“ON Jan 1, 2020, new reduced limits on sulphur in fuel oil brought about a 70 per cent cut in total sulphur oxide emissions from shipping, ushering in a new era of cleaner air in ports and coastal areas through the use of less-polluting fuels,” said the International Maritime Organization (IMO) in a statement issued a year later.

It continued: “The reduction in the upper limit of the sulphur content of ships’ fuel oil to 0.5 per cent (from 3.5 per cent previously) has significantly reduced air pollution from ships.”

And that is the widely accepted perception. Is it true? Yes. Is it the whole story? No.

In the several years of debate that preceded the introduction of mandatory sulphur reductions, globally and in IMO-designated Emission Control Areas (ECAs), there were warnings from various quarters that ship emissions, to some extent, counter global warming by reflecting sunlight. So, cutting polluting emissions would increase global warming.

Precious little attention was paid to these concerns. So mine was drawn to the subject line of the press statement that arrived last week: “New analysis of shipping emissions reveals that air pollution has a larger effect on climate than previously thought.”

Apparently, researchers based at the Oxford University’s Climate Processes Group have used novel methods of analysing satellite data to more accurately quantify the effect of human aerosol emissions on climate change. The results have been published in the journal Nature, and are very interesting.

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The Oxford team used satellite observations of clouds polluted by shipping as a model system to study the impacts of human aerosol emissions.

The results showed that clouds may react more strongly to air pollution than previously thought, becoming brighter and having a stronger cooling effect.

The researchers comment: “The effects of policies to reduce air pollution should be carefully considered when forecasting long-term climate change.”

The Oxford statement explains that human aerosol emissions have a cooling effect on the planet because they can make clouds brighter by providing extra condensation nuclei around which cloud droplets form. Brighter clouds reflect more of the sunlight that strikes them, deflecting it from the earth’s surface.

Large uncertainty

However, it is currently unclear how large this cooling effect is, particularly if the change in the brightness of the clouds cannot be seen in satellite images. This could be when the emissions are diffused, such as from a city’s traffic, or when there are winds that disperse them.

The cooling effect offsets some of the warming effect of greenhouse gases, and provides the largest uncertainty in human perturbations to the climate system.

To investigate this, the research team analysed data on ship emissions as a model system for quantifying the climatic effect of human aerosol emissions in general. Sometimes, when a ship passes underneath a cloud, its aerosol emissions brighten the cloud in a long line, similar to a contrail. These so-called ship tracks have been previously studied.

However, the vast majority of ships leave no visible tracks. This was the first study to provide a quantitative measure of the impact of invisible ship tracks on cloud properties, and thus their cooling effect.

The study’s key findings included confirmation that invisible shipping tracks had a clear impact on the properties of clouds they polluted. Surprisingly, the specific effects were different from those of visible shipping tracks.

Invisible ship tracks showed a smaller increase (roughly 50 per cent less) in the number of droplets in the clouds, but the amount of water increased more than in visible tracks. This implies, the researchers concluded, that for a given increase in droplets, the increase in water is larger than thought, equating to a greater cooling effect.

The study concluded: “The same may be true for aerosol emissions more generally – clouds may react more strongly to air pollution than previously thought, getting brighter and having a stronger cooling effect.”

The Oxford team explained that ship emissions often occur in remote ocean environments, and so provide unique opportunities to study the effects of aerosols in isolation from other human-induced factors that affect the climate.

This new study, led by Peter Manshausen (who is working on a DPhil, the Oxford equivalent of a PhD), used a global database of ship routes containing the locations of almost all ships at a given time – more than two million ship paths over six years.

Combining this data with historical weather observations, the researchers then simulated where all these ships’ emissions were, carried by the wind and entering the clouds.

Human health

Studying these locations in satellite data enabled them to measure the number of droplets and the amount of water in polluted and unpolluted clouds. Importantly, this method does not depend on the ship emissions being visible in satellite images.

Now we come to the implications of this research. According to the research team, the findings indicate that human health policies to reduce air pollution must be carefully considered when forecasting future climate-change scenarios.

The Climate Processes Group also found that ship tracks reduced pollution by around 25 per cent almost immediately after the IMO introduced stricter fuel regulations in 2020 to reduce air pollution caused by global shipping. That analysis used a machine-learning approach to automatically measure more than a million visible ship tracks from satellite images over a 20-year period.

A a co-author for the Oxford study, Dr Duncan Watson-Parris of the University’s Department of Physics, said: “Air pollution must be reduced for human health reasons. But our analysis demonstrates that such policies must be accompanied by determined action against global warming, to compensate for the loss of the cooling effect from human aerosols.”

One point made by industry observers before the sulphur reductions were mandated by IMO was that most particulate pollution generated by ships necessarily occurs away from land and human populations. IMO wasn’t prepared to take that into account when drafting its regulations on sulphur in marine fuel.

Now the results of a new study imply that shipping could very easily make a significant move towards limiting global warming, just by going back to burning high sulphur fuel when ships are a reasonable distance from coasts. That won’t happen. It would involve eating too much humble pie at IMO.

More generally, it is ironic that a policy strongly pushed by environmentalists has apparently made global warming worse. As the old saying goes: “Be careful what you wish for...”