On Monday, Rajendra Pachauri, chairman of the Inter-governmental Panel on Climate Change (IPCC), said there was a link between human emissions of greenhouse gases and rising temperatures.
He said: "If one looks at just the scientific evidence that's been collected, it's certainly becoming far more compelling. There is no question about it."
Pachauri was more forthright than at the last UN climate meeting in Montreal, Canada, in December, when he declined to say whether there was clearer scientific evidence that human activities were to blame.
The last IPCC report in 2001 said there was "new and stronger evidence" that gases released by burning fossil fuels in power plants, factories and cars were warming the planet.
Warming may herald catastrophic climate changes such as more heatwaves, droughts, floods and rising sea levels.
The IPCC, grouping research by about 2000 scientists, will present its next report to the UN in 2007.
"Given the gravity of the situation and the importance of taking action I hope that the global community will move a little more rapidly with some future agreements"
chairman of the IPCC
The report is the mainstay for environmental policy-making.
Still, Pachauri said it was too early to draw conclusions.
A BBC report last week said the IPCC would say in 2007 that only greenhouse gas emissions can explain freak weather patterns.
He said: "That's premature because the report is still nowhere near completion."
Action called for
Pachauri said the world needed to do more.
He said: "Given the gravity of the situation and the importance of taking action I hope that the global community will move a little more rapidly with some future agreements."
The UN's Kyoto Protocol, which obliges industrial nations to cut greenhouse gas emissions, entered into force last year after years of wrangling and weakened by a US pullout.
Pachauri said people living in island states, such as the Maldives in the Indian Ocean, Tuvalu in the Pacific or low-lying countries such as Bangladesh, were among those most at risk.
He said: "They are living in a state of fear. We must understand the reasons behind their fears. We're really talking about their very existence, the complete devastation of the land on which they're living."
New York, Shanghai, Buenos Aires and London could be swamped by rising seas, he said.
The IPCC report says that the cost of curbing greenhouse gases in the toughest case could delay world growth from reaching projected 2050 levels until 2051 or 2052.
In a speech at Oslo University, Pachauri said: "That's not a heavy price to pay. These (IPCC) projections are pessimistic."
"These (IPCC) projections are pessimistic"
chairman of the IPCC
He said more US companies, cities and states were acting to cap greenhouse gas emissions even though George Bush, the American president, pulled the US out of Kyoto in 2001, saying it was too costly and wrongly excluded developing nations.