"2018 is yet again an extremely warm year on top of a long-term global warming trend", Gavin Schmidt, director of GISS, said in a statement.
This year has also started with scorching temperatures, including Australia's warmest January on record, while North America had the complete opposite, experiencing a polar vortex that plunged parts of America into temperatures colder than that of Antarctica's.
The past five years have been the warmest in the modern record, the report said, and 2018's global temperatures were 1.5 degrees Fahrenheit, or 0.83 degrees Celsius, above the mean temperatures for 1951-1980.
Last year, the United States alone suffered 14 weather and climate disasters with losses exceeding $US1 billion ($1.4 billion) each, led by hurricanes and wildfires, according to the US National Oceanic and Atmospheric Administration (NOAA).
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NOAA and NASA contribute data to the WMO. The Bureau of Meteorology said there has been an increase in fire weather and the length of the fire season.
NASA declared Wednesday that 2018 was the fourth-hottest year in 139 years of records as average global temperatures rose alongside surging fossil fuel emissions.
"Over the next five years there is a one in 10 chance of one of those years breaking the (1.5C) threshold", Professor Adam Scaife of the Met Office told Reuters of the agency's medium-term forecasts.
"That is not saying the Paris Agreement is done for. but it's a worrying sign", he said. The United Nations defines the 1.5 degrees Celsius Paris temperature target as a 30-year average, not a freak blip in a single year.
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NASA and NOAA climate scientists said even though 2018 was a tad cooler than the three previous years that was mostly due to random weather variations. The Paris pact responded to a 1992 United Nations treaty under which all governments agreed to avert "dangerous" man-made climate change.
The report also noted that warming is being felt most in the Arctic, where sea ice continues to melt, raising the sea level.
Patrick Verkooijen, head of the Global Center on Adaptation in the Netherlands, told Reuters that the WMO report showed "climate change is not a distant phenomenon but is here right now".
He called for more, greener investments, ranging from defenses against rising seas to drought-resistant crops.
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