Delegates from more than 190 countries are gathered in Durban, South Africa, to debate the future of the greenhouse-gas limiting Kyoto Protocol, parts of which expire next year.
The United States in 2001 abandoned Kyoto and major economies such as Japan, Russia and Canada said they might not accept new mandated emission targets.
U.N. Secretary-General Ban Ki-moon said time is running out for countries to transform their economic from one based on fossil fuels to low-carbon alternatives.
"The world and its people cannot accept 'no' for an answer in Durban," he said in a statement.
A recent report from the World Meteorological Organization concludes that greenhouse gas emissions are at the highest level ever recorded. Critics, however, say alternative and renewable energy is more costly than conventional forms.
Ban said that with major food, energy and water crises looming, a sea change is needed for the overall economic sustainability and environmental health of the global community.
"We know the reasons: grave economic troubles in many countries, abiding political differences, conflicting priorities and strategies for responding to climate change," he said. "Yet let me emphasize: none of these uncertainties should prevent us from making real progress here in Durban."