Last month was the warmest December on record in Alaska, according to a federal report released Monday.

The statewide average temperature in December was 19.4 degrees, 15.7 degrees above the 20th century average, the National Oceanic and Atmospheric Administration's report said. Records for Alaska go back to 1925.

The report also found that 2017 was the third warmest year on record for the U.S. as a whole since record-keeping began in 1895. But December in Alaska specifically "was really quite remarkable," said Rick Thoman, climate science and services manager for the National Weather Service.

"Alaska, of course, being the only Arctic part of the U.S. … it's often referred to as polar amplification, that climate is warming much more rapidly at high latitudes," Thoman said. "We are the U.S.'s canary in that coal mine."

Last year was also the seventh warmest year in Alaska on record. The last four years are all in the top seven warmest on record.

"There's no comparable period like that," said Thoman. "This is unique in our 93-year temperature record."

December was "another month in this warm steamroller that keeps moving along," he said.

In the Bering Sea, the warmth has been "unrelenting," he said, with four straight years of significantly above-average sea surface temperatures.

Northern and Western Alaska were excessively warm last month, and Southeast Alaska's temperatures were closer to normal, Thoman said.

"Of course it's concerning," he said. "Alaska's changing and it's changing very rapidly. We're going to have to adapt."

Alaska's December statewide average temperature was 2.1 degrees warmer than its previous record in 1985.

"That's really quite astonishing. Usually you're breaking those by a tenth of a degree or two-tenths of a degree," Thoman said.

In the North Slope communities of Wainwright, Utqiaġvik, Nuiqsut and Deadhorse, every single day of December had temperatures above normal. Kuparuk had 30 days above normal and one day below normal. Across the state, though, there were relatively few daily record high temperatures last month, Thoman said.

"It's more the persistence, the slow grind of warmth," he said.

Five states had their warmest year on record in 2017: Arizona, Georgia, New Mexico, North Carolina and South Carolina, according to NOAA. Alaska was one of 32 states last year where annual temperatures ranked among the 10 warmest on record.