Be Careful What You Wish For: Sulfur Cut May Accentuate Global Warming !


  • New reduced limits on sulphur in fuel oil brought about a 70 per cent cut in total sulphur oxide emissions from shipping.
  • It will usher in a new era of cleaner air in ports and coastal areas through the use of less-polluting fuels.
  • However it is only the surface of the story.

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.

The Aerosol Effect

Researchers based at the Oxford University’s Climate Processes Group have used novel methods of analyzing 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. 

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 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.

Looming 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 gasses, and provides the largest uncertainty in human perturbations to the climate system. 

To investigate this, the research team analyzed 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. 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. The study concluded: “The same may be true for aerosol emissions more generally – clouds may react more strongly to air pollution than previously thought…”.

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.

Impact On Health

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. 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.”

IMO mandated 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. 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…”

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Source: Businesstimes


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