Not to be upstaged by the previous year, 2015 was globally the warmest year since records began in 1880, according to NASA and NOAA.
The average temperature across global land and ocean surfaces was 1.62 F (0.90 C) above the 20th century average, NOAA said. Surpassing 2014's record by 0.29 F (0.16 C), this is the largest margin by which the annual global temperature record has been broken.
Fifteen of the 16 warmest years on record have occurred since 2001.
In the contiguous United States, 2015 was the second warmest year on record and the third wettest.
"The warmth was spread throughout the globe with some important exceptions like in the North Atlantic," Thomas Karl, director for NOAA's National Centers for Environmental Information, said.
Global temperatures tied or broke existing monthly records for all but two months of the year.
"In previous times when we've set records, the gap has been significantly smaller than what we saw this year," Karl said, calling the margin "remarkable."
An El Niño pattern, which is marked by above-normal water temperatures in the central and eastern Pacific Ocean, will contribute to a warming of the global mean temperature, Gavin Schmidt, director of NASA's Goddard Institute for Space Studies, said. El Niño influences weather patterns around the globe.
Schmidt said that though El Nino assisted the rise in global temperatures, 2015 still would have broken the record without the warm-water phenomenon.
However, there is no evidence that the long-term warming trend has slowed, he said.
"A strong El Niño was a significant factor, as well as overall very warm ocean water temperatures," AccuWeather Senior Meteorologist Brett Anderson said. "However, natural factors alone can't explain this level of warming."
Anderson added that though the strong El Niño will weaken in the spring, effects will spread into much of 2016.
"There's a chance 2016 could be even warmer," he said.
Schmidt reiterated that thought due to 2016's warm start.