With their greenhouse gas emissions rising and regulations looming, shipping companies that transport nearly all of the world’s goods are looking at renewable fuel sources to power container ships.

The shipping industry is responsible for about 3 percent of global greenhouse gas emissions, according to the International Maritime Organization (IMO), and the European Commission estimates that its spew could grow between 50 percent and 250 percent by 2050. Yet the international climate accord hashed out in Paris in 2015 gave the industry a free pass, placing no requirements on the IMO, the body that regulates shipping, to reduce carbon emissions.

Nevertheless, the organization is working on a strategy to cut emissions, which it plans to unveil this year. If the IMO fails to come up with an acceptable plan by 2021, however, the European Union’s emissions reduction scheme will include shipping by 2023. IMO regulations go into effect in 2020 that will limit vessels’ sulfur emissions, a deadly byproduct of fossil fuels that’s found in particularly high concentrations in the cheap, lightly refined petroleum product known as bunker fuel favored by shippers.

These developments have the industry scrambling to adopt alternative fuels. Low-sulfur diesel and biofuels will probably make up the bulk of the substitution in the near term, but eventually, say shipping industry executives and energy experts, the fuel of the future for cruise liners, ferries and container ships will likely be hydrogen. Electricity generated from a hydrogen fuel cell to drive a motor produces no emissions, and when coupled with batteries and a device called an electrolyzer, the system can produce hydrogen from a resource that all vessels have easy access to: seawater.

Already, Compagnie Maritime Belge, based in Antwerp, has built a small passenger shuttle boat that runs on hydrogen called the Hydroville. While fuel cells, which convert hydrogen into electricity through a chemical reaction, will likely be the generator of choice in the future, the company’s research and development manager, Roy Campe, converted a diesel engine to run on compressed hydrogen. “We know how to maintain a [combustion] engine onboard and if something breaks we can fix it,” he told Oceans Deeply. “But with fuel cells it’s a different story.”

Campe and Alexandre Closset, chief executive of Swiss Hydrogen, which designs and builds hydrogen fuel cells for stationary and mobile applications and is currently working to install them on two research vessels, emphasized the challenging marine environment as one obstacle that will need to be figured out before fuel cells see widespread adoption on ships. “We have more work to do on air filters due to the salinity of the air,” Closset said. Campe noted that fuel cells’ high cost and lack of a backup fuel option (for systems that don’t include a costly electrolyzer) are other factors now limiting their use at sea.

But Scott Samuelsen, director of the National Fuel Cell Research Center at the University of California, Irvine, sees electricity produced from hydrogen as the inevitable choice of the shipping industry for three reasons.

Environmental regulation is one: Already, ports including the United States’ largest, in Long Beach, California, are placing stringent pollution limits – not just on ships docking there but also on the trucks that offload containers from the vessels.

The second is that hydrogen can be generated locally, either onboard or from wind and solar farms onshore deploying an electrolyzer, obviating the complex and fraught geopolitics around fossil fuel production. That method has an ancillary benefit of putting renewable energy power plants to use when the weather is favorable but utilities’ electricity demand is low.

The third is that in the long term, hydrogen fuel will mean lower operating costs for shippers as fuel cells drop in price due to economies of scale and petroleum becomes more expensive due to environmental regulations. “It will be an evolution and a changeover in the population of ships over many decades,” Samuelsen said.

As was the case with cars a decade ago, the first electrically powered vessels will be hybrids because of the current high cost of fuel cells and the immense amount of power required to propel a cruise liner or container ship. One such system would couple an array of fuel cells with a turbine engine running on liquefied natural gas. South Korea’s LG is expected to offer the first such hybrid fuel-cell gas turbine propulsion system in the next four years, Samuelsen said. The liquefied natural gas component of the system can later be converted to run on hydrogen.

Fuel cells can also power a ship’s ancillary electrical systems while the vessel is propelled by an electric motor hooked to a diesel generator and a battery. When the ship approaches port, it can run on the battery to reduce emissions. As fuel cells fall in price, Closset said, “You just replace the diesel generator with fuel cells, as the boat is already equipped with an electric motor.”

Full fuel-cell propulsion is probably still decades away, though. Swiss Hydrogen is part of a consortium that is working on a system for the Finnish research vessel Aranda. Scientists aboard Aranda collect air samples that must not be contaminated by engine fumes, and vibration from the engines can interfere with other observations scientists make onboard the ship. The hydrogen-powered system will enable maneuvering onsite, solving both problems at once. To propel the vessel, however, “we’d probably need 10 times more power,” Closset said.

Ferry operators and cruise lines have approached the company about developing a fuel-cell stack on such a scale. “They can promote to customers, ‘We’re using clean fuel, the ship doesn’t smell,’ and so forth,” Closset said.

But the container ships that produce the bulk of that 3 percent share of global emissions and transport 90 percent of the world’s products are another matter. The industry fiercely lobbied to be left out of the Paris agreement, according to a report last year from the United Kingdom-based organization Influence Map, which researches corporate influence on climate policy and other issues. And Closset said that low-emission systems need to be low-cost, “so it will be very difficult for them to introduce new technology.”

A previous version of this story misstated the type of fuel used in the Hydroville vessel. It is compressed hydrogen, not liquid hydrogen.