New research paints an even gloomier picture of global warming than the already grim report put out in early 2007 by the UN's Nobel-winning scientific panel, he told AFP at the margins of a major climate conference.

Director since 1981 of NASA's Goddard Institute for Space Studies, Hansen made headlines worldwide in 1988 with his US Congress testimony that climate change was already well under way, a finding highly contested at the time.

"What we have realised is that the dangerous level of atmospheric carbon dioxide is much lower than what we thought a few years ago," he said.

To prevent a devastating maelstrom of drought, extreme weather, famine and forced migration by century's end, the concentration of CO2 will have to be kept under 350 parts per million (ppm), he said.

"We are actually going to have to decrease the amount of CO2 in the atmosphere," he added, noting that the current level -- still rising -- is 385 ppm.

The UN Intergovernmental Panel on Climate Change had suggested that a ceiling of 450 ppm would be enough to forestall the worst effects of climate change.

Hansen's argument is a simple as it is sobering: continuing to drain Earth's store of fossil fuels -- oil, gas and coal -- will lead humanity straight toward climate calamity.

Only an abrupt and profound change in the way we consume energy can stop the global warming juggernaut.

A cap-and-trade system, under which progressively stricter "polluting right" exchanged in a carbon market, is likely to be reinforced at UN climate talks in Copenhagen this December as the favoured approach to slashing greenhouse gases.

The system is already in practice in the European Union, and has been proposed by US President Barack Obama for the United States.

But Hansen is highly sceptical that it can work.

"It takes about 10 years to negotiate it and get all the countries on board, and then you make all sort of compromises, so it turns to be very ineffectual," he said.

"If it's going to be cap and trade, I'd rather nothing came out of Copenhagen. I'd rather take another year and two and get it right."

The fact that prices in the EU carbon market have plummeted due to the global economic crisis lends weight to Hansen's doubts.

What would work, he argues, is a direct tax -- as close to the source as possible -- on fossil fuels.

"A carbon tax is the mechanism that allows you to make an international agreement globally effective in a short period of time," he said.

"You could start with the EU, United States and China -- that would be enough," he added, saying other nations confronted with a carbon-tax on their exports would quickly follow suit.

To create strong "green" incentives, the levy should be given directly back to the public on a per capita basis -- in the United States, he said, it would amount to several thousand dollars per household.

"A person with several large cars and a large house will have a tax greatly exceeding the dividend. A family reducing its carbon footprint to less than average will make money," Hansen wrote in December in an open letter to then president-elect Obama and his wife Michelle.

A third pillar of his climate proposal centers on coal, the most plentiful and polluting of the major fossil fuels. Hansen says all new coal-fired power plants should be banned, and older ones fitted with systems to capture carbon emissions and bury them underground.

Hansen's proposals got an unsolicited boost at the climate conference Tuesday from leading US economist William Nordhaus of Yale University, who told 2,000 experts that cap and trade "is inefficient and prone to market failure."

"It is better to change now, and quickly replace the cap and trade structure by a tax on green gas emissions."

After his 1980s testimony put him in the global spotligh, Hansen withdrew from the public arena to concentrate on science.

But 15 years later, the yawning gap between scientific certainty and public doubt on the climate threat, combined with the Bush administration's censuring of his institute's statements on climate, prompted him to get back in the game. "I decided that I didn't want my grand-children to say, 'grandpa understood what was happening but he didn't make it clear'," he said smiling.

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