The small size and relative newness of companies like First Solar, which is a fairly typical fi rm in the world of clean-tech pureplay stocks, can make it tricky for a person to invest in shares of individual companies in the sector. Pernick says that clearly not all the clean-tech start-ups will prosper longterm, but some will likely do very well. "Last year, solar, wind, biofuels, and fuel cells were a $55 billion global industry," he says. "We see that expanding to greater than $225 billion in the next 10 years. That's why investor interest is being piqued."

Carbon Offsets

Of all the odd concepts to come out of the green movement, carbon offsets have to be near the top of the list. The idea is that you pay someone else to plant a tree, invest in a solar-energy start-up, or otherwise help reduce the amount of carbon you emit into the atmosphere. Then, you can just go about your business more or less as usual, confident that you have offset whatever carbon you may be emitting.

Carbon offsetting has won the backing of sustainability champions. "Offsetting is a common solution," says Brian Mullis, president of Sustainable Travel International, a Hood River, Oregon, nonprofit education organization. "Investing in renewable energy is a good thing, as is preserving ancient forests and investing in replanting areas where deforesting has occurred."

The average citizen of a developed country produces anywhere from six to 23 tons of carbon per year. The cost of buying credits to mitigate those emissions varies widely, starting as low as $1 per ton and going as high as $30 per ton. At San Francisco- based TerraPass, a leading carbon-offsets retailer, credits cost about $10 per ton. So canceling out all the carbon your car emits during a year costs between $30 and $80, depending on the vehicle, according to Tom Arnold, TerraPass's chief environmental offi cer. Offsetting your entire house would run anywhere from $30 to $400, depending on size, location, and other factors.