First map of cargo ship pollution demystifies regulatory effects

So-called ‘ship tracks’ can offer an insight into the impact of emissions policy on air pollution – a major step forward for climate modelling.

Researchers at the University of Maryland, Baltimore County (UMBC), have published a new study which uses satellite data garnered between 2003 and 2020 to gauge the effects of fuel regulations on the level of pollution caused by cargo ships. 

According to the results, during the period investigated there were significant changes in the amount of sulfur pollution emitted by vessels once new regulations were introduced, first in 2015 and again in 2020. A clear sign that policy can have a substantial impact, this evidence can also be used to improve understanding of how pollutants and other particles interact with clouds – a major factor in gauging the extent to which we can expect the planet to warm in the coming decades. 

When pollutant particles created by ships enter low cloud cover, the size of individual water droplets decreases, but the total volume carried in the cloud doesn’t change. This means more droplet surface area, reflecting more solar energy back into space, keeping Earth artificially cooler than it should be based on the amount of greenhouse gases present in the atmosphere. 

Ocean air is generally very clean, apart from shipping routes. ‘Ship tracks’ – traces of pollutants left over, around, and in the wake of vessels – can easily be picked up by satellites. Crucially, the UMBC team found that tracks do not occur everywhere ships are travelling, but instead form only in areas with specific types of cloud cover at low altitude.

Emission Control Areas introduced in Europe, the U.S. and Canada successfully reduced this form of pollution, but hotspots developed on the edges of these zones as a result. Meanwhile, territories that didn’t have such schemes in place also saw increased levels of pollution. 

‘Ship pollution alone can create a substantial cooling effect because the atmosphere over the ocean is so clean,’ said UMBC’s Tianle Yuan. ‘There is a physical limit to how small cloud droplets can get, so at a certain point, adding more pollution doesn’t increase the clouds’ cooling effect.

‘But over the ocean, because the background is largely unpolluted, even a small amount of pollution from ships has an effect,’ he continued. ‘We can take advantage of the millions of ship track samples we have now to start to get hold of the overall aerosol-cloud interaction problem… ship tracks can be used as mini-labs.’

In order to conduct the study, Yuan and his team devised an algorithm to automate the collation of ship track satellite data. This has allowed them opportunity to create the first ever global map of these emissions over an 18 year period. The plan is now to make this available to researchers and scientists across the globe. 

Back in February, the world’s second-largest shipping company, Maersk, unveiled its intentions to introduce new renewable fuel-based charging buoys, which will be installed off the coast of major ports, allowing vessels to idle without direct emissions. 

Image credit: Borderpolar Photographer