Multi-model effective radiative forcing of the 2020 sulfur cap for shipping

Strict regulations on sulfur emissions from shipping, introduced in 2020, have drastically reduced SO₂ emissions from international shipping. As SO₂ is an aerosol precursor, this decline in anthropogenic emissions over the ocean will weaken the total aerosol effective radiative forcing (ERF) that historically has masked an uncertain fraction of the greenhouse gas induced warming.

Here, we use four global climate models and a chemical transport model to calculate the ERF resulting from an 80% reduction in SO₂ emissions from international shipping relative to 2019 emission estimates. The individual model means range from 0.06 to 0.09 W m^-2, corresponding to the ERF resulting from the increase in CO₂ concentration over the last 2 to 3 years. The impact of this one-year drop in shipping emissions in 2020 is overshadowed by the long-term effects of reduced anthropogenic SO₂ emissions over the past decades, also in oceanic regions.

Using a single model, we investigate sensitivities due to Dimethyl sulfide (DMS) emissions and calculate the forcing from the emission reduction in 2020 as represented by the most recent emission inventories.

As for aerosol ERF in general, the ERF due to the new shipping sulfur regulations has a large uncertainty range. Although not fully quantified here, this will very likely be high considering the contribution of uncertainties in shipping SO₂ emissions, the sulfur cycle, the modelling of cloud adjustments and the impact of interannual variability on the method for calculating radiative forcing.