BANGKOK, Thailand — Citing green hotels, coconut oil fuel for airlines and even recyclable golf tees, executives in one of the world's largest industries say they are trying to shrink tourism's oversized environmental footprint.

But with global travel projected to keep soaring, and those very leaders still eager to expand their own ventures, some doubt such efforts can significantly lessen global warming and other ecological woes.

"There are no simple solutions," Anna Pollack, head of a British tourism consultancy, told a two-day conference last week. "Tourism is both a victim of, and a contributor to, climate change."

More than 230 tourism executives, government officials and analysts attended the Pacific Asia Travel Association meeting in Bangkok, billed as the region's first to seek practical solutions to climate change.

Pollack, CEO of DestiCorp, urged delegates to face the hard realities of tourism's impact.

"We've had a free lunch so far," she said. "The industry hasn't paid for the water it's taken for golf courses, for emissions into the air, or the unique cultural and natural attractions tourists experience."

Based on U.N. World Tourism Organization data, she estimated that international travel will spawn 2.5 billion tons of carbon dioxide, the main "greenhouse" gas blamed for global warming, in 2020, up from 506 million tons in 1990.

Much of the greenhouse gas comes from the burning of fuel for airplanes, vehicles and ships.

International travelers are expected to nearly double to 1.6 billion by 2020, according to the U.N.

With travelers abroad already spending more than $2 billion a day, tourism is one of the world's largest industries.

Panelists outlined steps taken to stem or offset emissions from their enterprises, while also laying out plans for more hotels, travel promotions and airline routes.

Six Senses Resorts and Spa, an international luxury chain, has added a 2 percent carbon tax to its bills, while Britain-based TUI Travel asks clients to donate $2, which it matches and funnels into projects that offset emissions.

Edwin Fuller, who heads the international division of Marriott Hotels and Resorts, said the group is testing bamboo tees for its golf courses.

He said that at every property Marriott manages, it tries to persuade the owners to build all new hotels "green from the ground up" with energy-saving technology, eco-friendly materials and environmentally sound practices.

A representative of Virgin Air noted that in February one of its aircraft was the first to fly mostly on biofuel — in this case, a mixture of coconut and babassu nut oils.

Bhutan was offered as one model of sustainable tourism. The Himalayan kingdom issues a limited number of visas each year and charges high mandatory rates — up to $220 a day — for tourists.

Environmental groups were not invited to speak at the conference but some on the sidelines said the measures are inadequate for such a vast problem.

"Voluntary initiatives don't work," said Anita Pleumaron of the Bangkok-based Tourism Investigation and Monitoring Team. "Governments will have to take responsibility and enforce regulations."