Globally, 2018's temperatures rank behind those of 2016, 2017 and 2015.
Using computer simulations, the British weather office forecast that the next five years would average somewhere between 14.73°C to 15.27°C.
Weather extremes last year included wildfires in California and Greece, drought in South Africa, floods in India, while the new year saw Queensland and Tasmania threatened by record-breaking floods and bushfires.
An analysis of five worldwide datasets by the WMO showed that the global average surface temperature in 2018 was approximately 1° above the pre-industrial starting point.
According to NASA's Goddard Institute for Space Studies (GISS), the global temperature for 2018 was 1.5 degrees Fahrenheit warmer than the mean global temperature for the years 1951-1980.
"The 20 warmest years on record have been in the past 22 years", Mr Taalas said.
Similar reports on climate trends released by the US space agency NASA and the US National Oceanic and Atmospheric Administration (NOAA) confirmed that a year ago was the fourth warmest in modern times.
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Princess Ubolratana lived in the USA for 26 years until 1998, when she divorced Jensen and returned to Thailand. Tragedy struck in 2004, when the pair's second son, Khun Bumi Jensen, died in the Indian Ocean tsunami in 2004.
"2018 is yet again an extremely warm year on top of a long-term global warming trend", said GISS Director Gavin Schmidt.
This is also seen as a critical threshold for climate change, as it represents the lower bound of the average temperature rise. NASA and NOAA analyzed the same data independently and came to the same conclusion.
"Weather dynamics often affect regional temperatures, so not every region on Earth experienced similar amounts of warming", said NOAA.
Global average temperature anomaly map, showing that there are far more warm-than-normal than colder-than-normal regions on the planet.
The warming is driven largely by the continued emission of carbon dioxide and other greenhouse gases into the atmosphere by human activity, such as manufacturing, coal-fired power plant emissions, and deforestation.
All the results show the same "escalator-like" rise that scientists think is linked to the loss of sea ice, as well as an increase in extreme weather events around the world.
Nasa's data was compiled by 6,300 weather stations around the world and was compared against information stretching back to 1880.