As green as you try to be at home, a business trip can transform you into something of a carbon-emitting ignoramus, especially if you do a lot of flying. But it doesn't have to be that way. You (or your company) can buy carbon offsets to address some of the CO2 from your flights (Read the story "Clean Up" to learn more about carbon offsets). And on the ground, you have lots of options for green transport: public transportation, a green limo or a car rented from one of several environmentally aware firms.

Many of the major car-rental agencies, including Budget and Avis, have expanded their fleets to include hybrid, flex-fuel (cars that use more than 10 percent ethanol in the fuel mix) and/or low-emission vehicles. In the two years since Hertz spent $68 million to buy 3,400 Toyota Prius hybrids to launch its Green Collection, the company's fuel-efficient fleet has expanded to 35,000 cars. Hertz offers five types of fuel-efficient cars, all of which get at least 28 miles per gallon.

For every 1,000 standard rental cars Hertz has replaced with the Prius, the company says it will reduce carbon dioxide emissions by 3,000 tons per year. Green Collection cars are available at more than 50 domestic airport locations.

Even with Hertz's sizable investment, the leader in the greening of the car rental industry is Enterprise. It not only offers the biggest number of hybrids and other fuel-efficient cars but also an entire "platform" of corporate social responsibility initiatives focused on the environment. The company has donated $25 million to a plant science center to help develop renewable fuels, and has partnered with The National Arbor Day Foundation and the U.S. Forest Service to fund the planting of 50 million trees during the next 50 years.

Enterprise customers also have a chance to offset the CO2 produced by the rental car they drive. It costs $1.25 per rental to fund carbon-offset projects. Enterprise matches that dollar for dollar, up to $1 million a year.

How to Book a Green Car 
You have choices in booking a green car, if your company doesn't have a corporate account that requires you to book directly from a car-rental company.

Kayak has a hybrid filter in its search engine; you can also search for cars based on fuel efficiency.

Rezhub is a fairly new portal to environmentally sensitive car-rental companies, including Ace, Advantage, E-Z, Hertz, Thrifty and Dollar. The company donates 20 percent of all proceeds to environmental or volunteer organizations.

You can also book a hybrid through Orbitz if you're headed west; the site has partnered with Alamo, Avis, Budget, Fox, Hertz and National in Arizona, California, Nevada, Oregon and Washington.

An unlikely but creative marketing initiative was undertaken not by an online aggregator or car rental company but by an airport. The San Francisco International Airport authority launched the country's first Green Rental Car program, which rewards customers for renting a Honda Civic Hybrid, Nissan Altima Hybrid or Toyota Prius. Your reward for your green-mindedness is a $15 discount at the counter.

You can also try smaller firms, like Bio-Beetle on Maui, whose fleet of Volkswagen Beetles runs on bio-diesel made from used cooking oil, and eQocar in Burbank, Calif., which offers not just the Prius but also Yukons and the hybrid Lexus LS600 in its fleet of 45 cars.

The bottom line: Yes, you can expect to pay a higher rate for a green car. That Lexus costs a staggering $650 per day. You'll make some of that up, albeit very modestly, by better fuel efficiency. The benefit is really more about being a responsible consumer and perhaps a future hybrid owner: A car rental is a smart way to test-drive a hybrid or flex-fuel car in the real world, not just around the dealer's block.