Rajendra Pachauri, chairman of the United Nation's Intergovernmental Panel on Climate Change (IPCC), said at the opening session of the Copenhagen conference that policymakers should "avoid being overcome by the seeming hopelessness of addressing climate change."

"It is not hopeless," Pachauri said in a speech relayed on the IPCC website.

The IPCC meeting is seeking to adopt a concise report - encapsulating the three documents released over the past 13 months - on how to tackle and mitigate climate change.

The 100-page document "will provide the road map by which policymakers will hopefully find their way to a global agreement to finally reverse course on climate change," said Pachauri.

According to the Associated Press news agency, a leaked draft report presents a grimmer picture of the global warming situation than the IPCC's previous three reports, warning of "severe, pervasive and irreversible" climate impacts if world leaders fail to bring down the levels of the greenhouse emissions.

According to the draft report, the impacts of climate change are already visibly dangerous with rising sea levels, extreme weather, flooding and droughts in many parts of the world.

More than 800 scientists from around 80 countries participated in writing the report, making use of the work of more than 1,000 researchers and about 2,000 expert reviewers, the IPCC said.

"I do not discount those challenges," said Pachauri. "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," stressed the IPCC head.

UN members have pledged to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) and are looking to finalize a post-2020 deal in a Paris meeting in December 2015.

shs/glb (AP, AFP, dpa)