A man wearing a facekini swims in the Songhua river in a temperature of 33 degree Celsius in Jilin, Northeast China's Jilin province, July 9, 2015. [Photo by Wang Mingming/Asianewsphoto]

MORE RECORDS

The natural El Nino event, which warms the tropical Pacific Ocean, might be partly to blame for the 2015 heat record.

Karl, however, said 2015 would have likely been a record even without El Nino. "But El Nino pushed it way over the top," he added.

Gavin Schmidt, director of NASA's Goddard Institute for Space Studies, attributed 2015's temperatures largely to "the cumulative effect" of the long-term warming trend of our Earth, which is mainly caused by the emission of greenhouse gases from human activities into the atmosphere.

"The reason why this is such a warm record year is because of the long-term underlying trend," Schmidt told reporters at the same teleconference. "There is no evidence that long-term trend has slowed, paused, or hiatused at any point in the last few decades."

Since the El Nino will continue into this spring, he predicted that 2016 would be again "an exceptionally warm year and perhaps even another record."

Including the year 2015, 15 of the 16 warmest years on record have occurred during the 21st century, with the exception of 1998, which currently tied with 2009 as the sixth warmest year on record, both reports said.

CALL FOR ACTION

Astrid Caldas, a climate scientist at the Union of Concerned Scientists, said the long-term warming trend should be reversed if the world is to keep global warming below two degrees Celsius, the goal of the international climate agreement struck in Paris last December and the number that scientists believe will avoid irreversible changes to Earth's systems.

"This new record highlights how critical it was to get an agreement in Paris and the importance of countries not only following through on their commitments, but going further," Caldas said in a statement.

"A massive ramp-up of renewable- and low-carbon- energy will be essential to stay within two degrees Celsius of warming and avoid new records being set," she said.

Democratic presidential candidate Senator Bernie Sanders also sought the opportunity to urge movement away from fossil fuels.

"The debate is over," Sanders said in a statement. "Sixteen of the last 18 years have been the hottest ever recorded. Climate change is real and is caused by human activity."

"This planet and its people are in trouble. Unless we get our act together, we will see in years to come more droughts, more floods and more extreme weather disturbances," he said.