What Is Greener? Boat vs. Plane Emissions

Four cruise ships anchored offshore of Georgetown Harbor, Grand Cayman

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In 2019, after boycotting air travel on account of its colossal carbon footprint, Swedish climate activist Greta Thunberg set sail on a 15-day transatlantic voyage from the U.K. to New York for a U.N. Climate Action Summit. Her widely publicized endorsement of slow, carbon-neutral travel shone a light on the environmental impact of flying, ultimately leading to a whole flight-free movement. But alas, traveling a la Thunberg (i.e., via sailboat) is perhaps too technical and time-consuming to be considered a viable means of transportation, and trading airplanes for cruise ships can lead to an even bigger problem, considering boats are on par with planes in their greenhouse gas emissions. In some ways, watercraft can be even more polluting.

Several factors should be considered when weighing the emissions rate of boats versus planes, such as the vehicle's age, its fuel type and efficiency, the length of the trip, number of passengers, and so forth. Learn more about the different kinds of gasses passenger planes and cruise ships emit, the environmental impact of those gases, and which of these notoriously dirty modes of transport is greener.

Airplane Emissions

Airplane flying above palm trees, leaving vapor trails

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Of the reported 16.2% of global greenhouse gas emissions for which transportation, in general, accounts, air transport (of both people and freight) is responsible for 1.9%. A 2018 report from the International Council on Clean Transportation said passenger transport accounted for 81% of total aviation emissions—that's 747 million metric tons of secreted carbon dioxide per year. The International Council on Clean Transportation says if the aviation industry were a country, it would be the sixth top greenhouse gas emitter. In the U.S. alone, emissions from domestic flights have increased by 17% since 1990, and passenger air travel continues to have a positive growth rate globally, interfering with efforts to slow global warming. 

Carbon dioxide makes up about 70% of aircraft emissions. CO2 is the most widely understood greenhouse gas, which is produced by the consumption of jet fuel. The type of plane, number of passengers, and fuel efficiency are all factors in exactly how much CO2 a plane emits, but the Environmental and Energy Study Institute defines the ratio as about three pounds per pound of fuel consumed, "regardless of the phase of flight." A chunk of the gas emitted by a single flight, the nonprofit notes, can linger in the atmosphere for thousands of years.

In addition to CO2, though, burning jet fuel also generates nitrogen oxides, classified as indirect greenhouse gases because they produce both ozone, which warms, and methane, which cools. Although still a relatively small component of total aviation emissions, NOx emissions from air travel are increasing at a faster rate than CO2, more than doubling from 1990 to 2016. That increase can be attributed to a growing aviation industry—one whose primary environmental mission is to curb emissions from the more notorious CO2.

Of course, not all planes are created equal, and while none are truly eco-friendly, some are greener than others. The Airbus A319, for instance, outperforms the classic Boeing 737 of its size (the 300 model) in fuel efficiency. It consumes about 650 gallons of fuel per hour compared with the latter's 800 gallons per hour. The Airbus A380 was briefly marketed as a "Gentle Green Giant," but the ICCT notes that the Boeing 787-9 was 60% more fuel-efficient than the A380 in 2016.

The Effects of Radiative Forcing

The EESI says only 10% of gases produced by planes are emitted during the takeoff and landing (including the ascent and descent); the rest occur at 3,000 feet and higher. This is especially damaging because of radiative forcing, a measure of how much light gets absorbed by Earth and how much is radiated back to space. The contrails—vapor trails—planes leave in their wake cause radiative forcing and trap gases high in the atmosphere, where they cause more damage than at the ground level.

Boat Emissions

Cruise ship against New York City skyline at sunset

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Like planes, boats also emit a cocktail of toxic greenhouse gases—including but not limited to CO2 and NOx. The amount emitted, likewise, depends on the ship's size, age, average cruising speed, number of passengers, and length of trip. There are all sorts of watercraft, but when comparing the footprint of maritime transport—accounting for 2.5% of global greenhouse gas emissions—to that of air travel, it's perhaps most logical to analyze the vessel most similar in size to a passenger plane: a cruise ship.

Traditional cruise ships run on diesel, one of the most CO2-producing fuel types available. According to Sailors for the Sea, a nonprofit ocean conservation organization affiliated with Oceana, marine diesel generates 21.24 pounds of CO2 per gallon of fuel. What's more, cruise ships emit black carbon—soot produced by the combustion of fossil fuels and biomass—and almost six times as much as an oil tanker emits, at that. According to a 2015 report from the ICCT, cruise ships account for 6% of marine black carbon emissions despite making up only 1% of ships globally. The warming effect black carbon has on the climate is thought to be up to 1,500 times stronger than that of CO2.

The European Federation for Transport and Environment found in a continent-wide study on luxury cruise ship emissions that the amount of NOx released by these hefty liners was equivalent to 15% of Europe's entire car fleet. It also found that port cities throughout Europe suffered from air pollution caused by extraordinarily high levels of sulfur oxides generated by the ships. In Barcelona, for instance, ships are generating five times more SOx than cars. 

Large cruise ships designed for long-haul trips even have their own incinerators. The average cruise ship produces seven tons of solid waste every day, which leads to a reported 15 billion pounds of trash being dumped into oceans (as ash, mostly) per year. Besides the direct impact this has on marine life, the incineration process itself generates additional emissions of CO2, NOx, sulfur dioxide, ammonia, and other toxic compounds.

Ocean Acidification

In the same way planes intensify their emissions by belching greenhouse gases at altitude, emissions from ships are extra harmful because the CO2 that escapes their exhausts is promptly absorbed by seawater. Over time, this can change the pH of the ocean—a phenomenon called ocean acidification. Because increased acidity is caused by a reduction in the amount of carbonate, shells made of calcium carbonate may dissolve, and fish will find it difficult to form new ones. Ocean acidification also takes a toll on coral, whose skeletons are made of a form of calcium carbonate called aragonite.

Which Is Greener?

Cruise ships moored in sea in Nassau, Bahamas

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A 2011 case study of cruise ships in Dubrovnik, Croatia, estimated that the average CO2 emitted per person, per mile on a medium-sized 3,000-passenger cruise ship was 1,639 pounds (.74 metric tons), or about the equivalent of an economy-class return flight from London to Tokyo. By that calculation, a round-trip cruise from Port Canaveral in Orlando, Florida, to Nassau, Bahamas—a popular, 350-mile transatlantic route frequented by Royal Caribbean International, Carnival, and Norwegian Cruise Line—would equal about one million pounds (434 metric tons) of carbon emissions per person. That same return route, if traveled from Orlando International Airport to Nassau's Lynden Pindling International Airport in the economy class of a passenger aircraft, would add up to only 368 pounds (.2 metric tons) of CO2 emitted per person, according to the International Civil Aviation Organization's Carbon Emissions Calculator. And that's only emissions from carbon, not NOx or any other gases.

Of course, a case can be made that ferries and other, less-polluting boats provide eco-friendly alternatives to air travel. This could be the case for overwater routes that ferries can handle, such as the heavily trafficked route from Melbourne to Tasmania, Australia, or the shorter-but-equally-busy route between Morocco and Spain. But the slower-moving vessels that boast entire waterparks and golf courses on board are likely to always trump aviation in terms of greenhouse gas emissions.

Tips for Reducing Your Carbon Footprint While Traveling

  • Before booking a flight or a cruise, do your research on which airlines and cruise lines are taking steps to reduce their carbon footprints. Friends of the Earth regularly creates "cruise ship report cards" in which all the major cruise operators are given a grade based on air pollution reduction, sewage treatment, water quality compliance, and other factors. Atmosfair has released a similar ranking of airlines based on fuel efficiency.
  • Whether traveling by air or water, remember that the shorter the trip, the greener. Choose direct flights over ones with multiple stops to minimize mileage. 
  • Consider carbon offsetting your travel. Many airlines are now offering this as an additional service, but you can also donate to a carbon offsetting program of your choice, such as Carbonfund.org or Sustainable Travel International.
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