Switching from burning fossil fuels to burning ammonia is a promising way for the shipping industry to cut its carbon dioxide emissions. But if it isn’t careful, this swap could also boost harmful nitrogen pollution.

“That’s a major alarm bell that’s ringing,” says Xin Zhang at the University of Maryland.

The tens of thousands of ships on the oceans are responsible for about 3 per cent of global CO2 emissions, burning more than 300…