Endorsed by SOLAS since November 2019
Ship emissions are significant sources of polluting aerosols in coastal regions, causing hundreds of thousands of premature deaths per year globally. To address this, in 2015 the International Maritime Organization (IMO) ordered a reduction in the maximum ship sulphur emissions in coastal Sulphur Emission Control Areas (SECAs) of Europe and North America. From January 2020 this will be expanded to international waters, to reduce sulphur emissions from 3.5% to 0.5% of fuel mass.
This reduction in sulphur emissions from shipping is predicted to almost halve the number of premature deaths globally from sulphate aerosols but less sulphate in the atmosphere is also expected to reduce cloud brightness, reflecting less incoming solar radiation, which may ultimately lead to an increase in global average temperature.
ACRUISE will use a multidisciplinary approach to quantify the impact on atmospheric chemistry and climate due to the introduction of the IMO’s regulation of sulphur emissions from shipping in international waters in January 2020. This approach will combine in situ observations (aircraft and surface stations), satellite analysis and modelling across a large range of scales (from cloud, to regional to global) to quantify the chemical and climatic impact of the 2020 sulphur emission regulation.
Find more about ACRUISE at: https://www.pml.ac.uk/science/Projects/ACRUISE
The ACRUISE project’s recent works on ship emissions reveal new insights into the climatic impact of ship emissions and aerosol-cloud interaction: 1) Authors used a large database of ship paths and calculated where their emissions were advected and entered clouds. Then using satellite data, authors measured the cloud droplet number concentration and liquid water content in ship-polluted versus unpolluted clouds and found that ship emissions make nearby clouds more reflective regardless of whether a classic "ship track" is visible or not. Previous satellite-based studies that only focused on visible ship tracks might have underestimated the radiative forcing from shipping. 2) Another recent study used machined learning to find visible ship tracks in satellite images before and after 2020, when the IMO mandated the maximum fuel sulfur content to reduce by a factor of 7. Authors found that visible ship tracks decreased by 25% since the sulfur regulation.
Ongoing works within ACRUISE include analyses of flight and ground-based observations as well as model simulations across a range of scales, with the motivation to trace ships emissions from source to atmospheric processing and to impact on clouds.
Visible and invisible ship tracks. Color indicates the size of the cloud droplets. Ship tracks are visible in a) as the darker lines where aerosols have caused smaller droplet sizes. White boxes indicate authors' estimate of where ship emissions are advected by wind. Panel b) shows the example of a day with no visible tracks, even though ships have polluted some region (white boxes). Comparing these with the regions to either side (yellow) allows us to find the aerosol effect.
Reference: Manshausen, P., Watson-Parris, D., Christensen, M.W., et al. (2022). Invisible ship tracks show large cloud sensitivity to aerosol. Nature, 610, 101-106. https://doi.org/10.1038/s4 1586-022-05122-0
Watson-Parris, D., Christensen, M.W., Laurenson, A. and Stier, P. (2022). Shipping regulations lead to large reduction in cloud perturbations. Proc. Natl. Acad. Sci. U. S. A., 119 (41), e2206885 119. https://doi.org/10.1073/pnas.2206885119