The difference between NASA and the NOAA's temperature readings is due to how each collect their data, but by both's standards, the past four years mark the hottest in their 138-year archives.
1998, for instance, was at the time a record year for global temperatures, as it coincided with a very strong El Nino - but 2017's temperature now comfortably surpasses it.
The last three years were the hottest on record, the United Nations weather agency said Thursday, citing fresh global data underscoring the dramatic warming of the planet.
During an El Nino year - when a warming of the central Pacific changes weather worldwide - the globe's annual temperature can spike, naturally, by a tenth or two of a degree, scientists said. Last year's average temperature was eclipsed only by 2016's.
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.
"Global temperatures will continue to bob up and down from year to year, but the climate tide beneath them is rising fast".
NOAA and NASA analyses use temperature measurements from weather stations on land and at sea.
The globally averaged temperature in 2017 was about 0.46°C above the 1981-2010 long-term average of 14.3°C - a 30-year baseline used by national meteorological and hydrological services to assess averages and variability of key climate parameters, which are important for climate-sensitive sectors, such as water management, energy, agriculture and health. But NOAA uses slightly different methodology, so it concluded that 2017 was only the third-hottest year on record. "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".
The observed warming has been predicted within a few tenths of a degree in computer simulations going back to the 1970s and 1980s, several scientists said.
"Despite colder than average temperatures in any one part of the world, temperatures over the planet as a whole continue the rapid warming trend we've seen over the last 40 years", said Goddard Institute for Space Studies Director Gavin Schmidt in a press release.
The warming trend continued as President Donald Trump announced that the United States would withdraw from the 2015 Paris Climate Accord and repeal the Clean Power Plan, an Obama-era measure created to reduce emissions from power generation.