Shipping companies face a 2050 deadline set by the International Maritime Organization that seeks sharp cuts in vessels’ carbon-dioxide emissions. Photo: adrian dennis/Agence France-Presse/Getty Images Shipping operators are under pressure from governments and big customers such as Amazon.com Inc. to clean up vessels’ carbon emissions. But viable alternatives to fossil fuels are just taking shape. Supplies of methanol and ammonia—two cleaner-burning alternatives to crude-distilled bunker oil—are too limited to power the world’s 60,000 oceangoing ships, and those fuels are several times more expensive, companies say.