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Along one wall of Jim Hansen's wood-panelled office in upper Manhattan, the distinguished climatologist has pinned 10 A4-sized photographs of his three grandchildren: Sophie, Connor and Jake.

They are the only personal items on display in an office otherwise dominated by stacks of manila folders, bundles of papers and cardboard boxes filled with reports on climate variations and atmospheric measurements.

The director of Nasa's Goddard Institute for Space Studies in New York is clearly a doting grandfather as well as an internationally revered climate scientist. Yet his pictures are more than mere expressions of familial love.

They are reminders to the 67-year-old scientist of his duty to future generations, children whom he now believes are threatened by a global greenhouse catastrophe that is spiralling out of control because of soaring carbon dioxide emissions from industry and transport.

"I have been described as the grandfather of climate change. In fact, I am just a grandfather and I do not want my grandchildren to say that grandpa understood what was happening but didn't make it clear," Hansen said.

Hence his warning to Barack Obama, who today becomes the 44th US President. His four-year administration offers the world a last chance to get things right, Hansen said. If it fails, global disaster - melted sea caps, flooded cities, species extinctions and spreading deserts - awaits mankind.

"We cannot now afford to put off change any longer. We have to get on a new path within this new administration. We have only four years left for Obama to set an example to the rest of the world. America must take the lead."

After eight years of opposing moves to combat climate change, thanks to the policies of President George W. Bush, the US had given itself no time for manoeuvre, he said. Only drastic, immediate change can save the day and those changes proposed by Hansen - who appeared in Al Gore's An Inconvenient Truth and is a winner of the World Wildlife Fund's top conservation award - are certainly far-reaching.

In particular, the idea of continuing with "cap-and-trade" schemes, which allow countries to trade allowances and permits for emitting carbon dioxide, must now be scrapped, he insisted. Such schemes, encouraged by the Kyoto climate treaty, were simply "weak tea" and did not work.

"The United States did not sign Kyoto, yet its emissions are not that different from the countries that did sign it."

Thus, plans to include carbon trading schemes in talks about future climate agreements were a desperate error, he said.

"It's just greenwash. I would rather the forthcoming Copenhagen climate talks fail than we agree to a bad deal," Hansen said.

Only a carbon tax, agreed by the West and then imposed on the rest of the world through political pressure and trade tariffs, would succeed in the now-desperate task of stopping the rise of emissions, he argued.

This tax would be imposed on oil corporations and gas companies and would specifically raise the prices of fuels across the globe, making their use less attractive.

In addition, the mining of coal - by far the worst emitter of carbon dioxide - would be phased out entirely along with coal-burning power plants.

"Coal is responsible for as much atmospheric carbon dioxide as other fossil fuels combined and it still has far greater reserves. We must stop using it."

Programmes for building wind, solar and other renewable energy plants should be given boosts, with research programmes for new generations of nuclear reactors.

Hansen's calls for action stem from his special view of our changing world. He and his staff monitor temperatures relayed to the institute - an anonymous brownstone near Columbia University - from thousands of sites around the world, including satellites and bases in Antarctica.

These have revealed that our planet has gone through a 0.6C rise in temperature since 1970, with the 10 hottest years having occurred between 1997 and 2008: unambiguous evidence, he believes, that Earth is beginning to overheat dangerously.

Last week Hansen revealed his findings for last year which show that it was the coolest this century, although hot by standards of the 20th century. The finding will doubtless be seized on by climate change deniers and used as "evidence" that global warming is a hoax.

However, deniers should show caution, Hansen insisted: most of the planet was exceptionally warm last year. Only a strong La Nina - a vast cooling of the Pacific that occurs every few years - brought down the average temperature. La Nina would not persist, he said.

"Before the end of Obama's first term, we will be seeing new record temperatures. I can promise the President that."

Hansen's uncompromising views are, in some ways, unusual. Apart from his senior Nasa post, he holds a professorship in environmental sciences at Columbia and dresses like a tweedy academic.

Yet behind his unassuming, self-effacing manner, the former planetary scientist has shown surprising steel throughout his career.

In 1988, he electrified a congressional hearing when he announced he was "99 per cent certain" that global warming was to blame for the weather and that the planet was now in peril from rising carbon dioxide emissions. His remarks, which made headlines across the US, pushed global warming on to news agendas for the first time.

Hansen has continued his mission "to make clear" the dangers of climate change, sending a letter in December from himself

and his wife Anniek about the urgency of the planet's climatic peril to Barack and Michelle Obama .

"We decided to send it to both of them because we thought there may be a better chance she will think about this or have time for it. The difficulty of this problem [of global warming] is that its main impacts will be felt by our children and by our grandchildren.

"A mother tends to be concerned about such things."

As to the specific warnings he makes about climate change, these concentrate heavily on global warming's impact on the ice caps in Greenland and Antarctica. These are now melting at an alarming rate and threaten to increase sea levels by 1m or 2m over the century, enough to inundate cities and fertile land around the globe.

The issue was simple, said Hansen: would each annual increase of carbon dioxide to the atmosphere produce a simple proportional increase in temperature or would its heating start to accelerate? He believes the latter.

"I recently lunched with Martin Rees, president of the Royal Society, and proposed a joint programme to investigate this issue as a matter of urgency, in partnership with the US National Academy of Sciences, but nothing has come of the idea, it would seem," he said.

Hansen will continue to press his cause: a coal-power moratorium and an investigation of ice-cap melting. The world was now in "imminent peril", he insisted, and nothing would quench his resolve in spreading the message. It is the debt he owes his grandchildren, after all.

* Robin McKie is the Observer's science editor

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