Venture capitalists are people, just like you and me. They put their pants on one leg at a time and they're prone to making investing mistakes just like the rest of us. That said, it's hard to deny that some VCs are clearly better than others. John Doerr is one of Silicon Valley's more successful and higher-profile VCs, with big wins including early investments in Symantec,, and, more recently, Google.

This success, I believe, gives his words some weight. So when he says that global warming is real and "cleantech" is "the biggest economic opportunity of this century," my ears -- and yours -- should perk up.

The future for cleantech
It's not that the idea of cleantech as a big investment opportunity is new. The Motley Fool, I, and others have been writing about it for some time. Instead, it was Doerr's explanation of how cleantech can help address global warming that I found so interesting. He laid out four steps for solving global warming which, when viewed in aggregate, can provide investors with a useful framework for thinking about how to invest in cleantech.

First, Doerr said the U.S. government should adopt a mandatory goal of reducing greenhouse gases 25% by 2010. This is an ambitious goal and, in an election year, I don't think it's likely. Nevertheless, I do believe some controls are coming, and investors can profit by understanding which companies are getting ahead of the curve and positioning themselves to benefit from government mandates.

For instance, I have written before about Duke Energy's willingness to embrace mandates and explained how this progressive position -- when backed with strategic investments in cleaner coal-burning technologies and large-scale carbon sequestration and alternative fuel energy projects -- could position it ahead of its peers if and when government mandates on carbon emissions are imposed.

Many companies, however, are not waiting until such mandates are imposed; they're taking action now. The Wall Street Journal recently ran an article on the carbon market going mainstream. Among other things, it outlined how companies such as Procter & Gamble could benefit from a new regulatory environment on carbon dioxide emission. More interesting still is PG&E, which is aggressively adding to its renewable energy portfolio and in July added two utility-scale solar contracts totaling 106 megawatts.

Second, Doerr called for the adoption of renewable sources such as solar, wind power, and fuel cell technology. The first two are hardly bold calls, but growth potential is there. In May, the American Wind Energy Association estimated 5,600 megawatts of wind generation power would be installed in 2008 -- a record. Companies such as General Electric, which announced this past month that it was investing $100 million in three wind farms in New York, and American Superconductor (NASDAQ:AMSC), which is now licensing its massive 1.65 MW wind turbines to Turkey (one of the world's windiest countries), are both positioning themselves nicely to ride this wave.

Another interesting field to keep an eye on is geothermal power. In late July, Raser Technologies announced it had signed a commitment with Merrill Lynch (NYSE:MER) to finance a new 10 MW geothermal plant in New Mexico. It could be just the first of many such deals.

In the solar field, Intel (NASDAQ:INTC) recently revealed it would spin off an internal component that has been working on technology for use in solar panels, and First Solar (NASDAQ:FSLR) announced that it had begun construction of the first of four solar module manufacturing facilities in Malaysia, as well as a new 10 MW photovoltaic power plant in Nevada.

Third, Doerr said the United States needs to reinvigorate its biofuels industry. Earlier this spring, DuPont revealed that it was forming a joint venture with Danisco to develop and commercialize a low-cost technology solution for the production of next-generation biofuels. And, more recently still, Dow Chemical (NYSE:DOW) reported it was working with the Department of Energy to test a new catalyst to bolster ethanol production, and BP announced it was investing $90 million in Verenium to develop next-generation technologies for the efficient production of ethanol.

Finally, Doerr said there needs to be more investment in technologies that can remove existing carbon dioxide from the atmosphere. One interesting start-up to keep an eye on in this area is Global Research Technologies, but I would encourage investors to also focus on larger companies such as Siemens. Cleaning up vast amounts of carbon dioxide is a big problem and it could well take a big company to deliver the resources necessary to make a dent.

Invest intelligently
Investors looking for a more diversified approach to investing in renewable energy might want to consider the PowerShares WilderHill Clean Energy exchange-traded fund or, if a more specific sector -- such as wind power -- is more to your liking, an ETF along the lines of the new Global Wind ETF is an option. Alternatively, investors with a more conservative approach might want to look at a company like United Technologies. It can't be considered a pure cleantech play, but it is producing a variety of clean technologies, including vertical axis wind turbines and photovoltaic solar power arrays.

The bottom line is that, like Doerr, our Motley Fool Rule Breakers team believes cleantech will be big. And while there will be many technologies and companies taking part in the solution, Fools should be strategic about how they want to approach the opportunity. After all, just because the opportunity is large doesn't mean everyone's profits will be, too.

If you'd like to take a look at our ongoing cleantech research at Rule Breakers and read up on the companies recommended to date, you can do so free for 30 days. Click here for more information. There is no obligation to subscribe.

This article was originally published Nov. 17, 2006. It has been updated.

Fool contributor Jack Uldrich still puts his pants on one leg at a time, but they're nanomaterial pants that easily repel liquids and prevent staining. He owns stock in Intel. Dow and Duke Energy are Income Investor choices. Google and PowerShares WilderHill Clean Energy are Rule Breakers selections, and is a Stock Advisor pick. Intel is an Inside Value recommendation, while Symantec is a former Inside Value recommendation. The Fool has a strict disclosure policy.