On a bright September day on the harbor in Copenhagen, several hundred people gathered to welcome the official arrival of Laura Maersk.

Laura was not a visiting European dignitary like many of those in attendance. She was a hulking containership, towering a hundred feet above the crowd, and the most visible evidence to date of an effort by the global shipping industry to mitigate its role in the planet’s warming.

The ship, commissioned by the Danish shipping giant Maersk, was designed with a special engine that can burn two types of fuel — either the black, sticky oil that has powered ships for more than a century, or a greener type made from methanol. By switching to green methanol, this single ship will produce 100 fewer tons of greenhouse gas per day, an amount equivalent to the emissions of 8,000 cars.

The effect of global shipping on the climate is hard to overstate. Cargo shipping is responsible for nearly 3 percent of global greenhouse gas emissions — producing roughly as much carbon each year as the aviation industry does.

Figuring out how to limit those emissions has been tricky. Some ships are turning to an age-old strategy: harnessing the wind to move them. But ships still need a more constant source of energy that is powerful enough to propel them halfway around the world in a single go.

Unlike cars and trucks, ships can’t plug in frequently enough to be powered by batteries and the electrical grid: They need a clean fuel that is portable.