Barely 100 days before the world hopes to seal a global climate treaty, the UN's top climate scientist has given his personal endorsement to hugely ambitious goals for slashing emissions.

"As chairman of the Intergovernmental Panel on Climate Change (IPCC) I cannot take a position because we do not make recommendations," said Rajendra Pachauri when asked if he supported calls to keep atmospheric carbon dioxide concentrations below 350 parts per million.

"But as a human being I am fully supportive of that goal. What is happening, and what is likely to happen, convinces me that the world must be really ambitious and very determined at moving toward a 350 target," he said.

In its benchmark 2007 report, the IPCC said that the key for preventing dangerous global warming was to keep CO2 concentrations below 450 ppm.

Above that level, average global temperatures are likely to increase by more than 2C by the end of the century, a threshold that leaders from the G8 leading industrialised nations agreed last month must not be crossed.

But translating this global target into concrete national commitments at UN climate talks ahead of a landmark conference in December in Denmark has proven extremely difficult.

"How that 2C limit is going to be obtained is something the G8 nations have not addressed so far," Mr Pachauri said. According to the IPCC, whose report serves as a guidepost for the UN talks, this can only happen if global greenhouse gas emissions peak by 2015 and rich nations slash their carbon pollution by 25 to 40 per cent, compared to 1990 levels, by 2020. So far only the EU has come even close to meeting these goals, with the US seeking to shift the focus to its more long-term commitment of cut CO2 output by 80 per cent before 2050.

The IPCC report says that major emerging economies such as China and India would have to green their economies too, though at a slower pace. But a growing body of scientific evidence suggests that even these hard-to-reach goals may not be ambitious enough, prompting many of the nations most threatened by global warming to set the bar even higher.

More than 80 of the world's poorest and most climate-vulnerable nations have now declared that CO2 concentrations must be scaled back to below 350 ppm, and that temperatures cannot rise more than 1.5C by century's end.

The Association of Small Island States staked out this position last December, and was joined earlier this month by the 49-strong bloc of Least Developed Countries, mostly in Africa, along with south and southeast Asia.

"I think this is a good development," said Pachauri. "Now people - including some scientists - see the seriousness of the impacts of climate change, and the fact that things are going to get substantially worse than what we had anticipated."