The concept got mass exposure through former US Vice President Al Gore's film, "An Inconvenient Truth".

Socolow is sticking to his theory three years on, despite a surge in consumption of coal, the highest carbon fuel, and the emergence of serious tradeoffs from burning biofuels, which is derived from plants as an alternative to oil.

"We really don't have to wring our hands about the growth of coal," the professor of mechanical and aerospace engineering told Reuters in an interview this week. "But it means we have to move as quickly as possible."

Especially important is unproven "clean coal" technology, he said. This involves trapping the carbon dioxide produced by coal-fired power plants and piping it underground, a process called carbon capture and storage (CCS).

Following the unexpected leap in coal demand CCS may have to be deployed at more than 1,600 coal plants by 2050 -- a third more than the world's total coal power capacity now, he said.

Europe wants up to a dozen CCS demonstration plants in place by 2015 and the United States is also investing.

PUBLIC OPPOSITION

Worries about CCS include the possible costs of a delay in the phase-out of coal, in miners' lives, atmospheric pollution and the landscape.

Other low-carbon technologies including nuclear, wind and biofuels all face some public opposition given the possible scale of deployment needed.

A wind power "wedge" would require two million large windmills in place of coal plants by 2050, producing 25 times more wind power than available now, while a nuclear wedge would need a tripling of present capacity, Socolow estimated.

"We can do any of the wedges badly or well... It's about public acceptance. We've seen that with wind and nuclear in various parts of the world."

"What's impressive is that the public have said they don't want the risk of unpredictable climate change. If we lose that momentum it could take decades to get it back."

Together the seven wedges would cap carbon dioxide emissions at today's levels by mid-century.

Biofuels may be in trouble already. Soaring demand, built on subsidies, has raised food prices worldwide and led to the clearing of rainforests, analysts say.

But that may emerge as a temporary "aberration", Socolow said, referring to the chance of making biofuels in future without involving food crops.