IMO2020 regulations led to a loss of aerosol-induced cooling of +67 mW m-2 globally, while the concentration of ship-induced fine particulate matter simultaneously dropped by ~60% across continents. Sensitivity simulations to test the effects of hypothetical region-specific regulation strategies demonstrate that the strongest air quality improvements occur when IMO2020 regulations are enforced in coastal regions where population density is high, while open-ocean regulations have little effect on air quality. However, the largest loss of aerosol cooling is also attributable to FSC reductions in coastal regions, where ship traffic is dense and cloud albedo highly susceptible to aerosol perturbations. Consequently our results highlight a fundamental trade-off: efforts to reduce air pollution caused by the shipping sector simultaneously lead to a substantial loss of aerosol-induced cooling. The balance between air quality improvements and retaining the cooling strongly depends on the spatial distribution of ship traffic, population exposure and cloud cover. Future research should explore the trade-off across multiple models and for region-specific regulation strategies under different climate change scenarios.