Katrina Virts was looking at data from the World Wide Lightning Location Network , a network of sensors that tracks global lightning, and noticed it almost right away: a peculiar line of lightning strikes that stretched nearly straight across the Indian Ocean. Testing a theory, she and her colleagues at NASA’s Marshall Space Flight Center in Huntsville, Alabama, compared the lightning map to maps of exhaust plumes from a global database of ship emissions, and found a shocking correlation: Thunderstorms above two of the world’s busiest shipping lanes seemed significantly more powerful than storms in other areas of the ocean.

In a new paper that tries to understand how this works, Virts and a team of other atmospheric scientists looked more closely. They studied the locations of 1.5 billion lightning strikes from 2005 to 2016, and found nearly twice as many strikes on average over the major routes ships take across the northern Indian Ocean, through the Strait of Malacca and into the South China Sea, compared to adjacent areas of the ocean with similar climates.

Because the areas of increased lightning are far wider than the shipping lanes themselves, the researchers say the effect probably isn’t due to bolts striking ships directly. The increase in lightning also “cannot be explained by meteorological factors, such as winds or the temperature structure of the atmosphere,” they write.

Instead, they argue, the likely culprit is the ships’ emissions. The team hypothesizes that aerosol particles of soot and nitrogen and sulfur emitted in the engine exhaust of ships “act as the nuclei on which cloud drops form, and can change the vertical development of storms, allowing more cloud water to be transported to high altitudes, where electrification of the storm occurs to produce lightning.” As a statement published alongside the study explains:

Where the atmosphere has few aerosol particles–over the ocean, for instance–water molecules have fewer particles to condense around, so cloud droplets are large. When more aerosols are added to the air, like from ship exhaust, water molecules have more particles to collect around. More cloud droplets form, but they are smaller. Being lighter, these smaller droplets travel higher into the atmosphere and more of them reach the freezing line, creating more ice, which creates more lightning. Storm clouds become electrified when ice particles collide with each other and with unfrozen droplets in the cloud. Lightning is the atmosphere’s way of neutralizing that built-up electric charge.

Joel Thornton, an atmospheric scientist at the University of Washington and lead author of the study, said the effect was “one of the clearest examples of how humans are actually changing the intensity of storm processes on Earth through the emission of particulates from combustion.”

It’s “the first time we have, literally, a smoking gun, showing over pristine ocean areas that the lightning amount is more than doubling,” said Daniel Rosenfeld, an atmospheric scientist at the Hebrew University of Jerusalem, who was not connected to the study. “The study shows, highly unambiguously, the relationship between anthropogenic emissions—in this case, from diesel engines—on deep convective clouds.”

Thornton notes that ships burn dirtier fuels in the open ocean away from port, spewing more aerosols and creating even more lightning. One upside to this nearly continuous trail of shipping exhaust: Scientists hoping to better understand how aerosols affect cloud formation now have another place to focus their attention (not to mention more fodder for understanding and debating humans’ impact on the climate).