The six years since 2015 are the six warmest ever registered, as are 20 of the last 21, evidence of a persistent and deepening trend, the Copernicus Climate Change Service (C3S) reported.
Last year's record high -- a soaring 1.25 degrees Celsius above pre-industrial levels -- was all the more alarming because it came without the help of a periodic natural weather event known as an El Nino, which added up to two-tenths of a degree to the 2016 average, according NASA and Britain's Met Office.
"It is quite clear that in the absence of El Nino and La Nina impacts on year-to-year temperatures, 2020 would be the warmest year on record," Zeke Hausfather, director of climate and energy at the Breakthrough Institute in Oakland, California, told AFP.
During an El Nino, which occurs every two to seven years, warm surface water in the tropical Pacific Ocean can boost global temperatures. La Ninas -- such as one currently underway -- have the opposite cooling effect.
"2020 stands out for its exceptional warmth," said C3S director Carlo Buontempo.
"This is yet another reminder of the urgency of ambitious emissions reductions to prevent adverse climate impacts in the future."
In 2015, the world's nations vowed to cap global warming "well below" 2C, and 1.5C if possible.
A subsequent report from the UN's climate science advisory panel, the IPCC, left no doubt that 1.5C was the safer threshold.
With just over 1C of warming so far, the world has seen a crescendo of deadly droughts, heatwaves, flood-inducing rainfall, and superstorms made more destructive by rising seas.
2020 witnessed a record number, for example, of hurricanes in the Atlantic -- so many that the World Meteorological Organization (WMO) ran out of letters in the alphabet to name them.