by The huge cargo ships that cross the world’s oceans sometimes leave behind “tracks” – long wispy clouds that sweep across the sky, lasting for a few days at most before disappearing.
These ghost clouds look beautiful, but they are a visible sign of deadly air pollution. They form when tiny particles of sulfur dioxide from ships’ smokestacks interact with water vapor in the atmosphere, creating low-reflective clouds.
Sulfur pollution from ships causes thousands of premature deaths every year. But in what could be a cruel twist – especially from an industry responsible for around 3% of global greenhouse gas emissions – this form of pollution helps cool the planet by brightening clouds and absorbing the sun’s energy an expression away from Earth.
Therefore, when the International Maritime Organization (IMO), the United Nations body that regulates shipping, reduced the sulfur content allowed in ship fuel by 80% in 2020, it was a victory for human health. Around 30,000 premature deaths will now be avoided each year.
But “it was a silver cloud with a dark lining,” said Michael Diamond, an assistant professor at Florida State University’s Department of Earth, Ocean and Atmospheric Sciences. The regulations accidentally ended a massive geoengineering project. Ship tracks have diminished sharply, and with them, the cooling effect of this pollution.
As global temperatures rise, it has left scientists scrambling to determine whether these shipping regulations could inadvertently cause an alarming acceleration of global warming — a controversial hypothesis that has been dismissed by some experts.
It’s a debate that’s more urgent because of last year’s scorching heat. “Scientists are surprised by the outlier in 2023,” said Olaf Morgenstern, a scientist at the National Institute for Water and Atmospheric Research in New Zealand.
The heat was particularly noticeable in some parts of the oceans, where water temperatures in areas including the North Atlantic fell wildly off the charts.
Scientists say the rise in global temperatures was based mainly on two factors: the effects of El Niño, a natural climate phenomenon that tends to have a global warming effect, combined with a background of long-term global warming caused by the burning of fossil fuels.
But some have speculated that the heat has been so high that it has had other effects. Theories include a lack of Saharan dust reflecting sunlight, a change in wind patterns, and the January 2022 eruption of the Hunga Tonga underwater volcano, which injected enough water vapor into the atmosphere to fill 58,000 Olympic swimming pools. .
Of all the theories, however, the impact of shipping regulations is fast becoming one of the most debated. Scientists have long known that reducing this particle pollution would have a warming effect, but how much “is where the controversy begins,” Morgenstern said.
In November, leading climate scientist James Hansen co-authored a paper arguing that a reduction in shipping pollution was the primary cause of an alarming acceleration in global warming that exceeds what climate models predict.
“The IMO shipping regulations were an unintended science experiment,” Hansen told CNN. His research predicted that global temperatures would exceed 1.5 degrees Celsius of warming above pre-industrial levels in the 2020s and 2 degrees in the 2050s – a catastrophic level of warming that could trigger a large number of climate hotspots.
But other scientists have urged caution, particularly because the relationship between particulate pollution and clouds is extremely complex. Solving it 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 that the reduction in shipping pollution is likely to have a very small warming effect.
According to Forster’s calculations, the regulations will increase global warming by about 0.01 degrees Celsius, which could grow to about 0.05 degrees by 2050 — equivalent to about two additional years of human-caused emissions.
However, he said the uncertain effect of pollution on clouds means it is possible the impact of warming could be much greater – 0.1 or 0.2 degrees more by 2050.
Diamond, whose own work estimates that the regulations will bring levels of warming between 0.05 and 0.1 degrees in the next few years, said that this heat will not be a “stopper” but that it is important. Every fraction of a step matters as the world heads towards levels of warming that even humans will struggle to adapt to.
Diamond, along with most of the other scientists CNN spoke to, doesn’t believe the decline in shipping pollution was a major factor in last year’s global warming, especially since there is usually a lag time before it shows up. changes in the atmosphere in the atmosphere. The temperature of the world.
“But I think it might be more important regionally,” he said. Shipping is unevenly distributed, with large concentrations concentrated between Europe, North America and Asia, meaning that the effects of air pollution are also likely to be skewed.
In areas like the North Atlantic, where temperatures rose several degrees above normal in 2023, Diamond said, “shipping is a good explanation for part of why it was so warm.”
There are only a few years of data so far, and it will take time for scientists to sort out the exact impact of the fall on shipping pollution.
But particle pollution from all sources, including the burning of fossil fuels, has clearly had a cooling effect. Without it, the world would be about 0.4 degrees warmer, according to a 2021 report from the Intergovernmental Panel on Climate Change. And future pollution reductions 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 this isn’t an argument against reducing air pollution, Diamond said, it’s an argument for tackling it alongside reducing carbon emissions.
The cooling effect of air pollution is far greater than the heating effect of burning fossil fuels. It’s when we tackle air pollution without also reducing carbon emissions that “we can get into trouble,” Diamond said.
That is what is happening in this shipping industry, where hundreds of millions of tons of fossil fuels are still being driven across the oceans by huge container ships.
“We need to forget why the regulation exists,” Forster said. “It’s there to save people from air pollution.” Although reducing this pollution will have a small warming effect, immediate action to reduce emissions will reduce the rate of global warming and improve air quality, he said. We are not “somehow doomed,” he said.
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