Climate responses to a rapid phaseout of sulfur in shipping emissions: A large ensemble study

In 2020, motivated by improving air quality in major ports and shipping lanes, the International Maritime Organization imposed strict new regulations on the sulfur content of shipping fuel. This led to a rapid reduction in the number of observed ship tracks (linear tracks of clouds brightened by aerosol perturbations; Watson-Parris et al. 2022), and presumably a commensurate reduction in anthropogenic aerosol forcing. The magnitude of this forcing, and the resulting temperature change, are uncertain however. The recent confirmation that 2023 was the hottest year on record can only partly be explained by the onset of the El Niño phase of the El Niño-Southern Oscillation (ENSO). Such warming, in addition to the sizable warming in NH ocean basins- geographically collocated with shipping- raise the question of how much shipping emissions changes might have contributed to this signal, and any extreme weather events associated with it.   In this study we aim to answer this question by utilizing a large ensemble of fully-coupled Community Earth System Model version 2 (CESM2) simulations with and without the shipping emissions changes. We leverage the CESM2 large ensemble and choose 20 simulations with varying ENSO conditions from which to branch off with shipping emissions reduced to 20% of their baseline value. These are integrated forward for another 20 years, while non-shipping emissions follow the SSP3-7.0 scenario, in order to robustly explore the transient climate response. In this talk we will highlight the forced climate response, focusing on temperature (T), precipitation (P), and atmospheric circulation, both globally and in key regions such as the North Atlantic. Given the change in ENSO phase during 2023, we will also describe how this climate response is modulated by different ENSO conditions, the Atlantic Multidecadal Variability and other modes of climate variability. The underlying relevant climate processes, including cloud dynamics, radiative imbalances at the top of the atmosphere, and daily variability will be summarized to link our single model study to observed changes. References: [1] Watson-Parris, D., Christensen, M., Laurenson, A., Clewley, D., Gryspeerdt, E., Stier, P. “Shipping regulations lead to large reduction in cloud perturbations”. PNAS 119 (41) e2206885119: https://doi.org/10.1073/pnas.2206885119 (2022)