Shipping emissions: Ships release invisible contrails that slightly cool the climate

The effect of aerosol pollution from shipping and other sources on clouds has been underestimated and may need to be factored into future climate models


5 October 2022

A container cargo ship in the ocean

Ships release aerosol particles of air pollution as well as greenhouse gases

Tryaging/Getty Images

Streaks of invisible air pollution from the world’s shipping industry, which are unseen on satellite imagery, are changing the reflectivity of clouds. This means clouds exposed to air pollution may reflect more of the sun’s light and heat than we thought, which will have to be taken into account for future climate models.

Clouds play a complex role in the climate. Depending on how high they float in the atmosphere and their composition, they can either contribute to or detract from global warming. Understanding their interactions with the environment is crucial for accurate climate models.

Pollution from aerosols – fine particles suspended in air – influences the formation and properties of clouds. Previous studies looking at the aerosols from ship funnels have used satellite imagery to assess their effect on clouds – some paths are clearly affected, but many more clouds appeared unaffected.

Now, Peter Manshausen at the University of Oxford and his colleagues have found that clouds that had appeared unaffected are in fact affected by the aerosols. The researchers mapped the paths of two million ships in the Atlantic Ocean over a six-year period and used a global database for the winds on the trade routes the ships travelled along to see where their emitted aerosols would go. They then used another database of cloud properties to see how these clouds were affected.

The researchers found that at the locations they predicted the aerosols would travel to, there were tracks of clouds with fewer droplets but more liquid water. This slightly changes the overall reflectivity of the clouds and, more importantly, a larger water-to-droplet ratio implies a stronger cooling effect, so the clouds would reflect more radiation back towards the sun.

This leads to a cooling effect from human-derived aerosols that, measured as the amount of water between two points in the atmosphere, was found to be -0.76 watts per square metre, very different from the heating effect, at 0.2 watts per square metre, in the most recent Intergovernmental Panel on Climate Change report.

“If the aerosols cause the clouds to be brighter than we thought previously, that means that as we reduce air pollution for health reasons, we stand to expect more global warming from the lack of this cooling that comes from the aerosol cloud interactions,” says Manshausen.

The method the researchers used allows them to study the effect of aerosols on cumulus clouds, rather than stratocumulus clouds. In cumulus clouds, the emission-induced changes couldn’t be seen from satellites before.

This makes the finding important for informing more accurate climate models, says Leighton Regayre at the University of Leeds, UK.

“It’s really compelling, it’s an area that would have been neglected by other climate modelling groups,” says Regayre. “We do largely focus on the stratocumulus regions, rather than cumulus, because in the pre-industrial times, there were a lot more stratocumulus clouds. As we get warmer sea surface temperatures, there’s going to be, it seems like, less stratocumulus and more cumulus.”

The work doesn’t mean, however, that the emissions from ships have a cooling effect overall, says Manshausen. The emitted greenhouse gases will still warm the climate, while the emitted aerosols have a smaller cooling effect.

Journal reference: Nature, DOI: 10.1038/s41586-022-05122-0

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