The Prime Minister described the climate change challenge today as "one of the great human endeavours of our time".
Gordon Brown joined Climate Change Secretary Ed Miliband and UN Minister Baroness Kinnock to answer questions from young people just three days before crunch UN climate talks kick off in Copenhagen.
Mr Brown warned the panel of 16 to 25 year-olds that the world was "only halfway there" in getting agreement on both intermediate targets and finance for the poorest countries.
He said: "This is perhaps the greatest challenge that we face as a world.
"And this is the turning point that can either work for us or it can fail.
"This is one of the great human endeavours of our time - to bring the world together to deal with the problem that has been caused essentially by the richest countries but is now affecting some of the poorest countries in the world.
"If we do not act, all of us are going to be worse off as a result."
The event at the Natural History Museum in London involved around 50 young people from organisations including ActionAid, the National Union of Students and the UK Youth Climate Coalition.
Action taken unilaterally around the globe is already projected to take five billion tonnes of greenhouse gasses out of the atmosphere by 2020, he said.
"At the moment all the offers that we've got take us down from the projected 54 billion tonnes in 2020 to 49 billion.
"So we have got to get down from 49 billion to 44 billion.
"So we are looking for countries in the next few days to make offers that will get us this extra five billion or five giga-tonnes of greenhouse gas emissions - and that's a real challenge."
Mr Brown also told the young people that targets should not be "imposed" on developing countries by richer ones.
"If you reach a decision at Copenhagen, all the countries of the world have got to come together," he said.
"And it cannot be imposed by one group of countries on another - these days are over."
The Prime Minister sympathised with a member of the audience who said climate change and environmental sustainability should be a bigger part of the school curriculum.
He told her: "I think most people here will probably feel that they didn't get enough information or education about the environment when they were at school. I think it's changing a little now."
Mr Brown said his two sons were learning more about the environment and climate change than he ever did at the same age.
He also said that most of the correspondence he received from the public was on the issue of climate change.
"Therefore I feel that there is pressure on me from you and from others to do something about it," he said.
Baroness Kinnock said climate change should be taught at school as part of many subjects including geography and history.
On renewable energy, Mr Brown said the UK was now the world's "number one" power for offshore winds.
And he said the Government would be making an announcement in the next few days on a "big expansion" in the number of offshore wind installations.
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