President Donald Trump announced earlier this year that the US will pull out of the 2015 Paris climate accord unless his administration can secure a better deal.
Other countries have pledged to press on regardless at the November 6-17 meeting in the western German city of Bonn. The 23rd Conference of the Parties (COP) will be presided over by Fiji, one of the small island nations particularly vulnerable to the effects of global warming.
"This COP is more important than most people realize," Andrew Steer, head of the World Resources Institute, a Washington-based environmental think tank, said.
Steer said a growing number of commitments from governments, as well as from major companies and cities around the world was "broadly encouraging." He noted the falling cost of renewable energy a key technology required to help power-hungry economies kick the carbon habit.
Negotiators will try to agree on ways to measure each country's greenhouse gas emissions and to make sure everyone is playing by the same rules. Since the Paris agreement doesn't foresee sanctions for countries that fail to meet their targets, peer pressure is the main mechanism for ensuring that governments abide by their commitments and continue to increase their efforts in future.
The United States used to be the major force pushing for more open reporting and that will suffer because of the Trump administration's planned withdrawal from Paris, he said.
"Without US leadership the deal is likely to take a little bit longer and may not be as strong," said Purvis, president of the group Climate Advisers.
Negotiators have until the end of 2018 to come up with the final rule book; it would be subject to approval at the next climate summit in Katowice, Poland.
"The United States are not anymore the most important actor in this game of climate change," Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research, said.
"The most important actor is China, and India will soon become extremely important, as well as the European Union," Schellnhuber said.
Some are also placing their hopes in Emmanuel Macron. The French president is expected to fly in on November 15 to show his support during the final phase of the talks. Macron has called for a follow-up meeting in Paris next month.
Environmental campaigners are planning to highlight Germany's continued reliance on the heavily polluting source of energy with a protest at a coal mine near Bonn ahead of the talks.
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