Up to 25,000 people are expected to attend the UN's annual conference on fighting climate change in Germany.
This year will be among the three hottest on record, the United Nations said on Monday as almost 200 countries began talks in Germany to bolster a global climate accord that the United States plans to quit.
Temperatures this year would be slightly less than during a record-breaking 2016 and roughly level with 2015, as part of a long-term warming trend driven by greenhouse gases from burning fossil fuels, the UN's World Meteorological Organization (WMO) said.
"We have witnessed extraordinary weather," said Petteri Talaas, head of the WMO, pointing to extreme events including a spate of hurricanes in the Atlantic and Caribbean, monsoon floods in Asia and drought in East Africa.
TRT World's Ira Spitzer reports from Brandenburg.
Talaas said the dip from last year was largely because a natural El Nino event that released extra heat from the Pacific Ocean in 2016 had faded.
Delegates said sweltering temperatures and weather extremes were a spur for action at the annual conference in Bonn from November 6-17, which will to work on a detailed rule book for the 2015 Paris climate agreement and try to step up action before 2020.
"This is our moment of truth," Fiji's Prime Minister Frank Bainimarama, presiding at the Bonn talks, told delegates, urging them to "lock arms with all other nations and move forward together."
"Millions of people around the world have suffered and continue to suffer from extreme weather events," said Patricia Espinosa, the UN's climate chief. "The message cannot get any clearer. We must act right here, right now."
US President Donald Trump, who doubts mainstream scientific findings that man-made greenhouse gases are the primary cause of climate change, said in June he would pull out of the Paris agreement and instead promote US fossil fuels.
None of the speakers at the opening ceremony mentioned Trump by name. The meeting included a traditional Fijian ceremony and children parading with models of a whale, jellyfish and polar bear to urge more action.
US delegation shrinks
A UN list of delegates counts 48 Americans, mostly technical experts and many fewer than in recent years. The US delegation office in a tent village in Bonn has less space, for instance, than those for France or Italy.
A formal US pullout will take until November 2020 and delegates say there are wide uncertainties about how far Washington will balance Trump's pro-coal agenda with the conference's goals.
Thomas Shannon, a career diplomat who once called climate change "one of the world's biggest challenges," will head the US delegation.
The Paris climate agreement sets a goal of ending the fossil fuel era this century and to limit warming to "well below" two degrees Celsius (3.6 Fahrenheit) above pre-industrial times, ideally 1.5C.
The WMO report said average surface temperatures in 2017 were about 1.1C above the pre-industrial era in data from January to September.
Many scientists say the 1.5C limit is slipping out of reach because of insufficient action by governments. The United Nations says the world is on track for a temperature rise of about three degrees by 2100.