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, whose big wins include 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 that "cleantech" is "the biggest economic opportunity of this century," my ears -- and yours -- should perk up.

The future for cleantech
The idea of cleantech as a big investment opportunity is nothing new. Fools like me 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; when viewed in aggregate, they 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 presciently 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 boost the company ahead of its peers if and when the government imposes mandates on carbon emissions.

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 (NYSE:PG) could benefit from a new regulatory environment for carbon dioxide emissions. More interesting still is PG&E (NYSE:PCG), which is aggressively adding to its renewable energy portfolio. It recently 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 they still hold growth potential. 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 week its intentions to invest $100 million in three wind farms in New York, and BP (NYSE:BP), which also broke ground on a 100MW wind farm in Kansas this month, are both positioning themselves nicely to ride this wave.

In the solar field, IBM (NYSE:IBM) recently announced a research breakthrough in photovoltaic technology that could significantly reduce the cost of harnessing electricity from the sun, and MEMC Electronic Materials (NYSE:WFR) and SolarWorld have quite recently both signed multibillion-dollar contracts to supply silicon wafers to some of the world's largest solar manufacturers.

Third, Doerr said the United States needs to reinvigorate its biofuels industry. To a degree, this is already happening. Archer Daniels Midland now has a 50 million-gallon facility in production. And in May, DuPont (NYSE:DD) announced a joint venture with Danisco to develop and commercialize a low-cost technology solution for producing next-generation biofuels. With the advent of tougher EPA regulations requiring cleaner-burning diesel, the demand for biofuels could grow stronger in the near future. And both companies, by positioning themselves at the forefront of this biofuels "reinvigoration," could profit nicely from its expansion.

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 here 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. If you prefer a more specific sector, such as wind power, an ETF like the new Global Wind ETF (NASDAQ:PWND) is also an option. Alternatively, investors with a more conservative approach might want to consider companies 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.

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 their investments in it. After all, just because the opportunity is large doesn't mean everyone's profits will be, too.

If you'd like to see 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 on Nov. 17, 2006. It has been updated.

Fool contributor Jack Uldrich still puts his pants on one leg at a time, but they're stain-repellent nanomaterial pants. He owns stock in IBM and PG&E. Symantec is an Inside Value recommendation, and Duke Energy is an Income Investor choice. Google and PowerShares WilderHill Clean Energy are Rule Breakers selections, and is a Stock Advisor pick. The Fool has a strict disclosure policy.