Global warming fears, gas prices and government policies (like ethanol subsidies) are attracting even more investment in clean technologies. According to the Cleantech Network, venture capitalists invested $2.9 billion in the sector in 2006.

A decade ago, it was just $50 million. Rapid growth in investment and initial public offering values has set off talk of a dot-com style bubble.

For this video, we stopped in at the Greenvest 2007 conference in San Francisco to find out why some VCs (like Nancy Floyd of Nth Power who was investing in the sector when it was practically non-existent back in ’93) are still putting their money into cleantech.