2021 Arctic summer sea ice 12th-lowest on record: NASA

The scientists found that sea ice in the Arctic ocean and neighboring basins hit its annual minimum extent on September 16, after waning in the 2021 Northern Hemisphere spring and summer.

This year, the minimum extent of Arctic sea ice dropped to 4.72 million square kilometers. Sea ice extent is defined as the total area in which ice concentration is at least 15 per cent.

The average September minimum extent record shows significant declines since satellites began measuring consistently in 1978. The last 15 years (2007 to 2021) are the lowest 15 minimum extents in the 43-year satellite record.

Sea ice extent in 2020 and 2019 were the second and third lowest on record at 3.74 million square kilometers in 2020 and 4.14 million in 2019.

Less sea ice melted in 2021 even as the planet as a whole was warmer than usual -- with new temperature records in North America and Eurasia, drought in the US West, and episodes of intense melting on Greenland's ice sheet.

However, the scientists believe that this summer, plenty of ice was close to disappearing but never quite reached that point -- maintaining extent but not thickness.

"I don't see any inconsistency with the Arctic sea ice extent not breaking any records this year despite global temperatures being high," said Claire Parkinson, a sea ice scientist at NASA's Goddard Space Flight Center, in a statement. "The key is that the Earth is large and there are differences regionally."

"We don't expect sea ice to be lower every year," added Walt Meier, a sea ice researcher at the National Snow and Ice Data Center, "just like we don't expect temperatures to be warmer everywhere on Earth every year even with global warming."