The head of the UN's climate science panel urged national policymakers Monday not to lose heart in the face of a mighty challenge to tackle global warming.

"It is not hopeless," Rajendra Pachauri, chairman of the Nobel-winning Intergovernmental Panel on Climate Change, said on opening a five-day IPCC meeting in Copenhagen to complete a landmark report.

Policymakers should "avoid being overcome by the seeming hopelessness of addressing climate change," he said.

The meeting must approve a synthesis report distilling three massive volumes released over the past 13 months of the IPCC's Fifth Assessment Report on the available climate science.

Meeting behind closed doors, scientists and government representatives are to hammer out a "summary for policymakers" and approve a main document on Friday. These will be unveiled on Sunday.

Governments should make decisions "informed by the science", Pachauri said in a speech relayed on the IPCC website.

"I do not envy them. Their task is formidable," he added, and pointed to the "growing peril" of delaying curbs on greenhouse-gas emissions.

"I do not discount those challenges. But... solutions are at hand," he said.

"Tremendous strides are being made in alternative sources of clean energy. There is much we can do to use energy more efficiently. Reducing and ultimately eliminating deforestation provides additional avenues for action."

The IPCC's latest assessment report says evidence of man-made global warming is overwhelming, and that there are signs of climate change already on the march.

UN members have vowed to limit warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.

They eye a post-2020 pact, to be sealed in Paris in December 2015, as the mechanism for achieving it.

But heat-trapping carbon emissions are rising so fast that in the worst-case scenario sketched by the IPCC, the planet could be up to 4.8 C warmer by 2100 and sea levels up to 82 centimetres (32 inches) higher.

Worse floods, droughts, hunger, homelessness and conflict over resources will be the likely outcome.

- Risks and options -

Delay beyond 2030 will reduce the chances of reaching the 2 C goal without harsher and probably more expensive mitigation measures in the latter half of the century, says the Fifth Assessment Report.

But it also sets out policy options for reducing emissions, including a drive on energy efficiency, and a switch to cleaner or non-fossil sources.

This could be achieved at the cost of a tiny reduction in the expected growth of economic consumption -- a brake of 0.06 percentage points against a projected annual increase of 1.6-3.0 percent over the century.

Greenpeace campaigner Kaisa Kosonen said the three volumes staked out some major gains in knowledge compared with the Fourth Assessment Report, which earned the IPCC a co-share in the Nobel Peace Prize and catalysed interest in climate change.

Advances include scientific knowledge about worsening polar ice loss and ocean acidification and the falling cost of wind and solar power, she said.

New estimates also show that, to keep under 2 C warming, humans cannot emit more than another 1,000 billion tonnes of carbon -- a "budget" that is a vital tool for policymaking, she said.

"These are the new messages that must be included (in the summary). They are highly politically relevant," said Kosonen.