The leaked assessment by the group of international experts says there is now overwhelming evidence to show that the Earth's climate is undergoing dramatic transformation because of human activity.
A draft copy of the report by a working group of the Intergovernmental Panel on Climate Change (IPCC) states that concentrations of carbon dioxide, methane and other greenhouse gases are at the highest for at least 650,000 years.
It predicts that global average temperatures this century will rise by between 2C and 4.5C as a result of the doubling of carbon dioxide levels caused by man-made emissions.
These temperatures could increase by a further 1.5C as a result of "positive feedbacks" in the climate resulting from the melting of sea ice, thawing permafrost and the acidification of the oceans.
The draft report will become the fourth assessment by the IPCC since it was established in 1988 and was meant to be confidential until the final version is ready for publication next year.
However, a copy of the report has been made available by a US government committee and can be found on the internet by anyone who makes an e-mail request for a password to access the area on its website.
The US Climate Change Science Programme, which yesterday released its own report saying climate change was being affected by man-made pollution, said it wanted as many experts and stakeholders as possible to comment on the draft IPCC report.
The IPCC's chairman, Rajendra Pachauri, however, did not learn of the decision to, in effect, publish the report until it was posted online, according to the journal Nature. The IPCC assessment is written by scores of scientists - who can draw on the expertise of hundreds more researchers - to produce the most definitive and authoritative assessment of climate change and its impacts.
Global warming sceptics will get little comfort from the confident language in the draft report, which dismisses suggestions that climate change is an entirely natural rather than man-made phenomenon.
"There is widespread evidence of anthropogenic warming of the climate system in temperature observations taken at the surface, in the free atmosphere and in the oceans," it says.
"It is very likely that greenhouse gas forcing has been the dominant cause of the observed global warming over the past 50 years.
"And it is likely that greenhouse gases alone would have caused more warming than has been observed during this period, with some warming offset by cooling from natural and other anthropogenic factors." Since its last report in 2001, the IPCC's working group says it has amassed convincing evidence showing that climate change is already happening.
It also finds that climate change is set to continue for decades and perhaps centuries to come even if man-made emissions can be curbed.
"2005 and 1998 were the warmest two years on record. Five of the six warmest years have occurred in the past five years (2001-2005)," the report says.
Satellite data since 1978 shows that the Arctic sea ice has shrunk by about 2.7 per cent each decade, with even larger losses of about 7.4 per cent during the warmer summer months.
"The smallest extent of summer sea ice was observed in 2005. Average Arctic temperatures have been rising since the 1960s and 2005 was the warmest Arctic year," the draft IPCC report says.
"An increasing body of evidence suggests a discernible human influence on other aspects of climate, including sea ice, heat waves and other extremes, circulation, storm tracks and precipitation," it says.
Melting glaciers and polar ice sheets could cause sea levels to rise by up to 43cm by 2100, and the rise for the next two centuries is predicted to be nearly double that figure.
Man-made emissions of greenhouse gases have probably already caused the increase in sea levels observed over the past century, says the report.
"Anthropogenic forcing, resulting from thermal expansion from ocean warming and glacier and ice sheet melt, is likely the largest contributor to sea level rise during the latter half of the 20th century," the report says.
"Anthropogenic forcing has likely contributed to recent decreases in Arctic sea ice extent. There is evidence of a decreasing trend in global snow cover and widespread retreat of glaciers consistent with warming and evidence that this melting has also contributed to sea-level rise," it adds.
Evidence of climate change
* Arctic sea ice has shrunk by 2.7 per cent per decade since 1978 and by 7.4 per cent each decade during the summer months.
* Five of the six warmest years have occurred in the past five years, with 2005 and 1998 being the two warmest years on record.
* Global average sea levels rose at a rate of about 2mm a year between 1961-2003, and by an average of more than 3mm a year between 1993-2003.
* Mountain glaciers and polar land ice have in general melted faster than they have formed over the past 40 years.
* Permafrost temperatures have increased on average and the area covered by seasonally frozen ground has decreased by about 7 per cent over the past 50 years.
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