Nine of 11 experts, who were among authors of the final summary by the UN's Intergovernmental Panel on Climate Change in 2007 (IPCC), also said the evidence that mankind was to blame for climate change had grown stronger in the past two years.

Giving personal views of recent research, most projected on average a faster melt of summer ice in the Arctic and a quicker rise in sea levels than estimated in the 2007 report, the most authoritative overview to date drawing on work by 2,500 experts.

"A lot of the impacts we're seeing are running ahead of our expectations," said William Hare of the Potsdam Institute for Climate Impact Research.

Ten of 11 experts said it was at best "unlikely" -- or less than a one-third chance -- that the world would manage to limit warming to a 2 degrees Celsius (3.6 Fahrenheit) rise above pre-industrial levels.

"Scientifically it can be done. But it's unlikely given the level of political will," said Salemeel Huq at the International Institute for Environment and Development in London.

And David Karoly, of the University of Melbourne, said the world was "very unlikely" to reach the goal.

"The concentration of long-lived greenhouse gases in the atmosphere is already enough to cause warming of more than 2C above pre-industrial levels, and we are continuing to emit more and more greenhouse gases into the atmosphere," he said.

Officials from 175 nations are meeting in Bonn, Germany, for 11 days of negotiations lasting until April 8 on a new UN climate treaty due to be agreed in December. Reuters got 11 replies to five questions, sent to 35 IPCC authors.

The European Union, many developing nations and environmental groups say 2 Celsius above pre-industrial levels is the maximum to avoid the worst of rising sea levels, floods, droughts or heatwaves. Temperatures are already up 0.7 Celsius.

An alliance of 43 small island developing states, who fear being swamped, want temperatures limited to an even tougher goal of below 1.5 Celsius. They say rich nations should sharply cut greenhouse gas emissions, mainly from burning fossil fuels.

Removal of manmade sun-blocking smoke under clean air laws may add a 1 Celsius rise while oceans will warm further under a lag effect, underscoring how near the 2 degrees limit is already.

The IPCC said in 2007 that it was at least 90 percent certain that human activities, led by burning fossil fuels, were the main cause of warming in the past 50 years. Nine reckoned that evidence was stronger, two said it was unchanged.

Six of the scientists said world average annual temperatures would set a new record by 2015 -- and another four projected that it would happen by 2020 -- dismissing views from skeptics that global warming has stopped.

The hottest year since records began in the 19th century was 1998, according to the World Meteorological Organization.

And the scientists generally said that sea levels would rise faster than projected in the IPCC report, in a threat to many cities, islands and coasts from Bangladesh to Florida.

The IPCC said seas would rise by between 18 and 59 cms (7-24 inches) this century. But it pointed to big uncertainties about ice sheets in Greenland or Antarctica -- one IPCC estimate was that this ice could add up to 20 cms to sea level rise.

In the poll, the lowest projection for sea level rise by 2100 was 30-40 cms, the highest up to 140 cms.