The world has succeeded in confronting a dangerous pollutant. But did he accidentally heat the planet in the process?

(CNN) – Huge cargo ships plying the world's oceans sometimes leave “footprints” in their wake: long, thin clouds that stretch across the sky and last a few days at most before disappearing.

These ghostly clouds look beautiful, but they are a clear sign of deadly air pollution. They form when small particles of sulfur dioxide emitted from ship smokestacks react with water vapor in the atmosphere, creating low, highly reflective clouds.

Causes of sulfur pollution from ships Tens of thousands of premature deaths annually. But in what may seem like a cruel turn (especially by the industry responsible for… about 3% of global greenhouse gas emissions), this type of pollution also helps cool the planet by lighting up clouds and reflecting the sun's energy away from Earth.

That is why, in 2020, when the International Maritime Organization (IMO), the United Nations body that regulates maritime transport, reduced its size by one 80% content Allowing sulfur in bunker fuel was a victory for human health. It is estimated that every year some 30,000 deaths A baby born prematurely.

It was a “silver cloud with a dark lining,” said Michael Diamond, an associate professor in the Department of Earth, Ocean and Atmospheric Sciences at Florida State University. The regulations put an end to the large, accidental geoengineering project. Traces of ships It has been reduced significantly And with them the refreshing effect of this pollution.

As the planet warms, scientists are trying to figure out whether these navigation rules may be inadvertently fueling an alarming acceleration in global warming, a controversial hypothesis that has divided some experts.

It's a debate made more urgent by… Record temperature Since last year. “Scientists are amazed at how anomalous 2023 was,” said Olaf Morgenstern, a scientist at New Zealand's National Institute of Water and Atmospheric Research.

The heat was particularly evident in some parts of the oceans, where water temperatures were as high as the North Atlantic They shot tremendously.

Scientists say global temperature rise has been driven primarily by two factors: climate impacts Boya natural climate phenomenon that tends to have a greenhouse effect, combined with the context of long-term global warming caused by the burning of fossil fuels.

But some have speculated that the temperature has risen so abnormally that there may be other influences at play as well. Theories include a lack of desert dust to reflect sunlight, a change in wind patterns, and a January 2022 volcanic eruption. Underwater volcano Hunga Tongawhich injected enough water vapor to heat the planet into the atmosphere to fill it 58,000 Olympic swimming pools.

However, of all the theories, the influence of marine systems has quickly become one of the most discussed. Scientists have long known that reducing this particulate pollution would have a warming effect, but to what extent “is where the debate begins,” Morgenstern said.

In November, prominent climate scientist James Hansen co-authored a paper arguing that limiting climate change is possible. Maritime transport pollution It has been the main driver of the alarming acceleration in global warming beyond what climate models predict.

Hansen told CNN that the IMO's shipping regulations were an “unintended scientific experiment.” Their research predicted that global temperatures would exceed 1.5 degrees Celsius of warming above pre-industrial levels in the 2020s and 2 degrees Celsius in the 2050s, a catastrophic level of warming that could lead to a series of climate tipping points.

But other scientists have urged caution, not least because the relationship between polluting particles and clouds is so complex. Unraveling this problem “is one of the biggest challenges in climate science,” Diamond said.

Reducing pollution from shipping is likely to have very little impact on global warming, said Piers Forster, a professor of climate physics at the University of Leeds in the UK.

According to Forster's calculations, these regulations would increase global warming by about 0.01 degrees Celsius, which could grow to about 0.05 degrees by 2050, which is equivalent to about 0.05 degrees Celsius. Two additional years Of human-caused emissions.

However, he added, the uncertain impact of pollution on clouds means there is a possibility that the warming effect could be much greater: an additional 0.1 or 0.2 degrees by 2050.

Diamond, whose research suggests that regulations will bring levels of warming over the next few decades between 0.05 and 0.1 degrees, said this warming would not be a “wow factor,” but it is important. All part of the class Climate change is of great importance as the world moves towards levels of global warming to which even humans will find it increasingly difficult to adapt.

Diamond, like most scientists CNN spoke to, doesn't believe the decline in marine pollution was a major factor in last year's global warming, among other things because there is usually a time lag before changes in the atmosphere are reflected back on Earth. Temperature.

“But I think it could have had a much greater impact at the regional level,” he says. Maritime transport is unevenly distributed, with much of it concentrated between Europe, North America and Asia, which means the effects of air pollution can also be skewed.

In regions like the North Atlantic, where temperatures rose several degrees above normal in 2023, “shipping is a good explanation for why the temperature is warming,” Diamond said.

Data has only been available for a few years now, and it will take some time for scientists to unravel the exact impact of reduced pollution from shipping.

But it is clear that particulate pollution from all sources, including the burning of fossil fuels, has had a cooling effect. Without it the world would be 0.4 degrees warmer, according to the 2021 report of the Intergovernmental Panel on Climate Change. Reducing pollution in the future could have a significant impact.

Annika Ekman, a professor of meteorology at Stockholm University in Sweden, said her research found that a decline in human-caused particle pollution between 2015 and 2050 could cause the planet to warm by up to 0.5 degrees.

But Diamond said this is not an argument against reducing air pollution, but rather an argument for addressing it alongside reducing carbon emissions.

The cooling effect of air pollution is much greater than the heating effect of burning fossil fuels. When we fight air pollution without reducing carbon emissions, “we can have problems,” Diamond said.

This is what is happening in the shipping sector, where huge container ships continue to push hundreds of millions of tons of fossil fuels across the oceans.

“We must not forget why the regulations exist,” Forster said. “It's there to save lives from air pollution.” He added that although reducing this pollution will have a small impact on global warming, taking immediate action to reduce emissions will significantly reduce the rate of global warming and improve air quality.