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Polluting ships could be detected from their cloud tracks

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Researchers from Imperial College London, the University of Oxford, and University College London have used satellite imaging data to investigate the impact of shipping on cloud formation, finding a strong connection between the two.

The environmental impacts of shipping are considerable, particularly given the sheer amount of cargo that travels by sea. As ships burn fuel, they release pollution into both the sea and air, including sulphurous gases, heavy metals and other carcinogens. Airborne particles of sulphur and oxygen (sulphates) emitted by ships are known to modify some types of clouds.

These particles act as ‘seeds’ around which water droplets gather, leaving visible ‘ship tracks’ in the clouds behind them. Understanding how these clouds are affected by aerosols is valuable for building models to inform the climate mitigation effort, as these clouds influence climate change.

Although it can be difficult to connect emission of aerosols with their impact on clouds, ship tracks offered a comparatively straightforward starting point for the UK scientists to investigate the connection. They worked with satellite images of more than 17,000 ship tracks matched with the movements of individual ships (identified from their on-board GPS), finding that these changes in the clouds largely disappear in zones with restrictions on sulphur in shipping fuels.

“Ship tracks act like an experiment that would be impossible for us to do otherwise: we cannot inject sulphate aerosols into the atmosphere at such scale to see what happens,” Dr Edward Gryspeerdt, a research fellow at Imperial College’s Department of Physics, said in a statement. “Instead, restrictions on the amount of ship sulphate emissions can contain provide us with a perfect experiment for determining just how important the aerosols are in cloud formation.”

The researchers included data from a period in which areas around the coast of North America, the North Sea, the Baltic Sea and the English Channel restricted sulphur in ship fuel to 0.5 per cent. They saw that ship tracks almost entirely vanished in these areas under similar conditions.

They concluded that sulphate aerosols have the most significant impact on cloud formation, with other pollutants in ship exhaust – such as black carbon – having a comparatively small impact.

Their findings are likely to prove useful in helping monitor compliance with new sulphur regulations, which come into force in 2020. The International Maritime Organisation will require shipping companies to adhere to the 0.5 per cent restriction on sulphur content in shipping fuel, hopefully bringing about “major health and environmental benefits for the world”. Ships which flout this rule could be easily detected from satellite images of the clouds it passes beneath.

“Currently, it is hard for regulators to know what ships are doing in the middle of the ocean,” said UCL’s Dr Tristan Smith, a co-author of the study. “The potential for undetected non-compliance with the 2020 sulphur regulations is a real risk for shipping companies because it can create commercial advantage to those companies who do not comply.”

In September, it was reported that shipping companies have been spending billions of pounds fitting ‘cheating’ devices to hide their sulphur emissions while rerouting pollution into the sea in preparation for the new sulphur restrictions.

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