Although international shipping is critical to global trade, cargo ships are a major source of air pollution, including greenhouse gas emissions. At present, atmospheric emissions from fuel oil-powered marine ships include sulfur oxides (SO_x), nitrogen oxides (NO_x), particulate matter (PM), and carbon dioxide (CO₂). According to the International Maritime Organization (IMO), the United Nations agency responsible for preventing air pollution by ships, maritime shipping is responsible for 18 to 30 percent of NO_x, 9 percent of SO_x, and 3.5 to 4 percent of CO₂ emissions worldwide. In addition, the maritime risk management and environmental assessment organization RightShip projects that if no abatement action is taken, global CO₂ emissions may increase 50 to 250 percent by 2050 because of shipping growth. In April 2018, the IMO adopted a non-binding agreement to reduce CO₂ emissions. Suggested carbon reduction strategies include improving fuel quality and engine emission standards as well as using new technologies, such as fuel cells, biofuels, and advanced sail design. See also: Air pollution; Biosynthesis of fuels; Carbon dioxide; Fuel oil; Nitrogen oxides; Ship powering, maneuvering, and seakeeping

Cargo ships burn a heavy fuel oil, called bunker oil, which is a toxic material with a high sulfur content. Beginning in 2020, in order to reduce SO_x and NO_x emissions, IMO regulations will require cargo ships to use low-sulfur fuels and include NO_x emission controls for their engines. However, these standards will not reduce CO₂ emissions. (Copyright © EvrenKalinbacak/Getty Images)

The use of hydrogen fuel cells, in particular, shows great promise in decreasing ship-generated carbon emissions. Fuel cells convert chemical fuel into direct-current electrical energy, usually by combining hydrogen and oxygen and emitting water vapor and heat. When the hydrogen is produced using a natural energy source, such as solar energy, wind power, or biomass, fuel cells produce very low CO₂ emissions. However, if the hydrogen used in fuel cells is derived from non-renewable natural gas or petroleum, then CO₂ emissions (from hydrogen production) may exceed diesel-fueled propulsion systems. Hydrogen fuel cells have never been used for ship propulsion. Still, this did not stop Sandia National Laboratories from initiating a feasibility study for a high-speed passenger ferry powered by hydrogen fuel cells, called the San Francisco Bay Renewable Energy Electric vessel with Zero Emissions (SF-BREEZE). Sandia not only determined that such a vessel was feasible, the company also determined that by using renewable hydrogen, CO₂ emissions would be reduced by 75.6 percent, NO_x by 99.2 percent, and PM by 98.6 percent. Compared to a diesel-powered ferry, the cost of building a fuel-cell ferry would be about three times greater. See also: Energy sources; Fuel cell; Hydrogen

For cost optimization as well as proof of concept, a somewhat pared-down fuel-cell ferry called the Water-Go-Round is being built. The Water-Go-Round will be 21 meters (70 feet) in length and carry 84 people. When it launches in the fall of 2019, the Water-Go-Round will be the first fuel-cell ferry built in the United States. Passenger service in the San Francisco Bay area is expected to begin after a three-month testing period. To solve the hydrogen infrastructure issue, the ferry will be fueled from a truck. If this project is successful, hydrogen fuel cells might ultimately be used to power new—or fitted in old—ferries, tugboats, cruise ships, and cargo vessels. See also: Cruise ships; Ferry; Merchant ship; Ship design; Tugboats