But now's the time for Al Gore to come clean, critics say.

They are accusing the former US vice president of seeing a different kind of green - profiting from the climate change agenda on the way to becoming the first carbon billionaire.

Gore has made significant investments in environment-friendly projects like carbon-trading markets, solar power, biofuels, electric vehicles, sustainable fish farming, waterless lavatories and smart home electricity meters.

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But he has also invested in other businesses, including Google and Apple.

Since quitting politics, his personal fortune has risen from US$2 million (HK$15.6 million) to US$100 million.

"Al Gore is poised to become the first carbon billionaire," said Marc Morano of climatedepot.com. "As much as Gore's made now, it is going to be nothing compared to what he is going to make in five years if all these new carbon-trading mandates go through."

But Gore says global warming deniers are merely polluting the public's mind.

"I believe the transition to a green economy is good for our economy and good for all of us," he said.

At the very least, Gore puts his money where his mouth is.