Dr. Rajendra Pachauri, head of the Nobel Prize-winning UN Intergovernmental Panel on Climate Change, addressed the gathering, which began on Monday, underscoring the urgency to act on greenhouse gas emissions.
He said "The costs of responding to climate change will become progressively higher as time goes on, therefore we must take action now."
More than 15,000 participants, including 110 heads of state are expected to attend the event, making it the largest gathering ever on climate change.
The Copenhagen conference runs through December 18.
Dianne Penn, UN Radio.
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