As more vacationers have begun to contemplate the impact of their travels on the planet, from the greenhouse gas emissions of their flights to the litter they leave behind in scenic areas, mainstream travel publishers have devised a new category of books to address their concerns.

The new responsible and ethical travel guides, including Lonely Planet's recent "Code Green: Experiences of a Lifetime" and the upcoming "Green Travel: The World's Best Eco-Lodges & Earth-Friendly Hotels" from Fodor's Travel, aim to give readers a way to judge the sustainability of operations from lodges to wildlife treks. In a world where commercial enterprises are increasingly eager to tout their ecotourist credentials, these specialty books help travelers distinguish environmental ventures from orchestrated PR.

Some publishers, such as the U.K.'s Rough Guides and Australia's Lonely Planet, have integrated the concept into all their books and Web sites. They urge readers to reduce their global warming emissions and compensate for those they generate over the course of a vacation. Both companies' Web sites have a feature allowing visitors to calculate the global warming impact of any given trip and then donate money to Climate Care, a British group that compensates for carbon emissions by funding initiatives that cut greenhouse gases. Every Rough Guide, moreover, contains a section urging travelers to stay longer in a given location to minimize their climate impact.

Brice Gosnell, Lonely Planet's regional publisher for the Americas, said readers are demanding this service and have indicated that they welcome the changes guidebooks have made. "It's just about giving people the information they need to make appropriate decisions," he said.

Mark Ellingham, Rough Guides' co-founder, said guidebooks "should encourage our readers, and by extension, airlines and governments, to treat the issue with the gravity it demands."

U.S. travel guidebook publishers, such as Fodor's and Frommer's, have traditionally confined this sort of advice to books targeting countries where environmental activities are most popular, such as those in Latin America. Fodor's has an ecotourism chapter in its Costa Rica book, while Frommer's tackles the subjects in guidebooks on such countries as Belize, Panama, Brazil and Peru.

"In general, the U.S. market is just becoming aware of eco-travel, carbon footprint and the impact of travel on the planet," said Fodor's Travel publisher Tim Jarrell. He said that as Americans "increasingly become concerned about global warming, they will begin to examine different parts of their life."

Kelly Regan, Frommer's Travel Guides editorial director, said her company is working to educate readers about practical steps, such as reusing towels and linens to conserve energy and water. "It's a very small thing, but it can reap big benefits," she said.

Fodor's and Frommer's are expanding their responsible travel offerings, covering not only which hotels use solar power and sustainably harvested wood, but which tourist activities improve the welfare of the local communities they touch. Fodor's "Green Travel," which will be published in the spring, identifies three criteria as essential to responsible travel: environmental conservation, social and cultural awareness, and economic benefits for the communities tourists visit. Lonely Planet's "Code Green" and Rough Guides' "25 Ultimate Experiences" apply a similar standard to their trips.

Publishers also are exploring the possibility of introducing rating systems in their standard guides that would let readers know which accommodations are greener than others.

These changes may seem minor in light of the massive carbon emissions that global travel produces each year: A round-trip flight for two passengers from California to Europe produces about the same amount of carbon dioxide that a U.S. car emits on average in an entire year, and air travel is expected to be the single biggest contributor to human-induced climate change by 2020.

But Regan said these books speak to travelers' sense of urgency to see natural wonders before global warming makes them disappear.

THE NEW GREEN GUIDEBOOKS: A SAMPLING

"Code Green: Experiences of a Lifetime," Lonely Planet

It charts 82 trips and highlights the "responsible travel credentials" of each one, such as yurt stays in Tajikistan's Pamir Mountains, a village walk and home-stay in India's Kumaon region and a canoeing-camping trip to view hippos on Zambia's Zambezi River. It also touts destinations that have environmental attractions but are not in immediate danger of being overly exploited, usually because they're remote.

"Frommer's Cancun, Cozumel & the Yucatan 2008," Frommer's Travel Guides

It includes a section on ecotourism that refers readers to two groups, the Whale and Dolphin Conservation Society and Tread Lightly, that address the ethics of swimming with marine mammals. The book also provides information on visiting protected areas, such as the Rio Lagartos and Celestun refuges, both of which feature flamingos.

"25 Ultimate Experiences: Ethical Travel," Rough Guides

It describes 25 trips that minimize visitors' effect on the environment while helping local communities, including a zero-impact luxury ski camp in the Swiss Alps consisting of white-canvas geodesic domes and a beach lodge near Mozambique's Guludo village that employs 55 locals who will eventually run the resort. There's also advice on ethical clothes shopping, volunteering abroad and cultural sensitivity.

"Green Travel: The World's Best Eco-Lodges & Earth-Friendly Hotels," Fodor's Travel (April 2008)

It details eco-destinations in six regions of the world: North America and the Caribbean, Central and South America, Europe, Africa and the Middle East, Asia, and Australia. Explore ethical travel dilemmas such as renting a car, giving money to beggars and visiting countries ruled by repressive governments.