Regardless of how you slice it, Sperling says, long-distance transportation—and the carbon dioxide it produces—is where travelers as a group do the greatest damage to the environment.

"You can be an environmental saint—drive a hybrid car, recycle, conserve water—but if you take one flight, it actually blows your carbon budget right out of the water," says Elle Morrell, director of a green lifestyle program at the Australian Conservation Foundation. "A person flying round-trip from Sydney to New York creates as much carbon dioxide as another person [who doesn't fly] in a year."

It only stands to reason that water should be conserved in places where it's scarce, but in popular destinations such as Goa, India, or on Kenya's Mombasa Coast, hotels with lavish fountains and watered landscaping exacerbate painful shortages, according to Tricia Barnett, director of Tourism Concern, a Britain-based nonprofit that advocates for communities affected by tourism.

In Guanacaste, Costa Rica, officials are taking steps to import much-needed clean water from a rainier volcanic region. Meanwhile, one local golf course uses as much water per day as a village of more than 5,000 people, says Martha Honey, executive director of the Center on Ecotourism and Sustainable Development, in Washington, D.C.

But experts are quick to point out that the most obvious solutions to problems such as these—reducing our flights and using less water—aren't necessarily the best way forward. If travelers quit flying, for instance, wildlife and nature preserves in developing countries would likely fail, Honey says. "It would be devastating to the local economies and to the national parks in those places that depend on tourism."

When it comes to habitat destruction, however, environmentalists agree that tourism poses one of the greatest threats to the world's most fragile and biodiverse ecosystems. The reason is simple: The tropical settings travelers like to visit are also where teeming habitats are found. Cancún's shoreline used to be just such a place. Fifteen years ago, scuba instructor Wendy Aceves routinely spotted eagles, iguanas, and crocodiles in mangroves that today attract tourists who, by the dozen, zip around on motorized watercraft. Now, Aceves says, she no longer sees those native creatures.

Does this mean that travelers should avoid Cancún and other places like it? Some experts, including Jamie Sweeting of the Washington, D.C.–based environmental advocacy group Conservation International, say yes, developers should not be rewarded for decimating habitats. Sweeting laments, for instance, the "gross habitat degradation" that occurred to make way for three Jack Nicklaus golf courses at Cap Cana, in the Dominican Republic. He says that mangroves, swampland, and trees—all staples of the local ecosystem—were plowed under as "bulldozers flattened the entire coastline for tens of miles."