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WASHINGTON, DC: Bryan Comer leads the International Council on Clean Transportation (ICCT) marine programme - dedicated to providing policymakers with the data and analysis they need to avoid, reduce, and eliminate pollution from the global shipping sector. He suggests conventionally powered cruise ships actually emit more CO2 per passenger than airlines:

Cruising is a way to visit some of the most pristine and spectacular places on earth, and we hear a lot about how flying is bad for the climate, right? Right. But what if I told you that even the most efficient cruise ships emit more carbon dioxide per passenger kilometer (CO2/pax-km) than a passenger jet?

How do the CO2 emissions from cruising compare with those from flying? In our global shipping emissions inventory, we found that the world’s largest and most efficient cruise ships emit about 250 gCO2/pax-km. Based on the data in ICCT’s most recent transatlantic aircraft emissions inventory, the industry average carbon intensity ranges from approximately 10 gCO2/pax-km to 130 gCO2/pax-km, with longer flights tending to have lower carbon intensity. At 2,000 km, the average carbon intensity is approximately 80 gCO2/pax-km.

This is for CO2 only. Counting the climate impacts of short-lived climate pollutants (SLCPs) like methane, black carbon, and especially persistent contrails from aircraft would change the math. While still uncertain, scientists’ best estimate is that the total (CO2 plus SLCPs) climate impact of flying is about three times that of CO2 alone. But cruise ships also emit SLCPs, especially methane, and for simplicity here we’ll compare only CO2 emissions in our cruise-ships-versus-airplanes analysis.

Cruise ships double as floating hotels, so it’s only fair to also consider emissions from hotel stays for those who fly. According to a 2021 tool from Cornell, a 1-night stay in a 4-star hotel in the United States results in about 30 kilograms of carbon dioxide equivalent emissions (kgCO2e) per room per night. If we assume there are two people per room, we can cut that in half. It’s even lower for less fancy hotels. For simplicity, let’s assume that CO2e = CO2. So, if one person goes on a 5-night cruise that covers 2,000 km, at 250 gCO2/pax-km (the most efficient cruise ship line) that passenger is responsible for 500 kgCO2. The same person flying by jet would emit 160 kgCO2 on an average airline. Adding in the hotel emissions means an extra 15 kgCO2 per night, so 75 kgCO2, and the total is 235 kgCO2. In this example, even accounting for emissions from an equivalent-night hotel stay at a 4-star U.S. hotel, a passenger on a cruise ship emits about two times more CO2 than someone who flies and rents a hotel.

I know I said we’re leaving out methane, but it’s important to know that new cruise ships are increasingly opting for methane as fuel, in the form of liquefied natural gas (LNG). Clarksons estimates that, by gross tonnage, half of new cruise ships being built today are designed to run on LNG. While this reduces direct air pollution emissions, the types of engines that cruise ships use leak unburned methane into the atmosphere. This is called “methane slip” and the life-cycle greenhouse gas (GHG) emissions from these engines end up being higher than using low-sulfur marine gas oil, according to our research. To look into this more, the ICCT, in partnership with Explicit ApS and TNO, recently launched the FUgitive Methane Emissions from Ships (FUMES) project, and we’ll be measuring real-world methane emissions from LNG-fueled ships, including cruise ships. We expect to publish the results next year.

For those cruise ships that don’t run on LNG, many are currently using scrubbers that reroute pollution from the air into the water. The scrubber discharge water is contaminated with polycyclic aromatic hydrocarbons and heavy metals which have been linked to cancers and reproductive dysfunction in marine mammals. All this so that cruise ships can continue to use cheaper high-sulfur heavy fuel oil instead of more expensive low-sulfur fuels to comply with the International Maritime Organization’s (IMO) fuel sulfur regulations. Unfortunately, we estimate that using scrubbers results in higher direct and life-cycle CO2 emissions than using marine gas oil, as well as higher particulate matter emissions, including of climate-warming black carbon.

So, how can the climate impacts of cruising be reduced? Let’s start with some baby steps. First, cruise ships should stop using scrubbers and instead use low-sulfur marine gas oil at all times, or at least while the ship is in or near ports, estuaries, harbors, Marine Protected Areas, and Particularly Sensitive Sea Areas (PSSAs). This is critically important for PSSAs containing coral reefs, as they are already suffering the ill effects of ocean acidification and a warming climate. Second, all cruise ships that have shore power connections should plug in whenever it’s available, and ships without shore power connections should install them. Third, cruise companies should stop ordering LNG-powered cruise ships and instead start investing in ships that use non-methane alternative fuels, such as green methanol or even green hydrogen. One cruise ship is already testing a hydrogen fuel cell using methanol as the hydrogen source, although it gets most of its power from LNG engines. New cruise ships should be capable of running on non-methane fuels such as methanol, with at least a portion of their power from hydrogen fuel cells, batteries, and wind-assisted propulsion, and all new cruise ships should be equipped with shore power connections.

Ultimately, mandatory, enforceable regulations will be needed to ensure that the cruise industry invests in ships powered by zero-life-cycle-emissions fuels and energy. California already requires cruise ships to use shore power, and the Port of Vancouver has banned the use of scrubbers by ships at berth or at anchor, including cruise ships. Meanwhile, the IMO is developing regulations aimed at phasing out GHG emissions from international shipping, including cruising. However, these regulations are currently too weak to stop global GHG emissions from the sector from growing, and strengthening them will require years of negotiations among IMO member states and observer organizations, including the cruise industry, which has its own IMO delegation.

In the meantime, regions such as the European Union are implementing their own regulations that require the life-cycle GHG intensity of marine fuels to fall over time, starting in 2025. Also, starting in 2023, the European Union will begin charging ships for their carbon pollution through its Emissions Trading Scheme. The effects of regulation will take time to realize. In the meantime, the industry should take it upon itself to abandon the use of scrubbers, use marine gas oil whenever possible, test out fuels that have low or zero life-cycle emissions, invest in zero-emission technologies including shore power, batteries, fuel cells, and wind-assisted propulsion, and show that it can be a leader in decarbonizing the global shipping sector. Otherwise, it’s hard to justify a summer cruise if you know that it’s worse than flying.

P.S. To those who have already booked their tickets, maybe reconsider this year’s voyage to the North Pole in this LNG-powered ice-breaking cruise ship: https://us.ponant.com/le-commandant-charcot

The International Council on Clean Transportation (ICCT) is an independent nonprofit organization providing technical and scientific analysis to environmental regulators. Comer is a lead author of the Fourth IMO Greenhouse Gas Study and is an expert on Arctic shipping issues. His work highlights how maritime transportation can transition away from fossil-fueled ships to zero-emission vessels.
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