Four green barons of the business world explain how they made their fortunes in the eco space, in a piece on Reuters today.

The interviews show how a couple of them have done it through carbon trading (hmm) but the other two made it via the energy game and clean technology.

Bruce Khouri, co-founder of Solar Integrated Technologies made $5 million by cashing in shares in the company. When asked, “Is ‘the business of green’ a bubble?’ he said:

“No. For political reasons, the United States has been behind others on green issues, but once it catches up it will be a domino effect. In 20 years they won’t talk about regular roofing because it just won’t exist … there is so much rooftop real estate that is completely under-utilized.”

• Green certified homes sell faster, higher price