How bad for the environment are cruise ships?

Illuminating answers to environmental questions.
Sept. 1 2009 10:10 AM

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How bad for the environment are cruise ships?

A cruise ship in New York city. Click image to expand.
Where does waste from cruise ships go?

My family took a cruise to the Caribbean this summer along with a few thousand other passengers. How bad for the environment was our vacation?

The smorgasbord of activities available on a modern cruise ship can leave passengers with a delightful buzzor a deep-seated malaise,depending on their temperaments. Either way, all those spa treatments and all-you-can-eat buffets create a fair amount of waste.

Like any other oceangoing vessel, a cruise ship can affect both the water and the air with its waste products. First, there's the issue of bilge water, which collects in the lowest part of the ship and often contains oil from leaky engines as well as other contaminants. In general, a ship will treat bilge water before discharging it, but there have been cases of irresponsible ocean dumping in the past. Then there are the air pollutants that fly out from the ship's smokestacks—these include particulate matter, sulfur dioxide, and nitrogen oxides. (The latter two are the principal components of acid rain.)

Cruise ships do pose some additional environmental risks compared with other vessels. That's because a large cruise carries thousands of passengers, each of whom produces a personal waste stream that can end up in the ocean. (For comparison, a typical freighter might be staffed by a dozen or so crewmembers.) In a recent EPA survey of boats operating in Alaska, cruise ships reported generating an average of 21,000 gallons of sewage a day. Those ships also produced a daily average of 170,000 gallons of graywater—the stuff that drains from sinks, showers, and laundry machines. Graywater can contain detergents, oil, grease, and food waste as well as oxygen-depleting nutrients and various pathogens.

A complicated patchwork of federal, state, and international laws governs what cruise ships can discharge and where. Sewage, for example, needs to be treated if it's going to be flushed out within three miles of U.S. coastlines, though beyond that, it can be dumped in its raw state. (According to the international convention that regulates ship pollution, "on the high seas, the oceans are capable of assimilating and dealing with raw sewage through natural bacterial action." However, all members of the trade association that represents more than 90 percent of the global cruise industry have a policy not to dump untreated sewage anywhere.) Bilge water also must meet certain oil content standards before it can be released into the ocean. * For the past few years, environmental groups—with the support of Sen. Richard Durbin of Illinois and Rep. Sam Farr of California—have been pushing a piece of legislation called the Clean Cruise Ship Act (PDF), which would bring all the regulations under one umbrella and strengthen policies on oily bilge water, sewage, and graywater.

If you're shopping around for a green cruise, you should check whether your boat comes equipped with an advanced wastewater-treatment system (PDF). You might also check whether the ship can plug into the local power grid when docked rather than continuing to run its engines, a process known as "cold ironing." The infrastructure to provide cruiseships with shore-based power is already in place in Juneau, Alaska, and Seattle and is in the works in San Francisco and Los Angeles. * Cruise ship companies are also touting plenty of other environmental initiatives, ranging from laundry facilities that reuse water from air-conditioning systems to kitchens that serve local and sustainably sourced food.

What about carbon dioxide emissions? According to Carnival Corp.'s latest environmental report (PDF), the company's ships emit 1.17 pounds of CO2 per passenger mile, assuming full occupancy. Industry statistics suggest that the most popular cruises last seven days and that the Caribbean is the No. 1 destination. So let's say you've decided to take Carnival's weeklong Western Caribbean cruise, which leaves from Miami and stops at Grand Cayman, Cayman Islands; Isla Roatan, Honduras; Belize; and Cozumel, Mexico, before returning to Florida—a total of 1,826.5 miles. Your personal emissions for the voyage—not counting any time spent on land—would thus come to 2,137 pounds of CO2, or just about a ton. In 2006, fossil fuel-related CO2 emissions in the United States were 21.8 tons per capita—or about 119.5 pounds per person per day. So a seven-day cruise produces about 18 days' worth of carbon dioxide.

In contrast, a round-trip flight on a narrow-body jet from Miami to Grand Cayman would emit about 340 pounds of CO2, assuming an industry standard 80 percent occupancy. * That's a lot less than a cruise, but then, it's not an apples-to-apples comparison since a cruise ship provides transportation and accommodation and entertainment. Based on the Lantern's back-of-the-envelope calculations, though, the overall emissions would still be lower if you could keep your carbon footprint during your week in the Grand Cayman at less than double what it would be at home.

The Green Lantern thanks Marcie Keever of Friends of the Earth and Jackie Savitz of Oceana.

Is there an environmental quandary that's been keeping you up at night? Send it to, and check this space every Tuesday.

Correction, Sept. 1, 2009: This article originally indicated that San Francisco and Los Angeles were already providing cruise ships with shore-based power. (Return to the corrected sentence.)

Correction, Sept. 2, 2009: The article originally stated that the round-trip plane ride would emit 312.3 pounds of CO2. (Return to the corrected sentence.)

Correction, Sept. 3, 2009: The corrected paragraph notes that under international law, bilge water must meet certain oil content standards before it can be released into the ocean. It also includes the caveat that most cruise ships have a standing policy not to dump untreated sewage anywhere. (Return  to the corrected paragraph.) 



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