Leaders of 80 countries are expected to attend a special UN meeting in New York to discuss the effects of global warming and the ways to combat it.
The meeting comes just days after scientists reported that more Arctic ice melted this year than ever before.
The UN climate chief says a breakthrough is essential.
US President George W Bush will not be present, hosting instead a meeting of 16 “major emitter” countries in Washington on Thursday and Friday.
However, the attendance of 150 countries and 80 heads of state makes Monday’s meeting the most high-level UN gathering on climate change.
UN climate chief, Yvo de Boer, said: “I expect the meeting on Monday to express a sense of urgency in terms of negotiating progress that needs to be made.”
The BBC’s environment reporter, Matt McGrath, says this meeting will not solve the problems of climate change but UN Secretary General Ban Ki-moon hopes it will give impetus to negotiations on global warming that are due to take place in Bali, Indonesia, in December.
Mr Ban said: “Bali must advance a negotiating agenda to combat climate change on all fronts, including adaptation, mitigation, clean technologies, deforestation and resource mobilisation.”
The secretary general will deliver the keynote address at Monday’s summit, entitled “The Future in Our Hands: Addressing the Leadership Challenge of Climate Change.”
Our correspondent says the large turnout of heads of government, plus California Governor Arnold Schwarzenegger and former US Vice President Al Gore, is likely to encourage the UN that politicians are ready to commit to long-term, legally binding reductions on emissions of greenhouse gases.
Governor Schwarzenegger said ahead of the meeting: “California is moving the United States beyond debate and doubt to action.”
Mr Bush, who does not support binding emission targets, will not take part in the formal discussions but will be joining Mr Ban and other key leaders for dinner.