With encouragement from 150 world leaders ringing in their ears, government negotiators in Paris were on Tuesday left to turn the rhetoric into reality and agree a draft text of a global deal to slow climate change.
U.S. President Barack Obama and Chinese President Xi Jinping made common cause on Monday with other countries to stress the urgency of an agreement to slow a rise in global temperatures blamed for spurring floods, heat waves and rising sea levels.
But as the leaders left Paris, negotiators from 195 countries were left to work on a draft text of more than 50 pages still riddled with disagreements.
The main sticking point is how to come up with the billions of dollars needed to finance the cleaner energy sources that are badly needed if emerging countries are to develop without relying heavily on fossil fuels.
Many delegates said the large turnout at the U.N. climate summit in Paris, weeks after attacks by Islamic State militants killed 130 people, was a sign of hope after the last summit collapsed in failure in 2009 in Copenhagen amid rancour between rich and poor nations.
French President Francois Hollande said he was encouraged by the start of talks that are planned to run until Dec. 11.
"It's set off well but it has to arrive too," he told reporters. He said there were "two reefs. Either we overload the vessel and it sinks or we empty it and it goes nowhere."
The technical talks repeated little of Monday's grand language. Countries restated their negotiating positions with few hints of likely compromise.
China's delegate Su Wei "noted with concern" what he called a lack of commitment by the rich to make deep cuts in greenhouse gas emissions and help developing nations with new finance to tackle global warming.
"It's back to the nitty gritty," said Alden Meyer, of the Union of Concerned Scientists, adding the opening day was "all good but that does not resolve the crunch issues."
"It is still a text with many options," Peruvian Environment Minister Manuel Pulgar Vidal told Reuters, adding with a shrug "but everybody has shown their commitment to have an agreement."
The mood was brightened by major announcements including a plan by India and France to mobilise $1 trillion for solar power for some of the world's poorest people and a private sector initiative led by Microsoft co-founder Bill Gates to mobilize billions of dollars for new energy research and development.
"Leaders still have the scars of Copenhagen on their hearts and brains," Yvo de Boer, who was the U.N.'s climate chief in Copenhagen, said.
"The fact that so many leaders came back here on the opening day to send encouragement ... is a sign that they really want to move," he said.
A deal in Paris would be by far the strongest ever agreed to bind both rich and poor nations to limit greenhouse gas emissions, which scientists say have blanketed the earth, raised global temperatures and begun upending the planet's climate system.
French President Francois Hollande (2ndL) poses with participants in the Climate Generations area during the World Climate Change Conference 2015 (COP21) at Le Bourget, near Paris, France, December 1, 2015. At 2ndR, French Minister for Ecology, Sustainable Development and Energy Segolene Royal. Photo: Reuters/Philippe Wojazer
Liz Gallagher, of the London-based E3G environmental think-tank, said the opening day had "made an agreement more likely".
But she said the biggest gap was over climate finance. Developing nations want the rich to pledge rising amounts beyond the current goal of $100 billion a year by 2020 to help them obtain clean energy sources and adapt to the effects of climate change, such as more floods, droughts and intense storms.
Other disputes concern how to define a long-term goal for phasing out fossil fuels.
In June, developed nations in the Group of Seven (G7) signed up for a goal of decarbonizing the world economy by 2100. China and India say they need to rely on coal to lift millions from poverty and prefer a shift to low-carbon development this century.
So far, pledges made by about 170 countries to curb greenhouse gas emissions beyond 2020, made in the run-up to the Paris summit, are too weak to limit rising global temperatures to 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times. That is widely viewed as a threshold for dangerous and potentially catastrophic changes in the planet's climate system.