Worried about what you're doing to warm the planet? Wish there were a way to cut back on the greenhouse gases your lifestyle creates -- without cutting back on your lifestyle?

Maybe there is.

At least three national companies -- TerraPass, DriveNeutral and Carbon Fund.org -- will help you calculate how much carbon dioxide you generate, translate that into dollars and spend that money to combat global warming.

They fund electricity-generating windmills. They support programs that turn cow manure into energy. They plant trees. They go to the Chicago Climate Exchange -- sort of a stock exchange for pollution -- and buy carbon credits, effectively reducing the amount of carbon dioxide that companies on the exchange can release into the air.

Roanokers Sean and Amy McGinnis try to minimize their family's impact on the environment. They travel around town on bikes, towing their 3-year-old in a bike trailer. They keep their house cool in winter and warm in summer. When weather permits, they hang out their laundry rather than use the dryer. They use energy-saving light bulbs. Sean McGinnis rides the Smart Way bus to his job at Virginia Tech.

When they buy things, they consider the energy that went into and the greenhouse gases that came out producing various goods.

And last year, they bought a TerraPass, contributing money to offset the emissions of one of their two cars. They weren't thoroughly convinced about the program. But they put the sticker on their Honda sport utility vehicle anyway. At least it might raise awareness of the problem.

A manly business

Tom Arnold had an epiphany in Antarctica.

On a seven-day camping trip with 17 other Wharton School business students -- three of whom had never been camping before -- Arnold learned about stress. The stress of people working in challenging conditions. The stress of a continent in changing conditions.

For several days, temperatures climbed above freezing.

"Literally the entire island began to melt," Arnold said.

Glaciers became lakes. Rivers of melted ice roared to the ocean.

That's normal during the Antarctic summer, Arnold said, but it made him wonder: If this is what it's like now, what if the temperature rises?

It turns out the continent's temperature is rising -- 3 or 4 degrees over the past 50 years. Virtually every researcher Arnold met in Antarctica was studying climate change. Virtually every one was finding something unsettling.

Soon Arnold was in another class, a class challenged to develop a mechanism to help individuals offset the greenhouse gases their lifestyles generate. And so, in 2004, TerraPass was born.

Arnold said he believes about 10 percent of U.S. residents are potential TerraPass customers. Arnold wouldn't give specifics, but he said most of the 2,800 people who've bought into TerraPass so far are men.

"We're the first environmental product to weight male," Arnold said. "We're more male than the NFL."

His take on why that's so: "It's kind of a techie idea. It's kind of a geeky idea."

The company's typical customer is an educated man in his early 30s who drives a Honda Civic.

"This is not about Hummers and get-out-of-jail-free cards," Arnold said.

Competing for environmentalists

Like TerraPass, DriveNeutral grew out of a business school project, in this case the Presidio School of Management in San Francisco. Like TerraPass, DriveNeutral offers drivers a chance to atone for the carbon dioxide their cars emit. But DriveNeutral is much younger and the cost of conscience salving is much lower.

One reason is DriveNeutral has no paid staff. Another is the company spends all its money at the Chicago Climate Exchange. And buying carbon credits -- 250 pounds of carbon for $1 -- is cheaper than funding alternative energy projects.

So far, according to Jason Smith, the company's chief executive officer, a lot of DriveNeutral's customers have been friends and family of the company's student staff. DriveNeutral's first customer was the business school's provost. He signed on in September.

Smith plans to roll out a marketing campaign this spring, aiming at the San Francisco Bay area.

Both DriveNeutral and TerraPass concentrate on cars, and cars are significant contributors to greenhouse gases. A Ford Expedition pumps more than 7 tons of carbon dioxide into the air in a year. A Toyota Prius, with its electric-gasoline hybrid engine, emits more than 2 tons.

But a typical American family of four people and two cars is responsible for more than 76 tons of carbon dioxide annually, when everything is considered.

Carbon Fund.org, which began in 2003, considers just about everything in totaling up emissions. Its online calculator includes cars, heating, electricity and air travel. Unlike its competitors, Carbon Fund.org is a nonprofit and intends to stay that way. And, unlike its competitors, Carbon Fund.org allows its customers to designate where their money goes.

Don't want to buy carbon credits? Put your money into wind farms. Or tree planting projects.

Before the end of the year, according to Craig Coulter, the organization's director of partnership development, there will be an option to spend money on projects in developing countries, too.

While the companies are competitors, they're also comrades in a fight against climate change.

"We're basically after the same thing," Coulter said. "And it's so small now that if anyone grows it's a great success for all of us."

So Coulter should be happy about TerraPass' conversion of the McGinnises.

In the 10 months since the family bought a TerraPass, Sean McGinnis said, the company has kept them informed about what TerraPass is doing to combat global warming. That communication, plus the third-party verification built into the program, has convinced the McGinnises that buying a TerraPass does some good.

When it comes time to renew their TerraPass in March, Sean McGinnis said, they'll probably buy two.

On the Net: www.carbonfund.org www.driveneutral.com www.terrapass.com