Scientists and governments pored over the summary Thursday of an eagerly awaited UN report expected to emphasise the escalating threat from climate change.

To be released in Stockholm on Friday, it will be the Nobel-winning panel's first overview since 2007 of the scientific evidence for climate change.

A draft of the summary seen by AFP declares with the greatest emphasis to date that climate change is on the march and humans are responsible for it.

The report "will fire the starting gun" for negotiations on reaching a new global pact by the end of 2015 on curbing greenhouse gases, said Tim Gore of Oxfam International.

Those talks are supposed to enact a UN goal of limiting global warming to two degrees Celsius (3.6 degrees Fahrenheit) from pre-Industrial Revolution levels.

But the Intergovernmental Panel on Climate Change (IPCC) report will indicate that this goal can only be secured though a herculean effort to save energy or switch to cleaner sources.

Only one of the scenarios postulated in the report sees any possibility of safely anchoring the temperature rise to within 2 C by 2100.

It would require fossil-fuel emissions -- scaling new peaks almost every year -- to top out in 2020, then drastically decline over the next half-century.

In the worst projection, however, warming will be about 5.6 C (10.1 F) compared to the pre-industrial yardstick.

The report will predict sea levels to rise by between 26 and 81 centimetres (10.4 and 32.4 inches) by 2100, according to the draft.

In its last big review, published six years ago, the Nobel-winning group projected an 18-59 cm (7.2-23.6 inch) rise by 2100.

The big change comes from new evidence of melting from parts of the giant icesheets that smother Greenland and Antarctica.

The key document being unveiled on Friday is a 31-page summary of a massive technical text, the first volume of the IPCC's Fifth Assessment Report.

Governments around the world have a seat on the IPCC, and can vet the summary, given its implications for how states will address climate change. They cannot amend the main text, though.

Nations have called on scientists to tidy up a key section of the report to clarify why, over the past 15 years, temperatures have risen far slower than some computer models have suggested.

That phenomenon has been seized upon by sceptics to say that climate science is flawed or that global warming is a fraud by the green lobby.

Scientists attribute the "pause" to several factors, including variations in Earth's own climate system that are temporary but complex and still poorly understood.

'No sticking points'

The textual issue over this has been settled to the satisfaction of all, but delegates still have to pore over many other proposed amendments before the summary can be approved, said a source.

The challenge is "the volume of work, especially of the desire of some countries to get clarity," the delegate said. "But currently there are no sticking points."

As a political issue, climate change is in the doldrums.

A first attempt was made at a summit in Copenhagen in 2009 for a deal to tame carbon emissions and help poor countries exposed to worsening droughts, storms and floods.

That event was a near-fiasco and led to the goal being postponed to 2015. Today, willingness for concessions is low, especially in countries still struggling after the 2008 financial crisis.

Some experts say the IPCC report will be too conservative. "They (in the IPCC) are so nervous now," said one, referring to damage done to the panel's reputation when several errors were found in its landmark 2007 overview.

But the fact that the summary is explicitly approved by governments gives the report special weight, say others.

"These negotiations can be seen as the place where communications, science and politics meet," said Vanessa Bulkacz of Climate Action Network Europe, an alliance of green groups.

"After that, it's up to governments to use these persistent scientific facts as a springboard for real climate action."