The study was published in Geophysical Research Letters, led by researchers from Imperial College London, along with University College London and the University of Oxford.
Emissions from ships contain several chemicals, which act as ‘seeds’ around which water droplets gather, causing changes in cloud properties that are visible to satellites. This means that ships can affect clouds, leaving lines, known as ship tracks, in the clouds behind them as they sail.
However, the exact impact of these chemicals on the clouds is not precisely known. This is important as the kinds of clouds that the emissions affect can influence climate warming, and is therefore significant to capture in climate models.
These chemicals are also emitted from many sources, such as factories and cars, but with ship tracks the relationship is more straightforward, enabling researchers to tease out the links between aerosols and clouds more easily.
Lead researcher Dr Edward Gryspeerdt, from the Department of Physics at Imperial, stated regarding the study:
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. 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. By analysing a huge dataset of ship tracks observed from satellites, we can see that they largely disappear when restrictions are introduced, demonstrating the strong impact of aerosols
The team studied over 17,000 ship tracks from satellite observations and matched them to the movements of individual ships using their onboard GPS. The study covered the introduction of emission control areas around the coast of North America, the North Sea, the Baltic Sea and the English Channel, which restricted sulphur in ship fuel to 0.5%, leading to less sulphate aerosol emissions.
The researchers concluded that in these areas, ship tracks almost completely disappeared compared to prior the restrictions, under similar weather conditions. This indicates that sulphate aerosols have the most significant impact on cloud formation, as opposed to other components of the ship exhaust, like black carbon.
In addition, this finding also means that a ship that is not in compliance with the regulations, by burning the current high-sulphur fuels without exhaust treatment, could be detected because it would create a measurable difference in the satellite-observed cloud properties.
The team now want to more precisely relate known ship fuel compositions to ship tracks, enabling them to more accurately predict the influence of sulphur aerosols on cloud formation on a larger scale, ready to feed into climate models.