LONDON — Global temperatures could rise more than currently predicted by the end of this century, according to the most authoritative study of climate change so far.

Thousands of scientists involved with the Intergovernmental Panel on Climate Change are expected to conclude in a report to be published this week that global average temperatures could rise by between 1 and 6.3 degrees Celsius by 2100.

The upper end of the predicted range is half a degree higher than previously assumed by the IPCC, set up in 1988 by the United Nations to examine evidence of manmade global warming, and is likely to mean a far greater temperature rise at the poles.

A global average temperature rise of more than two degrees Celsius is already regarded by many scientists as dangerous to human society.

A rise of more than five degrees would be regarded as catastrophic to low-lying cities, which include New York and London.

The main message to the world’s politicians from the thousands of scientists involved in the IPCC’s Fourth Assessment Report, to be published in Paris on Friday, will be the increased confidence attached to the statement that the warmer temperatures of the past 50 years are mainly attributable to man’s activities.

The report comes after Prime Minister Blair held out hopes of a major breakthrough in a post-Kyoto climate accord in an address to the World Economic Forum in Davos.

He said that after talks with President Bush, President da Silva of Brazil, and Germany’s Chancellor Merkel, he was confident that progress could be made at the planned G8 summit in the German Baltic town of Heiligendamm in June.

Mr. Blair said the key breakthrough was to get the developing economies such as India, China, and Brazil — which are members of the Kyoto treaty but have no emissions reductions targets — to agree to binding commitments for a new treaty starting in 2012.

Observers pointed out this was highly unlikely without binding commitments from America, where Mr. Bush opposes “cap and trade” measures despite growing backing for them in the Democrat-dominated Congress.

The IPCC report is expected to predict that snow will disappear from all but the highest mountains, glaciers will recede and oceans will become more acidic, eventually leading to the destruction of coral reefs.

It will say that ocean temperatures have already risen 1.8 miles below the surface.

Scientists from the IPCC are expected to conclude unexpectedly that sea level rise may be lower than predicted — 23.6 inches by the end of this century compared to an upper prediction of 35.4 in previous assessments.

There was some controversy that the sea level predictions were not higher, but insiders point out that the cut-off point for the inclusion of new papers was the summer and that some of the latest science about the melting of land-based ice in Antarctica was only published in October.

Even though the report itself has been finalized, the language used in the policy makers’ summary — which is to be decided line by line in Paris this week — remains unfixed and is likely to be fought over by scientists holding different views.

Meanwhile, there is evidence of disagreement between Europe and America over the wording of another IPCC report, this time on mitigating climate change, expected on May 4.

The American government has suggested that “modifying solar radiance” by the use of giant mirrors to reflect the sun’s rays could help to reduce warming.