Professor James Hansen, often described as the world’s most prominent climate scientist, has also chosen to be one of its most controversial.

His view that the world has already passed a dangerous threshold for a safe future, because there is too much carbon dioxide in the atmosphere, has led him to step up attacks on politicians and the oil business.

He has been well known internationally since 1988, when he caused a sensation by being the first to warn the US Congress that climate change was a threat to his country and the planet.

Unlike many scientists who produce their results and then remain silent, Professor Hansen has always demanded action from successive US administrations. But his new calculations about the danger the planet faces have led him to more strident warnings.

Politicians who have been stung by his criticism have been attacking back. For example Joe Oliver, the Canadian natural resources minister, who is keen to export oil from Albertan tar sands via a new pipeline to the US, said Hansen was irresponsible to claim the project would mean it was “game over for the planet”.

Oliver said: “This is exaggerated rhetoric. It’s frankly nonsense. I don’t know why he said it, but he should be ashamed of having said it.”

But Hansen is a hard man to criticize, because of the quality of his science and his broad knowledge of climate change. His status comes from his 32-year tenure as the head of the NASA Goddard Institute for Space Studies in New York City. He retired earlier this year.

He is currently adjunct professor at the Department of Earth and Environmental Sciences at Columbia University and even at 72 is active in writing, speaking and campaigning about climate change.

The reason is his belief that the maximum safe level of atmospheric CO2 for the planet is 350 parts per million (ppm). This figure is much lower than many other scientists believe the atmosphere can stand before climate change gets out of control.

Some favour 400 ppm as the danger level, and others have settled for 450 ppm, a figure most politicians are happy with because it delays the need for action by them for a long time.

Professor Hansen’s anxiety is that we have already passed the 400 ppm figure and there is no sign of government action to do anything to curb a continuous rise to 450 ppm and beyond.

He says he once accepted that 450 ppm might be tenable, but that recent science has changed that. Speaking on YouTube to promote his book Storms of my Grandchildren, he said: “Now we look at the Earth’s history more carefully and have better data on how the Earth responded to changes in global temperature and changes in the atmospheric composition in the past … we see we have already passed into the dangerous range.”

If we continue with business as usual, Professor Hansen says, Arctic ice will be gone in summertime, disappearing mountain glaciers will mean the summer water supply to hundreds of millions of people will be lost within 50 years, and the sub-tropical regions will expand, making places like northern Australia and the southern and south-west US hard to live in.. – Climate News Network