But to help atone for that environmental sin, some drivers are turning to groups on the Internet that offer pain-free ways to assuage their guilt while promoting clean energy.
It involves buying something known as a carbon offset: a relatively inexpensive way to stimulate the production of clean electricity. Just go to one of several carbon-offset Web sites, calculate the amount of carbon dioxide produced when you drive, fly or otherwise burn fossil fuels, and then buy an offset that pays for an equivalent amount of clean energy.
Of course, emissions could be reduced the old-fashioned way — by flying less, turning off the air-conditioning or buying a more fuel-efficient car. But that would probably require some sacrifice and perhaps even a change in lifestyle. Instead, carbon-offset programs allow individuals to skip the sacrifice and simply pay for the right to pollute.
"To some extent, it's a way for people to buy their way into heaven," said Chip Giller, who is president of Grist.org, an online environmental magazine. "On the other hand, this is such a big macro problem that this is one of the few things people can do to really make a difference."
While offsets do not actually eliminate pollution, they do enable groups like Carbonfund.org to use the money to stimulate the production of clean electricity, which is more costly than burning coal or oil. An independent organization ensures that the money supports real projects. Some groups subsidize existing power production; others finance the building of new wind turbines or solar collectors.
In the two years since they appeared, such voluntary offset programs have become increasingly popular. But it is not clear whether they actually do any good, or are just one more way for Americans to feel good about doing things that pollute the atmosphere.
Take Biff Cuthbert. Running an organic clam farm for a few years taught him all about being green. But when he recently needed a new vehicle to haul musical equipment for his folk band, as well as his two Akita dogs, Mr. Cuthbert ignored his environmental conscience and bought a cream-colored 2004 Land Rover, which gets 12 miles to the gallon.
Feeling a pang of conscience about driving such a gas-guzzler, Mr. Cuthbert paid $79.95 to Terrapass.com, a group that helps finance non-polluting solar, wind and methane-driven energy projects. In exchange, he got a sticker for his windshield verifying that he is offsetting some of the 16,766 pounds of carbon dioxide his Land Rover will emit this year.
"It rounds the edges off of the guilt a little bit, I guess," said Mr. Cuthbert, 62, of Guilford, Conn. "It's a little like having your cake and eating it too," Mr. Cuthbert said.
Web sites like terrapass.com, carbonfund.org, nativeenergy.com and self.org focus on automobile emissions because drivers can become aware of their carbon footprint every time they fill up. An average car produces about 10,000 pounds of carbon dioxide a year.
Tom Arnold, one of the founders of Terrapass, a carbon-offset seller that, unlike most others, actually intends to make a profit, said that just as in the dawn of recycling a generation ago, the idea of carbon offsetting is being embraced at first by "greenies" who already plant trees, eat organic foods and buy fuel-efficient cars. As these environmentalists accept the altruistic good of doing something for the planet that will not immediately benefit them, they hope to spread the ideal of living a "carbon-neutral" life to nonbelievers.
Carbon offsets for power plants are already traded at an exchange in Chicago and will be part of a regional pact that New York and seven other northeastern states have formed to reduce greenhouse gas emissions.
The Rockefeller Brothers Fund bought offsets to cover the amount of pollution produced by all 1,479 people who attended conferences at its Pocantico Conference Center in Westchester County last year. Ben & Jerry's buys enough offsets to cover its manufacturing and retail operations, and even the Rolling Stones have bought offsets to make their concerts carbon neutral. The whole idea is to cap, and eventually reduce, the amount of greenhouse gases produced in the United States. But with gasoline prices nearing $3 a gallon, getting people to shell out even more to drive can be easier said than done.
Take the Gormans of Stamford, Conn., a family of self-professed environmentalists. Cynthia Gorman, 52, works for a transportation safety company in New Rochelle, N.Y., and commutes the 20 miles each way in her dark green 2001 Toyota Prius hybrid. A dedicated recycler, she said she buys a carbon offset from Terrapass every year because driving the hybrid on the highway "only gets maybe 42 miles per gallon." Hybrids are more efficient in the city, where their electric motors take over.
But when Ms. Gorman pulls back into the driveway of her Stamford home, she parks her Prius, with its Less Gs license plates, next to her husband Tony's dark green 2000 Dodge Durango, an S.U.V. that gets 13 miles per gallon, if that.
"For all that I'm doing, he's screwing up everything," Ms. Gorman said. Mr. Gorman does not try to hide his responsibility. He is a hunter, and a reggae producer, and he needs the S.U.V. to haul things. His license plate reads "Ire Mon" — a reggae singer's way of saying "It's O.K."—which is how he feels about his choice of wheels.
Mr. Gorman said that his wife sent him Web information about carbon offsets a while ago but that he did not understand what it was about and just let it fall through the cracks.
Mr. Gorman said he would probably buy his own pass at some point, but he worried that this green guilt thing could go too far. "It can really get you so depressed that you never leave the house," he said. "Look, sometimes you have to make a compromise. Yeah, I'm an environmentalist, but I need that vehicle."
A few days ago, Ms. Gorman took the decision out of his hands and bought a Terrapass for his Durango.
People who run offset programs insist that while guilt is part of the motivation for those who join, the movement really is more about a way of living. "What we're about is getting folks to live a net zero climate-neutral lifestyle," said Tom Boucher, president of Native Energy, which helps build wind turbines on Indian lands.
Ann C. Seligman, 45, lives in Manhattan and owns a 2004 Prius, so her "carbon footprint" is already small. Nonetheless, her husband, Michael, gave her a $99 carbon pass from www.carbonfund.org for her birthday. "We live in a basement apartment, not far from the East River, so global warming may come back to haunt us," she said.
Charles R. Church, 65, a retired lawyer in Manhattan, pays for a Native Energy $6-a-month pass even though he uses public transportation and rarely takes his 2002 Volvo station wagon out of its garage.
"The threat of global warming is becoming part of the zeitgeist and I'm happy about that," Mr. Church said. Offsetting his carbon footprint does not cost much and does not take much effort, he said, adding, "It just strikes me as the right thing to do."