Is there such a thing as a sustainable cruise? After all, cruise ships emit three times as much carbon dioxide as airplanes, according to an article in the Telegraph. The fact that many passengers fly to their cruising destination only compounds the carbon footprint.

Then there’s the issue of pollution. According to the environmental group Oceana, the average cruise ship produces the following immense amount of pollution every day:

— 25,000 gallons of sewage from toilets;
— 143,000 gallons of sewage from sinks, galleys and showers;
— 7 tons of garbage and solid waste;
— 15 gallons of toxic chemicals; and
— 7,000 gallons of oily bilge water.

That said, not all cruises are equal. This past fall the environmental group, Friends of the Earth (FoE) released an environmental report card in which it ranked 10 major cruise lines in the areas of sewage treatment, air pollution reduction, water quality compliance, and accessibility of environmental information. The top scorer, Holland America Line, earned an overall grade of B. Norwegian Cruise Lines and Princess Cruises were not far behind with final grades of B-. At the bottom of the heap were Royal Caribbean International and Disney Cruises, both with overall grades of F.

Still, there are signs that the cruise industry is changing for the better. In an article for G Living, Jennifer Buonatony wrote, “Over the past five years, the major cruise lines have spent an average of $2 million dollars per ship to upgrade vessels with better systems for dealing with waste management and emissions. And while the industry has grown 7.6% annually over the last decade, cruise ships have cut waste almost in half, which is an amazing figure.”

Among other environmental initiatives, Norwegian Cruise Line recycles thousands of gallons of used cooking grease by donating it to organic farmers in Hawaii and Miami. Carnival Cruise Lines has switched to eco-friendly detergents for dry cleaning and has developed a new, energy-efficient and nonpolluting engine for its newer ships.

Princess Cruises and Holland America have reduced air pollution from idling ships by modifying the engines on several of their ships so that they can be plugged into onshore hydroelectric power while in port. Holland America has also installed low-flow toilets and showerheads in staterooms and switches to soy-based inks for all printed materials on board. Rather than throwing away unneeded furniture, linens, travel-sized toiletries, and other items, Holland America donates them to charities around the world.

Its failing grade from FoE notwithstanding, Royal Caribbean has adopted a variety of initiatives to reduce energy use and pollution, including adjusting its itinerary and cruising speeds to save fuel. Its new mammoth Oasis of the Seas (5,400 passengers) has made some major green improvements. In an article for Mother Nature Network, Shea Gunther reported, “The Oasis of the Seas is far from green, but it does make some strides in the greener direction. The ship’s huge liquefied natural gas fueled engines come equipped with pollution scrubbers that completely eliminate all SO2 emissions, cut NO2 emissions by 80 percent and CO2 by more than 20 percent. Manufacturer Wärtsilä claims Oasis of the Seas will use 25 percent less power than smaller but similar cruise ships. The ship will also process its own waste on board, reusing the wastewater and dumping nothing into the ocean. It’s the first cruise ship to have a large tropical park filled with thousands of plants and natural features.”

Smaller cruise lines, such as Lindblad Expeditions and Adventure Smith Explorations, have also adopted measures to be more sustainable, including offsetting their carbon footprint, purchasing seafood from environmentally friendly sources, and adopting new sustainable technologies suitable for smaller vessels. Norway’s Hurtigruten cruise line recently signed a cooperative agreement with National Geographic’s Center for Sustainable Destinations to protect the destinations along its route.

One thing is clear: the cruise industry isn’t about to disappear. Even with the poor state of the economy, an estimated 15.2 million passenger nights were booked on cruises in North America alone in the second quarter of 2009, an increase of 1.5 percent over the previous year, according to a report by the U.S. Department of Transportation, Maritime Administration. The Port of Seattle, Washington, a major departure point for cruises to Alaska, reported a record number of cruise ships and passengers in 2009.

“We know when it comes to vacations, cruising is the best deal financially for many people,” Marcie Keever, director of FoE’s Clean Vessels Campaign, told the New York Times. “We just think travelers should try to choose one that is trying hard to do better environmentally.” The bad news is, the cruise industry still has a long way to go to achieve sustainability. The good news is, it’s beginning to change.