If you look at a map of lightning near the Port of Singapore, you’ll notice an odd streak of intense lightning activity right over the busiest shipping lane in the world. As it turns out, the lightning really is responding to the ships, or rather the tiny particles they emit.

Using data from a global lightning detection network, my colleagues and I have been studying how exhaust plumes from ships are associated with an increase in the frequency of lightning.

For decades, ship emissions steadily rose as increasing global trade drove higher ship traffic. Then, in 2020, new international regulations cut ships’ sulfur emissions by 77 percent.

Our newly published research shows how lightning over shipping lanes dropped by half almost overnight after the regulations went into effect.

That unplanned experiment demonstrates how thunderstorms, which can be 10 miles tall, are sensitive to the emission of particles that are smaller than a grain of sand.

The responsiveness of lightning to human pollution helps us get closer to understanding a long-standing mystery: To what extent, if any, have human emissions influenced thunderstorms?

Aerosol particles can affect clouds?

Aerosol particles, also known as particulate matter, are everywhere. Some are kicked up by wind or produced from biological sources, such as tropical and boreal forests. Others are generated by human industrial activity, such as transportation, agricultural burning and manufacturing.

It’s hard to imagine, but in a single liter of air – about the size of a water bottle – there are tens of thousands of tiny suspended clusters of liquid or solid. In a polluted city, there can be millions of particles per liter, mostly invisible to the naked eye.

These particles are a key ingredient in cloud formation. They serve as seeds, or nuclei, for water vapor to condense into cloud droplets. The more aerosol particles, the more cloud droplets.

In shallow clouds, such as the puffy-looking cumulus clouds you might see on a sunny day, having more seeds has the effect of making the cloud brighter, because the increase in droplet surface area scatters more light.

In storm clouds, however, those additional droplets freeze into ice crystals, making the effects of aerosol particles on storms tricky to pin down. The freezing of cloud droplets releases latent heat and causes ice to splinter.

That freezing, combined with the powerful thermodynamic instabilities tha