Average surface temperatures in 2017 were 1.1 degree Celsius above pre- industrial times, creeping towards a 1.5C ceiling set as the most ambitious limit for global warming by nearly 200 nations under the 2015 Paris climate agreement.
Using those inputs, the United Nations said that the average global surface temperature previous year was 1.1 degrees Celsius (1.98 degrees Fahrenheit) above pre-industrial levels.
Yet scientists at both federal agencies reached the same grave conclusion: The Earth is heating up.
This is a developing story.
Among extreme weather events past year, the Caribbean and the United States suffered a battering from hurricanes, the Arctic ended 2017 with the least sea ice for mid-winter and tropical coral reefs suffered from high water temperatures. Records go back to 1880.
WMO secretary-general Petteri Taalas said: "The long-term temperature trend is far more important than the ranking of individual years, and that trend is an upward one". The National Oceanic and Atmospheric Administration (NOAA), which uses different metrics, found that 2017 was the third-warmest year on record.
Researchers are placing blame for the trend squarely on climate change, especially because it appears the impacts of greenhouse gas emissions are able to overcome most natural warming patterns like El Nino.
Every state in the contiguous US and Alaska had above-average annual temperatures in 2017, NOAA said. But El Niño conditions can also curb the formation of powerful storms, and with no El Niño in the picture - and with warmer-than-average ocean waters - 2017's Atlantic hurricane season was unusually active, and devastating.
"The Arctic has warmed 2 and a half degrees C since the middle of the century", he said. "So much of the difference in 2017 between the groups that find it in second place and third place has to do with how the Arctic is handled".
NOAA and NASA analyses use temperature measurements from weather stations on land and at sea. But another way to track the planet's warming is to analyze the temperature of the atmosphere at a significantly higher elevation, in the so-called "lower troposphere" extending from a little above the planet's surface to several miles into the air.