In their annual “report card” of the polar region, they warn that the average air temperature was 1.6 degrees above normal in 2017, the second highest on record after 2016.
Parts of the Arctic Ocean were as much as 4 degrees warmer than normal.
As a consequence sea ice is melting from above and below. Maximum ice cover in the spring was the lowest on record, the data shows.
The bleak assessment comes from the US National Oceanic and Atmospheric Administration, pulling together the work of 85 scientists in 12 nations.
They conclude that the Arctic shows “no sign of returning to the reliably frozen region it was decades ago.”
Dr Timothy Gallaudet, acting NOAA administrator, said: “The rapid and dramatic changes we continue to see in the Arctic present major challenges and opportunities.”
With the ocean free of ice for longer each year the region is being increasingly industrialised.
Commercial trawlers are moving into polar seas.
The first container ship sailed over the North Pole this summer cutting several days off the journey from Asia to Europe, and clear waters make it easier for oil and gas companies to drill into vast reserves beneath the sea.
But Arctic animals are suffering.
Increased human activity is disturbing whales, such as the narwhal, that use sound to communicate.
And the melting ice is forcing polar bears to come ashore to look for food.
Sky Ocean Rescue has filmed in the Arctic this summer, following UN Patron of the Oceans Lewis Pugh in his most perilous swim to date – one kilometre along the edge of the ice pack.
He did so to raise the alarm over the effects of global warming in the region.
He said: “I think the average person realizes that climate change is happening.
“I don’t think the average person appreciates how quickly it’s happening.
“And lastly I don’t think they realise the impact it’s going to have on them and their children.