With MSI images taken on 3 August 2025, the EarthCARE Data, Innovation and Science Cluster (DISC) highlights the satellite’s power to probe what happens when ship fumes meet ocean clouds. The first figure below shows a natural colour Red Green Blue (RGB) image over the Pacific Ocean. The region between 45–50 °N clearly exhibits ship tracks embedded within low-level marine stratocumulus clouds.
These clouds are affected by combustion aerosols from ships.

Ships and sulphur

Ships burn sulphur-containing fuels and release substantial amounts of sulphur dioxide (SO₂) into the marine boundary layer of the atmosphere. This SO₂ is rapidly oxidized to sulphate aerosol, which serves as an effective cloud condensation nucleus (CCN).

The enhanced aerosol load from ship emissions increases the number of cloud droplets in liquid-phase clouds. As these droplets form on a larger number of nuclei, their effective radius decreases. This mechanism, referred to as the Twomey effect, occurs on very short timescales.

The second figure shows a zoomed-in subset of the MSI RGB image between 46–50 °N (a), as well as the cloud optical thickness (COT) (b) and effective radius (REFF) (c) from the MSI M-COP cloud product (MSI_COP_2A). The cloud droplet number concentration (d) is derived from COT and REFF following a relationship introduced by Gryspeerdt et al., 2019.

The results confirm an increase of the COT (b) and cloud droplet number concentration (d) relative to the unperturbed background. The cloud droplet number concentration is increased by about 30 cm-3, whereas the cloud REFF (c) is supressed by about 1-2 µm.

The combination of a higher droplet number concentration and smaller droplet sizes increases the total cross-sectional area of cloud droplets, enhancing the cloud’s optical depth.

As a result, more incoming solar radiation is reflected to space, leading to an increase in cloud albedo.

This phenomenon, commonly termed marine cloud brightening, contributes to a net cooling of Earth’s surface.

EarthCARE adds global perspective of clouds and climate

In 2020, the International Maritime Organization (IMO) introduced new regulations that reduced the sulphur content of ship fuels by about 80%. As a result, emissions of sulphate aerosols from ships strongly decreased, as have visible ship tracks.

The resulting reduction in the cooling effect of the aerosol-modified clouds appears to be linked to an acceleration in global warming since 2020.

In addition to monitoring the effects of shipping pollution with MSI, further synergistic measurements from the Cloud Profiling Radar (CPR) -developed by the Japan Aerospace Exploration Agency (JAXA)- and Atmospheric Lidar (ATLID) will also enable better understanding of the vertical structure of these clouds.

“It’s exciting to see how well the multispectral imager puts clouds like ship tracks into context,” says Anja Hünerbein of the Leibniz Institute for Tropospheric Research (TROPOS). “Such detailed information on different cloud types and how they are changing globally due to pollution will be critical in efforts to better model future climate change.”

Measuring these effects worldwide is precisely why EarthCARE is in space. Its fourth instrument, the broadband radiometer (BBR), views Earth from three different angles to measure the amount of sunlight and thermal radiation leaving the atmosphere. Along with EarthCARE’s other instruments, we will see how clouds and aerosols affect Earth’s heating and cooling on a global scale.

Thank you to the EarthCARE DISC for their contribution to this article. You can find more information about all the EarthCARE products via the dedicated EarthCARE data page on Earth Online.