News•January 7, 2016

December Heat Boosts 2015 to 2nd Warmest Year for U.S.

2015 is officially in the books as the second-hottest year ever recorded for the U.S., with a major boost provided by the incredible warmth that bathed the eastern half of the country in December, the National Oceanic and Atmospheric Administration announced Thursday.

How U.S. temperatures progressed throughout 2015.

That second-place finish comes as both NOAA and NASA are expected to announce that the year was the hottest on record globally. While that record heat was helped in part by one of the strongest El Ninos on record, it was mainly due to the contributions of manmade global warming, scientists have said.

The U.S. temperature ranking “is emblematic of what will ultimately be the warmest year for the globe,” Deke Arndt, chief of NOAA Climate Monitoring Branch, said during a press conference.

By the end of November, the U.S. was having its fifth hottest year-to-date, but December turned out not only to be record warm, but incredibly so: It was 6°F (3°C) above the 20th century average and bested the previous record holder (December 1939) by nearly a full degree. That surge of heat bumped the year as a whole up to the No. 2 slot in records going back to 1895, with a temperature that was 2.4°F (1.3°C) above the 20th century average. The only year warmer was 2012, by 0.9°F (0.5°C).

This was the 19th consecutive year that the overall U.S. temperature exceeded the 20th century average, NOAA said.

December’s warmth was fueled by a combination of the influences of El Nino, climate patterns over the North Atlantic and manmade warming, as winters overall in the U.S. have been heating up. During the month, record warm temperatures outpaced record cold ones by an astounding 21-to-1 ratio, and 29 states had their hottest December on record.

“December 2015 really blew away the competition,” NOAA climatologist Jake Crouch said.

And not only was December record warm, it was also record wet, the first time a month has set both records simultaneously, Crouch said.

Month-by-month temperature anomaly for every climate division in the U.S.
Credit: NASA

While the east baked in December, the west was relatively cool — the opposite of the pattern that prevailed during the first half of the year. During those months, the West roasted under a high-pressure ridge that helped fuel record-breaking drought while the East took repeated blows from outbreaks of Arctic air. The result of that temperature pattern flip was that every state had an above-average annual temperature.

Four states — Florida, Montana, Oregon, and Washington — saw their all-time warmest year, while three more — Alaska, California, and Idaho — had their second warmest. (California had its record warmest year in 2014.) Florida was an outlier in the eastern half of the country for much of the year due to a combination of influences, including drought during the warm spring and summer months and record warm ocean temperatures offshore.

Drought was also a factor in the warmth earlier in the year in the West, but despite the record-breaking levels it reached, the year as a whole was the third wettest on record for the country, behind only 1973 and 1983. Much of that precipitation fell in the central and southeastern parts of the country, such as the torrential rains that caused flooding in Texas and Oklahoma in May and South Carolina in October. Both of these were among the 10 weather and climate disasters that exceeded $1 billion in damage during the year.

“The fact is that we live in a warming world,” Arndt said. “We’re going to be dealing with more extreme heat events and more extreme rainfall events, and I think that really showed up in the results from the year as a whole.”

While parts of the U.S. were relatively cool at various times during the year, there were very few similar spots on the globe. Overall the planet saw incredible warmth, particularly in the oceans. El Nino helped to push Pacific Ocean temperature to levels rivaling the strongest events on record, but a Climate Central analysis showed that the overwhelming contributor to the year’s record warmth is the excess heat that has built up in Earth’s atmosphere because of the accumulation of greenhouse gases emitted by human activities.

Of the 15 warmest years on record, 14 have occurred in the 21st century. The last time there was a record cold year was 1911.

While 2015 will be the hottest year on record, that mark may not stand for long. The U.K. Met Office has said that with the strong El Nino in place, 2016 could end up besting 2015, which would mean three years in a row of record warmth.

That record-setting likely won’t continue year-after-year, but it shows just how much Earth’s temperature has soared.

Editor's note: This story was updated to correct when 14 of the 15 warmest years on record have occurred, which was in the 21st century, not the 20th.