The Arctic shattered heat records in the past year, as unusually warm air triggered massive melting of ice and snow and a late fall freeze, US government scientists said on Tuesday.
The grim assessment came in the Arctic Report Card 2016, a peer-reviewed report by 61 scientists around the globe that is issued by the National Oceanic and Atmospheric Administration.
The report spans from October 2015 to September 2016, a period when the Arctic's “average annual air temperature over land areas was the highest in the observational record,” it said.
“Rarely have we seen the Arctic show a clearer, stronger or more pronounced signal of persistent warming and its cascading effects on the environment than this year,” said Jeremy Mathis, director of NOAA's Arctic Research Program.
The Arctic region is continuing to warm up more than twice as fast as the rest of the planet, which is also expected to mark its hottest year in modern times.
Climate scientists say the reasons for the rising heat include the burning of fossil fuels which emit heat-trapping gases into the atmosphere, as well as the El Nino ocean warming trend, which ended mid-year but exacerbated the warming.
The Arctic's annual air temperature over land was 3.5 degrees Celsius higher than it was in 1900.
The sea surface temperature in the peak summer month of August 2016 was 5 degrees Celsius above the average for 1982-2010 in the Barents and Chukchi seas and off the east and west coasts of Greenland.
“It is apparent that the record-breaking delay in the freeze up of the sea ice cover in the fall of 2016 is associated with unprecedented warm air and ocean surface temperatures,” said the report.
This warming trend has also led to young, thin ice cover that melts easily.
Scientists added a section to the report on noteworthy records set in October and November 2016, even though that was beyond the report's typical time span. The report said this extra section had not yet been peer-reviewed.
“The Arctic sea ice minimum extent from mid-October 2016 to late November 2016 was the lowest since the satellite record began in 1979,” it said.
It was also 28 per cent less than the average for 1981-2010 in October.
More of the ice that freezes in the winter is thin, and made up of only a single year's worth of freeze rather than the thicker, more resistant ice built up over multiple years.
In 1985, almost half (45 per cent) of the Arctic sea ice was called “multi-year ice.” 
Now, just 22 percent of the Arctic is covered in multi-year ice. The rest is first-year ice.