The community of nations was mustering in Copenhagen on Monday for a 12-day climate conference framed as a fight back against a peril menacing generations to come.
After opening ceremonies and statements, due to start at 0900 GMT, negotiators from 192 countries set out on a marathon of unprecedented scale and importance.
With the whip of time cracking at their heels, delegates have to craft a blueprint for tackling "greenhouse" gases blamed for trapping solar heat and disrupting Earth's climate system.
They must also put together a funding mechanism for helping poor nations most exposed to the ravages of climate change, from drought to floods, vicious storms to rising seas.
By late next week, their bosses will be arriving.
More than 100 heads of state or government -- including US President Barack Obama, Premier Wen Jiabao of China, Indian Prime Minister Manmohan Singh, Brazil's President Luiz Inacio Lula da Silva and the heads of the European Union (EU) -- are set to attend a climax summit on December 18.
Their goal is to seal an ambitious political agreement in outline form.
Further negotiations would take place in 2010 to fill in the details and -- if all goes well -- from the end of 2012, the new pact would take effect.
"Negotiators now have the clearest signal ever from world leaders to craft solid proposals to implement rapid action," commented Yvo de Boer, executive secretary of the UN Framework Convention on Climate Change (UNFCCC), staging the talks.
"Never in 17 years of climate negotiations have so many different nations made so many firm pledges together.
"So whilst there will be more steps on the road to a safe climate future, Copenhagen is already a turning point in the international response to climate change."
Analysts, though, say the outcome is hard to predict, given the deep gap between the demands of developing countries and the willingness of rich countries to dig both into
their pockets and carbon emissions.
Antonio Hill of the British development charity Oxfam said that anger and suspicion among poorer countries could be eased by a big show of financial goodwill.
Developing countries need funds to ensure they are weaned off high-carbon energy and can shore up their defences against the impacts of climate change.
"The price of success in Copenhagen is 200 billion dollars (per year)," he said.
"We need to see this figure sparkling overhead in Christmas lights by the end of the summit. It's peanuts compared to the 8.4 trillion dollars we found to save drowning banks."
The Bella Center, the venue for the conference, has been declared UN territory. Around 34,000 delegates, journalists and observers from grass-roots organisations registered to attend the talks, but only 15,000, the maximum permitted under safety regulations, are being admitted.
Police at the weekend warned they would act swiftly to quell any violent protest. More than half of all of Denmark's police force has been deployed to the capital.