One of the coldest places on Earth is so hot it's melting. Glaciers, sea ice and a massive ice sheet in the Arctic are thawing from the toasty air above and warm water below. The northern polar region is heating up twice as fast as the rest of the planet and that's setting off alarm bells.
"The melting of the Arctic will come to haunt us all," said German climate scientist Stefan Rahmstorf. While global leaders set a goal of preventing 2 degrees Celsius (3.6 degrees Fahrenheit) of man-made warming since pre-industrial times, the Arctic has already hit that dangerous mark. Last year, the Arctic Circle was about 3.6 degrees (6.5 degrees Fahrenheit) warmer than normal.
Causes of warming
Earth is getting hotter because of the build-up of heat-trapping gases spewed into the air by the burning of coal, oil and natural gas, according to decades of peer-reviewed research. Scientists have long predicted the Arctic would warm first and faster than the rest of the globe. Real-time measurements are proving them right.
The Arctic is mostly ocean covered with a layer of ice; changes from ice to water often kick in a cycle that contributes to global warming. Sea ice is white and it reflects the sun's heat back into space. But when it melts, it's replaced with Dark Ocean that strongly absorbs it, said former NASA chief scientist Waleed Abdalati, who heads the environmental research program at the University of Colorado.
That heat gets transferred back up to the atmosphere in the fall and winter. As that happens, water vapour - a greenhouse gas - hangs around, trapping more heat. More clouds form around that time, also acting as a blanket, said Mark Serreze, director of the National Snow and Ice Data Center in Boulder, Colorado.
Role of winter
Winter is crucial. Three times in the past two cold seasons, air temperatures near the North Pole were near or even a shade above freezing. That's about 50 degrees warmer than it should be. From last November through February, Barrow, Alaska - the northernmost US city - was 7 degrees Celsius (13 degrees Fahrenheit) warmer than the 20th-century average, and much of the Atlantic Arctic off Norway and Greenland was as hot.
Warm winters weaken sea ice, which floats on the ocean surface. It's supposed to recover, spread more across the Arctic and get thicker in the winter so it can withstand the warmth of the summer. But a warmer winter means less protection when the heat hits.
In September 2016, the time of year the spread of ice across the Arctic is at its lowest, Arctic sea ice was the second lowest day on record, about 40 percent below the lowest day measured in 1979 when satellite records started. Between those two days 37 years apart, the Arctic lost enough sea ice to cover Alaska, Texas and California combined.
Then it didn't grow back that much this winter, setting record low amounts from November through March, when sea ice reaches its peak spread.
Beyond the Arctic
Of all the global warming warning signs in the Arctic, "it is the sea ice that is screaming the loudest," Serreze said.
That's a problem because a growing body of studies connects dwindling sea ice to wild weather. The reduced winter sea ice interacts with warmer oceans to change conditions in the air that then triggers a potent noticeable shift in the jet stream, the giant atmospheric river that controls much of our weather, said Rutgers University climate scientist Jennifer Francis. This theory is still debated by scientists, but increasingly more researchers are agreeing with Francis.
It's not just sea ice on the decline. Glaciers in the Arctic are shrinking. And the massive Greenland ice sheet is slowly but steadily melting and that can add a big dose to sea level rise. Since 2002, it has lost 4,400 billion tons (4,000 billion metric tons) of ice.
Then there's the Arctic carbon bomb. Carbon dioxide and methane - which traps even more heat than before - are stuck in the permafrost in places like Alaska and Siberia.
"Roast the Arctic and you create a mess everywhere on Earth," said Princeton University climate scientist Michael Oppenheimer.