In a report released ahead of the COP 24 climate summit in Poland, the World Meteorological Organization pointed out that the 20 warmest years on record have been in the past 22 years, and that "2018 is on course to be the 4th warmest year on record."
"This would mean that the past four years – 2015, 2016, 2017 and 2018 – are also the four warmest years in the series," the UN agency said in its provisional report on the state of the climate this year.
The "warming trend is obvious and continuing," WMO chief Petteri Taalas told reporters in Geneva.
The report shows that the global average temperature for the first 10 months of the year was nearly 1.0-degree Celsius above the pre-industrial era (1850-1900).
"It is worth repeating once again that we are the first generation to fully understand climate change and the last generation to be able to do something about it," Taalas warned.
With levels of greenhouse gases in the atmosphere, the main driver of climate change, at a record high, "we may see temperature increase of 3-5C by the end of the century," Taalas said.
"If we exploit all known fossil fuel resources, the temperature rise will be considerably higher."
Delegations from nearly 200 countries are due in Poland next week for the latest COP24 climate summit, aimed at renewing and building on the Paris deal and limiting global warming.
World leaders have been trying to breathe new life into the 195-nation agreement amid backsliding from several nations - most notably the United States - over commitments made when it was signed in December 2015.
It is to take effect in 2020 and calls for limiting global warming to less than 1.5C above pre-industrial levels.
But experts warn that global warming is on track to surpass three degrees by 2100 and urge governments to do more than first planned to rein it in.
"Every fraction of a degree of warming makes a difference to human health and access to food and fresh water, to the extinction of animals and plants, to the survival of coral reefs and marine life," WMO deputy chief Elena Manaenkova stressed in a statement.
"It makes a difference to economic productivity, food security, and to the resilience of our infrastructure and cities," she said.
"It makes a difference to the speed of glacier melt and water supplies, and the future of low-lying islands and coastal communities."
The report shows that the global average temperature for the first 10 months of the year was nearly 1.0-degree Celsius