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The world has been extremely hot this year, so much so it’s felt apocalyptic. July was the hottest month in 120,000 years, the water off of Florida is basically a hot tub, corals are bleaching, and El Niño is coming. While the greatest culprit has obviously been the very real threat of global warming due to man-made greenhouse gas emissions, the journal Science reports that scientists have discovered another, unexpected factor: ship tracks.
In 2020, the International Maritime Organization (IMO) enacted a new global standard requiring an 86 percent reduction in fuel sulfur content. The reasoning behind this was that, as the IMO outlines, “Sulphur oxides are harmful to human health, causing respiratory, cardiovascular and lung disease. Once released in the atmosphere, SOx can lead to acid rain, which impacts crops, forests and aquatic species and contributes to the acidification of the oceans.”
The policy appears to have succeeded in its goal of reducing sulphur emissions. According to the World Economic Forum, the result was a 10 percent reduction in global emissions of sulphur dioxide. However, scientists have discovered another, unintended consequence of the policy: a reduction in ship tracks.
“Ship tracks, produced by cargo ships, are lines of aerosol clouds over expansive ocean spaces,” writes a blog post by Harvard University. “Exhaust pumped from the cargo ships mixes with ocean clouds, where aerosol particles induce a brightening effect due to water vapor condensation.”
A NASA study co-authored by Tianle Yuan, an atmospheric physicist at NASA’s Goddard Space Flight Center, analyzed decades of satellite imagery and found that ship-track density fell by more than 50 percent in the main shipping corridors after the IMO regulation went into effect. A decrease in shipping as a result of the COVID-19 pandemic also played a role, but paled in comparison to the effect of the sulphur regulation.
Ironically, the problem is that the bright clouds produced by ship tracks reflect sunlight back in the atmosphere and have actually had a cooling effect on the Earth. In fact, according to another study, it’s not just the visible ship tracks studied by Yuan that had this effect, but also “invisible” tracks that had a brightening effect on clouds. The result is that the increase in light due to the IMO regulations has warmed the planet by 0.1 watts per square meter, according to Science.
Though this unintended consequence sounds dire, it does potentially bode well for possible future geo-engineering efforts to combat climate change. Some scientists have suggested injecting salt particles back into the air to produce the same effect as the ship tracks, a process called marine cloud brightening. Michael Diamond, an atmospheric scientist at Florida State University, told Science that the decline in ship tracks as a result of the IMO policy “suggests pretty strongly that if you wanted to do it on purpose, you could.”
