Mankind's response to climate change will shift how the world gets its energy and is already making green barons out of early investors in renewable energy, clean technologies and carbon trading.

Reuters spoke to four entrepreneurs who are cashing in on the energy revolution and who say there is more money to be made.

BRUCE KHOURI, co-founder of Solar Integrated Technologies, based in Los Angeles, has at the age of 48 made $5 million by cashing in shares in the company.

He still has a $11 million stake in the company, which makes lightweight solar panels for commercial roofs. He saw the opportunity while running his own industrial roofing firm.

Q: How did you get rich?

A: It hasn't been easy but we transformed an old-world roofing material into a renewable energy technology. It's a miracle Solar Integrated is still here but a pioneer charging across the prairie is bound to get hit by a few arrows.

As long ago as the early 1990s Khouri saw a market for flexible solar panels which could be laminated on to large roofs, such as warehouses. He did not found Solar Integrated until 2001 once tax and subsidy incentives made the market more attractive.

Q: Is 'the business of green' a bubble?

A: 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.

And 50 years from now every bit of a building that is struck by the sun will be generating power in some way.

PEDRO MOURA COSTA, co-founder of Oxford-based EcoSecurities, 44, made 4.8 million pounds ($10 million) when he sold some shares in the firm which helps convert emission cuts into tradable carbon credits. His remaining shares are worth about 37 million pounds ($73 million).

Q: How did you get rich?

A: I saw the carbon market could be big business and the Kyoto Protocol confirmed my views. But I didn't expect it to take 10 years to come into force.

Moura Costa was working as a forester in Malaysia when he saw the potential for an international carbon credit market.

He spent the early 1990s advising on a project to plant trees in Borneo to compensate for extra carbon pollution from new power plants in the Netherlands.

Q: Is 'the business of green' a bubble?

A: No. It's become quite obvious we do something now or it will be an irreversible trend with catastrophic consequences.

The only chance of it being a bubble is if we lack the political commitment to drive emission reductions worldwide -- and if we do that we might as well forget about any environmental effort whatsoever because climate change is hitting us hard and the trend is likely to accelerate. I think it's very unlikely political support will go away.

DAVID SCAYSBROOK, founder of Novera Energy, a 43-year old Australian, made 3 million pounds ($6 million) when he cashed in some of his shares in the wind power and landfill gas firm he founded in 1998. He has about 3 million pounds ($6 million) worth of shares invested in Novera and carbon cutter Camco International, which he advises.

Q: How did you get rich?

A: Three things had pushed up share valuations in the wind power industry, he said.

First, people were more worried about energy security and producing energy themselves. Second, the cost of traditional energy sources such as oil and gas had gone up. Third, tax breaks, subsidies and emissions caps had prompted even more conservative investors to finally move off their perch.

Q: Is 'the business of green' a bubble?

A: The scale of investment to date is nothing compared to what is coming.

The bubble aspect is ill-informed investors chasing pipe-dream technology. For example, there are hundreds of firms competing for the next generation of technology in solar panels but it won't necessarily be the best technology that wins.

NEIL ECKERT, chief executive of Climate Exchange, which runs the main European exchange for carbon trading, has shares worth about 18 million pounds ($36 million). He is also non-executive chairman of Trading Emissions and Econergy, both involved in emission-cutting projects and generating revenue from carbon credits.

Q: How did you get rich?

A: Despite the high paper value of his holdings in Climate Exchange, Eckert has yet to cash in. He already made millions selling his shares in Brit Insurance, which he set up and ran for 10 years until 2005.

Q: Is 'the business of green' a bubble?

A: No. We have the biggest opportunity to replace fossil fuel, which has a market capitalization of hundreds of billions of pounds, but it's vital we listen to the scientific consensus and create a financial solution.

I believe we have a chance to meet the stated mitigation targets much quicker than people think ... but it depends on whether people believe money can be made.

(Additional reporting by Gerard Wynn)