Radiative forcing from the 2020 shipping fuel regulation is large but hard to detect

In 2020, the International Maritime Organization imposed a sharp reduction in the level of sulfur in ship fuel to reduce emissions of sulfate aerosols and sulfur dioxide — pollutants associated with premature deaths from asthma, lung cancer, cardiovascular and pulmonary diseases, to improve air quality in coastal and large port communities. By decreasing sulfur pollution, the cleaner fuels also reduced the reflectivity of low-level marine clouds that aerosol particles in the exhaust had previously brightened – reducing the cooling effect of clouds.
Soon, observers noticed a decrease in “ship tracks,” those distinct, bright white, linear clouds that form in the trail of large ships, allowing more sunlight to heat the ocean surface. But quantifying the warming effect of increased sunshine on the ocean surface using conventional climate models has proven challenging because they simply can’t represent clouds well enough. We devise an innovative Machine Learning approach to capture and reproduce the complex physical and chemical processes that control cloud formation and unmask the true climate impact that these ship-induced, sunlight-reflecting clouds once had.
To estimate the cooling effect that these ship tracks used to have, an ensemble of neural networks was trained with multiple years of global cloud observations from satellites and model reanalysis of the meteorological fields collected before 2020 over three major shipping corridors in the Pacific and Atlantic oceans where ship tracks are the most prevalent. The neural networks were then tasked with simulating counterfactual cloud fields, representing the “observation record” beyond 2020 as if the IMO2020 regulation had never taken place (as a “business as usual” scenario). Differencing the counterfactual fields from the true 2020-2022 observational record of cloud radiative effect reveals a warming effect of 0.074 ±0.005 W m-2 as a result of this event. Furthermore, by comparing this event against the natural variability in clouds, using statistical significance tests, we concluded that changes in cloud radiative effect whether intentional or due to climate change will be difficult to detect above natural variability, a finding that has direct implications for evaluating the potential of marine cloud brightening.
These results raise concerns that future reductions in aerosol emissions, despite the important health benefits, will exacerbate global warming due to reduced aerosol radiative forcing (a net cooling effect). Our findings imply a strong, regionally-dependent masking effect of natural variability on the detectability of proposed deliberate modification to the cloud radiative effect. Such effort, if attempted, will need to be substantial in order to overcome the low detectability.
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