Delegates at the weeklong talks in Bonn, Germany, are expected to start line-by-line editing a 20-page draft that still contains multiple options on how to reduce the greenhouse gas emissions that scientists say are warming the planet.
Some 150 countries, including top emitters China, the United States, the European Union and India, have already made voluntary pledges to cut or curb their emissions after 2020, when the deal is supposed to take effect.
"We don't want to make a picture, we want to make a movie," Netherlands climate envoy Michel Rentenaar said, insisting that the Paris agreement cannot freeze the current level of ambition on climate action.
The UN talks have made significant strides since a 2009 climate summit in Copenhagen failed to live up to expectations. For the first time all countries now agree they need to act against climate change, which scientists say is already transforming the planet through melting glaciers, rising sea levels, intensifying heat waves and warmer, more acidic oceans.
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"We have gone very far from the Copenhagen atmosphere," said Pa Ousman Jarju, Gambia's environment minister. As always in the UN talks, which are based on consensus, there's a risk that a handful of countries block progress, he said.
It would be a major setback if some countries reject the draft in Bonn, leaving negotiators with little time to come up with a new one before Paris.
Several delegates said they didn't expect that to happen, though they couldn't rule it out.
The World Resources Institute, a Washington-based environmental think tank, said negotiators must be prepared to get "their hands dirty" in Bonn and deal with the sticky issues before the Paris summit begins on November 30.