"Climate change is the most serious sustainable environment challenge of our time. We must act now," South African International Relations minister Maite Nkoana-Mashabane said in opening the two-week meeting.
Topping the agenda in Doha is the launch of a followup commitment period for the Kyoto Protocol, the world's only binding pact for curbing greenhouse gas emissions.
Delegates must also set out a work plan for arriving in the next 36 months at a new, global climate deal that must enter into force by 2020.
Negotiators, to be joined in the last four days by cabinet ministers from over 100 nations, will be under pressure to raise pre-2020 emission reduction targets, and rich nations to come up with funding for the developing world's mitigation actions.