The greening of planes, trains, and automobiles

Trains

Rail is already the most electrified transport subsector, according to the IEA, and makes up only a slim 1 percent of transport emissions. So this is the smallest, most-solved problem of the batch.

Trains, like trucks, are heavy beasts that need a lot of power. But many trains already run on electric wires or rails. Others use a fuel, usually diesel, but turn that into electricity on board in order to power an electric motor (which has better torque than a fossil-fuel-fed engine). It’s a relatively simple step to swap out that diesel for something else, like hydrogen or batteries, to supply power for an already-existing electric motor. “Trains are pretty easy to electrify,” says chemical engineer Hartej Singh, who analyzes decarbonization for the nonprofit Rocky Mountain Institute in Washington, DC.

Expanding electric rail, the IEA says, is a good idea—especially if it replaces flights. Today, every mile a passenger travels on a train has, on average, one-fifth the emissions of the same distance traveled on a plane. But, the IEA notes, putting in a new electric rail line is an expensive proposition.

Ships

Shipping—accounting for about 10 percent of transport emissions—has a particular need to go extraordinary distances and lengths of time before refueling: Crossing an ocean calls for weeks-long journeys of thousands of miles.

Shipping currently predominantly uses bunker fuel—a high-sulfur variant of fossil fuel often described as the gunk left over at the bottom of an oil barrel. But that’s changing fast, thanks to International Maritime Organization goals, adopted in 2023, to hit net-zero emissions by close to 2050. “That’s basically the entire global fleet needing to transition off fossil fuels,” says Tristan Smith, an engineer at University College London who studies shipping. For now, just 1.2 percent of the ships in the global fleet use lower-emission fuels, but 21 percent of the new ships on order are designed to run on these alternatives. Smith sees a clear path ahead to get where shipping needs to go.

For now, one popular low-emission alternative is bio-methanol (made from plants). But this is a short-term distraction, says Smith: There’s simply not enough land to grow enough biofuel stock for the global fleet. For the long term, he’s betting on ammonia—NH₃—as the best solution.

This is a hydrogen-rich liquid fuel that provides a lot of oomph. Plus, we already know how to make it and move it around; globally some 150 million metric tons are produced every year, mainly for fertilizer. Ammonia counterintuitively works out to be cheaper than straight hydrogen, notes Smith (even though it has hydrogen as an ingredient), because pure hydrogen comes with the extra energy and cost burden of putting it under pressure and keeping it cold to store it. Ammonia, by contrast, is relatively easy to keep liquid. And though ammonia requires more storage space than fossil fuels, this matters less for ships than for, say, cars.

You do have to redesign your engine to run on ammonia, though. Ammonia is hard to ignite, and the engine needs catalysts to remove other pollutants, like the greenhouse gas nitrous oxide. All of this is being tackled: The Green Pioneer, run by Australian mining and green energy company Fortescue, is the first ship to trial an ammonia-burning engine (with some diesel in the mix), along with refueling strategies and safety protocols. The main problem with ammonia, says Smith, is that it’s toxic, so spills are nasty. All in all, Smith sees a clear path ahead for ammonia. “We can see a situation where there’s an explosion of ordering [of ammonia-engine ships] from the middle of next year,” he says.

Ammonia will put yet more strain on the demand for green hydrogen. By 2030, the IEA calls for an additional 8 million metric tons of hydrogen for ammonia-based transport fuels, on top of the 11 million metric tons of straight hydrogen for transport uses. There’s billions of dollars of investment already being plowed into green ammonia, Smith says. “We need lots of billions.”

There are additional ways to reduce emissions from ships—including shipping less cargo to begin with, improving logistics to run fewer ships shorter distances, designing sleeker hulls, and even putting up modern sails. That includes strange rotating poles called Flettner rotors that can help to propel a ship in a way similar to how a spinning baseball moves sideways in the air. All of them could help to whittle down shipping’s carbon burden.