In 2009, Dr. James Corbett, a professor of marine policy at the University of Delaware, came up with what he describes as a thought experiment. He compared the sulfur dioxide produced by the largest container ship burning the dirtiest bunker fuel with that of an automobile burning the cleanest fuel. He concluded that under this hypothetical case, 15 of the world’s largest container ships produced as much sulfur pollution as all the world’s 760 million cars. A memorable factoid, but of questionable veracity.

BBC’s program/podcast More or Less: Behind the Stats with Tim Harford, examines the shortcomings of this thought experiment. To listen to the podcast, click the banner below. Otherwise, read on.

The figure is alarming and was picked up first by the Guardian, who managed to slightly manage the message. They wrote that “just 15 of the world’s biggest ships may now emit as much pollution as all the world’s 760m cars.”

The article mentioned sulfur but didn’t clearly specify that they were referring only to sulfur pollution. There is more to pollution than just sulfur, sadly. The analysis didn’t even consider carbon dioxide, the primary greenhouse gas, for example.

The problem was that the memorable phrase, “just 15 of the world’s biggest ships may now emit as much pollution as all the world’s 760m cars,” spread widely among excitable journalists and is repeated in one form or another to this day. It was arguably untrue in 2009 and is completely false in 2021.

For those wishing to dig deeper, CE Delft, an independent environmental research and consultancy organization, examined various forms of the claim and concluded that most were wholly false.

On the other hand, while the conclusion drawn from Corbett’s thought experiment may have been exaggerated in 2009, it did point out that shipping was creating significant sulfur pollution. 

To return to 2009, why did cars create so much less sulfur pollution than ships? The fact is that they weren’t always so clean. Between 1997 and 2006, sulfur was cut from 300 ppm in gasoline in the US to 30 ppm. (By 2017, sulfur was being reduced in US gas to 10 ppm.)  During the period, ships hadn’t gotten any dirtier, but cars had gotten much cleaner.

What has happened in shipping since 2009? Shipping is catching up. The International Maritime Organization (IMO) has mandated that by last year, 2020, the upper limit on the sulfur content of ships’ fuel oil was reduced to 0.50% (from 3.50%), except in designated Emission Control Areas, where the limit was already 0.10%.

As a gauge of where we are and where we are going, a quote from Forbes in 2019:

Pound for pound, water transport is by far the most environmentally friendly way to move goods. But with the maritime industry’s vast scale, its environmental footprint remains substantial. To reduce shipping’s impact, new International Maritime Organisation regulations are requiring ocean-going ships to shift to cleaner fuels—and the industry has committed to further improvements by 2030 and 2050. 

“In a few short years we’ve gone from the shipping community suggesting it had little role in what’s happening to our atmosphere to embracing changes in a way that’s leading to active and exciting innovation,” says James Corbett, a professor of marine science and policy at the University of Delaware.