The world successfully tackled a dangerous pollutant. But did it accidentally warm the planet in the process?

CNN —

The huge cargo ships that criss-cross the world’s oceans sometimes leave “tracks” in their wake — long, wispy clouds that trail through the sky, lasting for a handful of days at most before disappearing.

These ghost clouds look beautiful, but they are a visible sign of deadly air pollution. They form when tiny sulfur dioxide particles belched out from ships’ smokestacks interact with water vapor in the atmosphere, creating low-lying, highly reflective clouds.

Ships’ sulfur pollution causes tens of thousands of premature deaths a year. But in what may seem a cruel twist — especially from an industry responsible for around 3% of global greenhouse gas emissions — this typeofpollution also helps cool the planet by brightening clouds and reflecting the sun’s energy away from the Earth.

So, when in 2020 the International Maritime Organization (IMO), the United Nations body regulating shipping, slashed sulfur content permitted in ships’ fuel by 80%, it was a victory for human health. An estimated 30,000 premature deaths will now be avoided each year.

But it was “a silver cloud with a dark lining,” said Michael Diamond, assistant professor at Florida State University’s Department of Earth, Ocean and Atmospheric Science. The regulations ended a vast, accidental geoengineering project. Ship tracks reduced sharply, and with them, the cooling impact of this pollution.

As global temperatures soar, it has left scientists trying to unpick whether these shipping regulations may be inadvertently fueling an alarming acceleration of global warming — a controversial hypothesis that has divided some experts.

It’s a debate made more urgent by last year’s record-breaking heat. “Scientists are amazed at the outlier that 2023 was,”