Giant cargo ships crisscrossing the world’s oceans sometimes leave “trails” behind; long, wispy clouds that move across the sky, lasting a few days at most before disappearing.
These ghost clouds look beautiful but are a visible sign of deadly air pollution. They form when tiny sulfur dioxide particles blown out of ships’ smokestacks interact with water vapor in the atmosphere to form low-lying, highly reflective clouds.
Sulfur pollution on ships causes tens of thousands of premature deaths a year. But such pollution, which seems like a cruel development, especially from an industry responsible for around 3% of global greenhouse gas emissions, also helps cool the planet by brightening clouds and reflecting solar energy away from Earth.
So it was a victory for human health when the International Maritime Organization (IMO), the United Nations’ shipping regulatory body, reduced the permissible sulfur content in ship fuel by 80% in 2020. An estimated 30,000 premature deaths will now be prevented each year.
But it’s a “silver cloud with a dark lining,” said Michael Diamond, an assistant professor in the Department of Earth, Ocean and Atmospheric Science at Florida State University. The regulations put an end to a major and accidental geoengineering project. Ship routes have decreased sharply and with it the cooling effect of this pollution.
As global temperatures rise, scientists are trying to figure out whether these shipping regulations are unintentionally fueling an alarming acceleration of global warming; It’s a controversial hypothesis that divides some experts.
It’s a debate made all the more urgent by last year’s record-breaking heat. “Scientists are astounded by the anomaly in 2023,” said Olaf Morgenstern, a scientist at the National Institute of Water and Atmospheric Research in New Zealand.
The heat was especially pronounced in some parts of the oceans; Water temperatures in regions including the North Atlantic have become extremely high.
Scientists say that the increase in global temperature is mainly due to two factors: the effects of El Niño, a natural climate phenomenon that tends to create a global warming effect, combined with the background of long-term global warming caused by the burning of fossil fuels.
But some suggest that the temperature was abnormally high and that other effects may also be at play. Theories include a lack of dust reflecting sunlight from the Sahara, a change in wind patterns, and the January 2022 eruption of the Hunga Tonga underwater volcano, which injected enough planet-warming water vapor into the atmosphere to fill 58,000 Olympic-sized swimming pools. .
But of all the theories, the impact of shipping regulations is quickly becoming one of the most debated. Morgenstern said scientists have long known that reducing this particle pollution would have a warming effect, but how much “is where the debate begins.”
In November, leading climate scientist James Hansen co-authored a paper arguing that reducing marine pollution is the main driver of the alarming acceleration of global warming that is going beyond what climate models predict.
Hansen told CNN that the IMO maritime regulations were “an unintentional scientific experiment.” His research predicted that global temperatures would exceed 1.5 degrees Celsius warming above pre-industrial levels by the 2020s and 2 degrees Celsius by the 2050s; This is catastrophic warming that could trigger multiple tipping points in the climate.
But other scientists have called for caution, especially because the relationship between pollution particles and clouds is extremely complex. Solving this is “one of the biggest challenges in climate science,” Diamond said.
Piers Forster, professor of climate physics at the University of Leeds in the United Kingdom, said the reduction in shipping pollution would likely have a very small warming effect.
According to Forster’s calculations, the regulations will increase global warming by about 0.01 degrees Celsius, and this figure is expected to rise to about 0.05 degrees by 2050; This is equivalent to approximately two more years of anthropogenic emissions.
But he added that the uncertain impact of pollution on clouds means the warming effect could be much larger; 0.1 or 0.2 degrees more by 2050.
Diamond, whose own study predicts the regulations will bring warming levels of 0.05 to 0.1 degrees over the next few decades, said this temperature would not be “flashy” but would be significant. As the world accelerates towards levels of warming that even humans will increasingly struggle to adapt to, every fraction of a degree matters.
Diamond, like most other scientists interviewed by CNN, does not believe the decline in ship pollution was a significant factor in last year’s global warming; especially since there is often a time lag before atmospheric changes are reflected in the atmosphere. Earth’s temperature.
“But I think it might be a little more important regionally,” he said. Transport is unevenly distributed, with much of it concentrated between Europe, North America and Asia, meaning air pollution impacts are also likely to be skewed.
In regions such as the North Atlantic, where temperatures rise several degrees above normal in 2023, “shipping is a good explanation for part of why this is so warm,” Diamond said.
There are only a few years of data so far, and it will take time for scientists to reveal the exact impact of the decline in ship pollution.
However, it is clear that particle pollution from all sources, including the burning of fossil fuels, creates a cooling effect. Without this, the world would be about 0.4 degrees warmer, according to a 2021 report from the Intergovernmental Panel on Climate Change. Reducing pollution in the future could have a big impact.
Annica Ekman, professor of meteorology at Stockholm University in Sweden, said her research found that reductions in human-caused particle pollution between 2015 and 2050 could warm the planet by up to 0.5 degrees.
But Diamond said this is not an argument against reducing air pollution, but rather an argument for combating it as well as reducing carbon emissions.
The cooling effect of air pollution far outweighs the warming effect of burning fossil fuels. Diamond said “we could be in trouble” by tackling air pollution without also reducing carbon emissions.
This is what is happening in the shipping industry, where huge container ships are still being pushed across oceans with hundreds of millions of tons of fossil fuels.
“We must not forget why management exists,” Forster said. “It exists to save lives from air pollution.” He said reducing this pollution would have a small warming effect, but urgent action to reduce emissions would both reduce the rate of global warming and improve air quality. “We are not on a dead-end path,” he added.
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