"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 encapsulating the three massive volumes, released over the past 13 months, of the IPCC's Fifth Assessment Report on the available climate science.
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."
UN members have vowed to limit warming to two degrees Celsius over pre-industrial levels.
