Pay as you go

In the continuing discussion about greenhouse gases, travel plays a heavy.

Globally, all forms of motorized transportation account for 22 percent of carbon dioxide emissions. Air travel has been singled out as being particularly harmful to the atmosphere. So what is a conscientious traveler to do?

"Travel itself isn't bad, and we can't ask people to stop traveling," said Brian Mullis, president of Sustainable Travel International, a nonprofit dedicated to minimizing environmental and cultural damage from tourism. "But we can be cognizant of how we travel, and make choices that have less impact."

Some choices are easy to understand. If you take fewer vacations but stay longer, you'll fly less frequently and contribute smaller amounts of greenhouse gas. Likewise if you use mass transit instead of renting a car or taking taxis. Walking or riding a bike has zero impact on the environment, and has the added benefit of being good for your body.

But if you wish to continue your high-flying ways, yet want to mitigate the damage, you'll have to consider purchasing carbon offsets.

In theory, a carbon offset allows someone to negate the emissions they create by paying for a reduction in emissions (or a counterbalance to them) somewhere else.

The idea has become mainstream. Some airlines -- including Delta and Continental -- allow passengers to purchase carbon offsets when they buy their tickets. (A domestic round-trip flight offset for an individual would cost between $10 and $20.)

Offsets are a $100 million global market that is growing rapidly. In the United States, which accounts for 60 percent of that market, there are no legal standards for what constitutes a legitimate offset, and no federal regulation of the industry.

"When a consumer is spending money to try to be carbon-neutral, they want to confirm the money is going to a worthwhile project," said Joe Pouliot of the World Wildlife Fund. "There are a lot of choices out there on how to do that."

Too many, some believe. The Federal Trade Commission sought comments in January as a first step toward possible regulation of the carbon-offset industry. Among those expressing concern to the commission were the attorneys general of California and 10 other states. They cited a lack of national standards and the potential for fraud in selling something so undefined.

"Offsets are a complicated commodity," said Derik Broekhoff, a policy expert at the World Resources Institute who believes federal regulation is overdue. "There's no way to expect an individual retail consumer to have the time to investigate what constitutes a legitimate offset program."

Voluntary measures

Even with those criteria in hand, the topic is a shifty one. Experts disagree on how much carbon dioxide a tree absorbs in its lifetime, for example. "I'm hard-pressed to come up with an example of outright fraud," said Broekhoff, "but without consistent standards, it's easier to fudge things."

The World Wildlife Fund (WWF) issued a report in March that ranked the Gold Standard, a nonprofit that sets principles for carbon offset companies, as the best available program for guaranteeing worthwhile offsets. To get Gold Standard approval, a carbon offset project must have third-party verification that it is creating a real reduction in emissions and meeting other standards.

But the WWF and many other environmental organizations advocate offsets only as the final step in a three-pronged approach to cutting emissions that is described as "avoid, reduce, then offset."

That approach resonates with Mullis of Sustainable Travel International.

"The first option is to support legislation that supports reductions in greenhouse gasses," Mullis said. "Second is to make choices that conserve energy and choose travel providers that are eco-conscious. Third, when you do travel, use offsets."