United Nations Secretary General Ban Ki-moon has opened high-level talks at the climate change conference in Bali with a call to action.
He said that if no action were taken, the world would face impacts such as drought, famine and rising sea levels.
Delegates are hoping to agree a "Bali roadmap" leading to further cuts in greenhouse gas emissions when the Kyoto Protocol targets expire in 2012.
The US and Canada are among countries opposed to further binding targets.
The UN itself wants developed countries to commit to cuts of 25-40% from 1990 levels by 2020.
'No plan B'
"We gather because the time for equivocation is over," said Mr Ban.
"Climate change is the defining challenge of our age. The science is clear; climate change is happening, the impact is real. The time to act is now."
The newly-elected Australian Prime Minister Kevin Rudd handed documents to Mr Ban confirming his government's ratification of the Kyoto Protocol.
"The community of nations must reach agreement," he told delegates.
"There is no plan B; there is no other planet any of us can escape to."
The Australian decision leaves the US as the only industrialised nation outside the Kyoto process.
Security at the summit was enhanced because of the car bomb attack on UN premises in Algeria, which left at least 26 people dead.
Negotiators have been trying to map out a two-year process that would result in a further set of emissions cuts to replace the current Kyoto Protocol targets.
Broad building-blocks have already been agreed, but much of the detail remains contentious, in particular how much weight to give to the heavy emissions cuts recommended by the UN's panel of scientists.
In a major assessment this year, the Intergovernmental Panel on Climate Change (IPCC) has concluded that emissions should peak and begin to fall within 10-15 years in order to avoid damaging impacts.
While acknowledging the science, the US argues for voluntary agreements rather than a global system of binding cuts, and argues that the text coming from the Bali conference should not contain numerical targets.
"We want to launch a process that is open and does not predetermine or preclude options," Under Secretary of State Paula Dobriansky told reporters.
Germany's Environment Minister Sigmar Gabriel issued a sharp rebuke to the US position.
"I do not need a paper from Bali in which we only say 'OK, we'll meet again next year again', he said.
"How can we find a roadmap without having a target, without having a goal?"
There has also been debate about adaptation - how to help developing countries protect their societies and economies against the worst impacts of climate change.
Studies indicate that the sums needed run into tens of billions of dollars per year, but funds committed so far amount to tens of millions of dollars.
"The main issue we've been trying to get across is that climate change is already hitting the poorest - it's not something for the future, it's something that's happening now," said Andy Atkins from the development charity Tearfund.
Mr Atkins said that in Niger, farmers have seen a rainy season shrink from three months each year to just six weeks.
"People in Bali are accepting adaptation will have to be part of a deal," he said. "But it's no good accepting that something is a big problem and then failing to do anything about it."