The U.N. report is meant to guide almost 200 nations meeting from Nov. 6-17 in Bonn, Germany, to try to bolster the 2015 Paris climate pact despite a planned U.S. pullout.
"2017 is set to be in top three hottest years," the U.N.'s World Meteorological Organization (WMO) said, projecting average surface temperatures would be slightly less sweltering than a record 2016 and roughly level with 2015, the previous warmest.
And 2017 would be the hottest on record without a natural El Nino event that releases heat from the Pacific Ocean about once every five years, it said. El Nino boosted global temperatures in both 2015 and 2016.
"We have witnessed extraordinary weather," WMO Secretary-General Petteri Taalas said in a statement of 2017, pointing to severe hurricanes in the Atlantic and Caribbean, temperatures above 50 degrees Celsius (122F) in Pakistan, Iran and Oman, monsoon floods in Asia and drought in East Africa.
"Many of these events - and detailed scientific studies will determine exactly how many - bear the tell-tale sign of climate change caused by increased greenhouse gas concentrations from human activities," he said.
The Bonn meeting is due to work on a "rule book" for the Paris Agreement, which seeks to end the fossil fuel era in the second half of the century by shifting the world economy to cleaner energies such as wind and solar power.
"These findings underline the rising risks to people, economies and the very fabric of life on Earth if we fail to get on track with the aims and ambitions of the Paris Agreement," said Patricia Espinosa, head of the U.N. Climate Change Secretariat which hosts the Bonn talks.
U.S. President Donald Trump, who once dismissed global warming as a Chinese plot to undermine the U.S. economy, plans to withdraw from the Paris pact and focus instead on bolstering the U.S. fossil fuel industry. China and the United States are the top emitters.
Many scientists say the 1.5C limit is already slipping out of reach because of insufficient action by governments to cut emissions so far.
"We'll need a lot of luck and drastic action to stay below 1.5 degrees," Professor Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research told Reuters.
Among extremes, the WMO pointed to a battering from hurricanes Harvey, Irma and Maria.
While it said there was no clear evidence whether climate change made hurricanes more frequent, when storms do take place warmer temperatures aggravate downpours, and higher sea levels can worsen storm surges, it said.
Among other extremes, monsoon floods killed 1,200 people in India, Bangladesh and Nepal, the WMO said. Severe wildfires, stoked by heat and drought, killed 64 people in Portugal, at least 41 in California and 11 in Chile.
A few places had record cold against the overall warming trend, such as -25.4C (-13.7F) in Bariloche, Argentina, in July.
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