Heavily-laden ships carrying trade around the world leave behind a trail of storms and lightning.

That’s the hypothesis of a team of researchers at the University of Washington, who found that emissions from ships were triggering intense lightning storms.

The scientists at the Department of Atmospheric Sciences analyzed data from the World Wide Lightning Location Network (WWLLN) and noticed a certain pattern of lightning strikes across the Indian Ocean.

The pattern could not be put down to atmospheric conditions. However, once they compared the lightning strikes with shipping routes an explanation began to emerge: the ships themselves were causing electrical activity in the sky.

Image: Thomton et al/Geophysical Research Letters/AGU

The pollution link

“We already know that ships influence cloudiness and clouds,” the study’s lead author Joel A. Thornton told the World Economic Forum. “But what hadn’t been looked at before was the connection to what happens higher up in the cloud – where thunderstorms and lightning occur.”

Using WWLLN data, the team looked at the locations of 1.5 billion lightning strikes from 2005 to 2016 and plotted them against shipping channels across the ocean. What they found was that thunderstorms directly above two of the world’s busiest shipping lanes were significantly more powerful than storms in areas of the ocean where ships don’t travel.

In fact, lightning is more than twice as likely to occur directly above heavily-trafficked shipping lanes in the Indian Ocean and the South China Sea.

Image: REUTERS/Jamal Saidi

So how are ships triggering storms?

As a ship travels it belches exhaust fumes into the air. “It is that particulate matter from the ship’s exhaust that changes the properties of the cloud to give rise to more lightning,” explains Professor Thornton.

The ship's emissions make cloud droplets smaller, lifting them higher in the atmosphere. This creates more ice particles, which leads to more lightning.

“We thought about whether the ships themselves could somehow be inducing lightning by changing the flow of air or temperature of the air around them, but from what we’ve seen from previous literature it doesn’t seem like that’s enough to cause the lightning."

Professor Thornton didn’t think that the same process could happen on land.

“It gets a lot more complicated on land, it’s harder to isolate the effect of pollution on storms. Although there is evidence that suggests you are more likely to get lightning over cities.

The effect of the pollution at sea also stopped over time, he added. In other words, there reached a saturation point whereby even if you added more particles there wouldn’t be a continual rise in electrical activity in the clouds.

Referring to his scientific paper, he said: “It opens up whole new research avenues for us to consider how humans are changing the nature of storms and lighting.”

The impact of shipping pollution

Commercial shipping is vital to the movement of goods around the world, and a cornerstone of global trade. Around 90% of world trade is carried by ships.

This map tracks the movements of the global merchant fleet over the course of 2012, showing the sheer scale of maritime traffic.

The designations employed and the presentation of material on this map do not imply the expression of any opinion on the part of the World Economic Forum concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

However, international shipping is also a big contributor to greenhouse gases. According to the European Union, it is a large and growing source of emissions, contributing up to 2.5% of global greenhouse gas emissions every year.

Pollution from ships is also harming human health. A report published last year found that increased emissions in East Asia were the cause of up to 37,500 premature deaths per year.

Reducing emissions

Ship emissions include pollutants such as sulphur dioxide (SO2) and nitrogen oxides (NOx). A single large cruise ship can reportedly emit more sulphur than 7 million cars.

But last October, the International Maritime Organization (IMO) announced that the shipping industry had agreed to cap its sulphur emissions by 2020. The cap would “have a significant beneficial impact on the environment and on human health, particularly that of people living in port cities and coastal communities, beyond the existing emission control areas,” said the IMO in its press release.

In the future, 3D printing could reduce the need for much of the shipping involved in global trade. Customers could order goods and have them printed locally.

Professor Thornton is upbeat about the changes that the new IMO agreement will bring: “We’re going to see a large change in the pollution coming from the ships in these regions. We should see a response in the lightning, with it reducing along with the reduction of emissions. And the effect will be immediate.”