The 195-nation conference in Paris had been scheduled to wrap up on Friday, but was extended another day after ministers failed to bridge deep divides during a second consecutive all-night round of negotiations.
Still, Foreign Minister Laurent Fabius, who is presiding over the talks, voiced confidence the event would culminate with a much-awaited pact.
“We are almost at the end of the road and I am optimistic,” said Fabius, whose hopes were echoed by many negotiators and observers despite potential deal-breakers still up in the air.
Fabius said he would submit the deal at 9am (0800 GMT) and was “sure” it would be approved. “It will be a big step forward for humanity as a whole,” he said.
World leaders have billed the Paris talks as the last chance to avert disastrous climate change: increasingly severe drought, floods and storms, as well as rising seas that would engulf islands and populated coasts.
The outcome of a laborious two-decade-long process, the post-2020 accord would commit all nations to curb greenhouse gases that trap solar heat, warming Earth’s surface and disrupting its delicate climate system.
Highlighting the urgency of the moment, US President Barack Obama and his Chinese counterpart, Xi Jinping, spoke by phone on Friday about the Paris negotiations, according to China’s foreign ministry.
Xi said the world powers “must strengthen coordination with all parties” and “make joint efforts to ensure the Paris climate summit reaches an accord”, according to a statement on the ministry’s website.
The planned accord would seek to revolutionise the world’s energy system by cutting back or potentially eliminating the burning of coal, oil and gas, whose carbon dioxide is the big warming culprit.