Has Reducing Ship Emissions Brought Forward Global Warming?

Ships brighten low marine clouds from emissions of sulfur and aerosols, resulting in visible "ship tracks". In 2020, new shipping regulations mandated an similar to 80% reduction in the allowed fuel sulfur content. Recent observations indicate that visible ship tracks have decreased. Model simulations indicate that since 2020 shipping regulations have induced a net radiative forcing of +0.12 Wm-2. Analysis of recent temperature anomalies indicates Northern Hemisphere surface temperature anomalies in 2022-2023 are correlated with observed cloud radiative forcing and the cloud radiative forcing is spatially correlated with the simulated radiative forcing from the 2020 shipping emission changes. Shipping emissions changes could be accelerating global warming. To better constrain these estimates, better access to ship position data and understanding of ship aerosol emissions are needed. Understanding the risks and benefits of emissions reductions and the difficultly in robust attribution highlights the large uncertainty in attributing proposed deliberate climate intervention. Ships have a unique climate effect due to brightening of low marine clouds, resulting in visible "ship tracks". These ship tracks are due to clouds interacting with ship emissions, particularly sulfur. Recently, regulations have drastically reduced allowable ship sulfur emissions. This has resulted in a decrease in observable ship tracks. Modeling and observations indicate that the reduction in ship sulfur emissions could have slightly warmed the planet starting in 2020. These changes are remarkably co-incident with observed patterns of cloud changes and may have accelerated global warming. Recent regulations on ship sulfur emissions have decreased ship tracks and resulted in +0.12 Wm-2 of radiative forcing Observed cloud anomalies are correlated with observed ocean temperature anomalies and shipping radiative forcing Reduced ship emissions may have accelerated global warming contributing to recent warm Northern Hemisphere surface temperatures