The biggest climate meeting in history, with 15,000 participants from 192 nations, opened in Copenhagen on Monday with hosts Denmark saying an unmissable opportunity to protect the planet was "within reach."
"The world is depositing hope with you for a short while in the history of mankind," Danish Prime Minister Lars Lokke Rasmussen told delegates at the opening ceremony of the talks, seeking to agree the first U.N. climate pact in 12 years.
He said that 110 world leaders, including U.S. President Barack Obama, would attend a summit at the end of the December 7-18 meeting to agree deep cuts in greenhouse gas emissions for the rich by 2020 and raise billions of dollars for the poor in aid.
"A deal is within our reach," Rasmussen said.
The presence of so many world leaders "reflects an unprecedented mobilization of political determination to combat climate change. It represents a huge opportunity. An opportunity the world cannot afford to miss," Rasmussen said.
"The ultimate responsibility rests with the citizens of the world, who will ultimately bear the fatal consequences, if we fail to act," he said.
But the summit will have to overcome deep distrust between rich and poor nations about sharing the cost of emissions cuts.
Activists asked delegates arriving at the conference centre, with a large wind turbine nearby, to go through a green gateway marked "Vote Earth" or a red one marked "Global Warming." They told off anyone choosing red.
Others handed out free coffee to delegates, pamphlets about global warming and buttons urging wider use of public transport.
The attendance of the leaders and pledges to curb emissions by all the top emitters -- led by China, the United States, Russia and India -- have raised hopes for an accord after sluggish negotiations in the past two years.
South Africa added new impetus, saying on Sunday it would cut its carbon emissions to 34 percent below expected levels by 2020, if rich countries furnished financial and technological help.
World leaders did not attend when environment ministers agreed the existing U.N. climate pact, the Kyoto Protocol, in 1997.
Plans by world leaders to attend have brightened hopes since Rasmussen said last month that time had run out to agree a full legal treaty in 2009. The aim for Copenhagen is a politically binding deal and a new deadline in 2010 for legal details.
British Prime Minister Gordon Brown, writing in the Guardian newspaper on Monday, said: "The British government is absolutely clear about what we must achieve. Our aim is a comprehensive and global agreement that is then converted to an internationally legally binding treaty in no more than six months."
He added: "If by the end of next week we have not got an ambitious agreement, it will be an indictment of our generation that our children will not forgive."
Some 56 newspapers from 45 countries including The Guardian, Le Monde, El Pais and Toronto Star on Monday published a joint editorial urging world leaders to take decisive action.
"Humanity faces a profound emergency. Unless we combine to take decisive action, climate change will ravage our planet," it said.
"The politicians in Copenhagen have the power to shape history's judgement on this generation: one that saw a challenge and rose to it, or one so stupid that we saw a calamity coming but did not avert it."
A Pinprick In Rising Temperatures
The Kyoto pact binds industrialised nations to cut emissions until 2012 and even its supporters admit it is only a pinprick in rising world temperatures, especially since Washington did not join its allies in ratifying the pact.
This time, the idea is to get action from all major emitters including China and India to help avert more droughts, desertification, wildfires, species extinctions and rising seas.
The meeting will test how far developing nations will stick to entrenched positions, for example that rich nations must cut their greenhouse gases by at least 40 percent by 2020 -- far deeper than targets on offer.
The United Nations wants developed nations to agree deep cuts in greenhouse emissions by 2020 and come up with immediate, $10 billion (6.1 billion pounds) a year in new funds to help the poor cope. He wants developing nations to start slowing their rising emissions.