WASHINGTON -- Last year was the planet's warmest year since modern record keeping began in 1880, two major US government agencies responsible for monitoring global climate trends said Wednesday.

The average temperature across global land and ocean surfaces for 2015 was 0.90 degrees Celsius above the 20th century average, surpassing the previous record set in 2014 by 0.16 degrees Celsius, said a report by the US National Oceanic and Atmospheric Administration (NOAA).

This marks "the fourth time a global temperature record has been set this century," the report said. "This is also the largest margin by which the annual global temperature record has been broken."

US space agency NASA, which used much of the same raw temperature data, but different analyzing methods, said globally-averaged temperatures in 2015 shattered the previous mark set in 2014 by 0.13 degrees Celsius.

"Only once before, in 1998, has the new record been greater than the old record by this much," a NASA statement wrote.

Anyway, the 2015 temperatures reflected a long-term warming trend of the Earth, which is mainly caused by the emission of greenhouse gases from human activities into the atmosphere.

Both reports said including 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.

"Climate change is the challenge of our generation," NASA Administrator Charles Bolden said. "It is a key data point that should make policy makers stand up and take notice -- now is the time to act on climate."

In 2015 alone, 10 months had record high temperatures for their respective months, said the NOAA report. Only January was the second warmest January on record and April third warmest.

Besides, the five highest monthly departures from average for any month on record all occurred last year.

The record warmth occurred broadly around the world, including Central America, the northern half of South America, parts of northern, southern, and eastern Europe stretching into western Asia, a large section of east central Siberia and regions of eastern and southern Africa.

A combination of strong El Nino, which warms the tropical Pacific Ocean, and human-caused climate change was largely to blame for the 2015 heat record, experts said.

"2015 was remarkable even in the context of the ongoing El Nino," said NASA's Goddard Institute for Space Studies Director Gavin Schmidt.

"Last year's temperatures had an assist from El Nino, but it is the cumulative effect of the long-term trend that has resulted in the record warming that we are seeing," Schmidt added.