(CNN) Last year was the Earth's warmest since record-keeping began in 1880, the U.S. National Oceanic and Atmospheric Administration and NASA said Wednesday.

It's been clear for quite some time that 2015 would steal the distinction of the hottest year from 2014, with 10 out of the 12 months last year being the warmest respective months on record -- and those records go back 136 years.

While it wasn't necessarily a surprise that 2015 finished in first place, its margin of victory was startling -- it lapped the field, with the average temperature across the entire planet 1.62˚F (0.90˚C) above the 20th century average, more than 20% higher than the previous highest departure from average.

This was aided by a December that looked and felt more like a March or April for much of the Northern Hemisphere, where traditional winter holidays had weather that was neither traditional nor winter-like.

In fact, December became the first month to ever reach 2 degrees Fahrenheit above normal for the globe. In the United States, December was both the warmest and the wettest on record -- no other month has ever held both distinctions for the country.

It is somewhat ironic that this news comes out of Washington on a day the city prepares for what could be one of the biggest snowstorms in its history -- but big snows can occur even in the warmest years. Remember Boston last year ? Despite the snowiest winter on record for Boston, the state of Massachusetts still ended the year with temperatures far above average.

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Why was 2015 so warm? The biggest culprit was a major El Niño , which has joined 1997-1998 as the strongest El Niño ever observed . El Niños, which are characterized by significant warming over topical ocean waters in the Pacific, not only warm the ocean but also pump lots of excess heat into the atmosphere, raising global temperatures.

El Niño years tend to be warmer than non-El Niño years (neutral or La Niña years). El Niño was a major driver of the heat this year, but certainly not the only factor. The change also was "largely driven by increased carbon dioxide and other human-made emissions into the atmosphere," a NASA press release said. This is evident in that recent neutral or even La Niña years have been hotter than previous strong El Niños.

Much like sports writers who start their preseason predictions immediately following the final buzzer of the previous season's championship, many climate scientists and weather forecasters are already saying 2016 could push the chart-topping temperature climb even higher , with El Niño lingering into spring and the continued influence from man-made climate change.

The odds would certainly favor that, as 15 of the top 16 warmest years have occurred since 2000 (1998 being the lone pre-21st century year on the list). The last time we had a year become the coldest on record was 1911.