'Polar bears may disappear.' Arctic sea ice keeps shrinking. Here's what that means for the planet

Arctic sea ice shrank to its second-lowest level on record last week, according to statements released Monday by the National Snow and Ice Data Center and NASA.

Sea ice affects Arctic communities and wildlife such as polar bears and walruses, and it helps regulate the planet’s temperature by influencing the circulation of the atmosphere and ocean.

"If you decrease the amount of sea ice, you start warming up the Arctic, and when you start warming up the Arctic, you start changing the circulating of the jet stream, which brings weather to us here," NASA scientist Nathan Kurtz said.

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Sea ice extent was measured at 1.6 million square miles Sept. 18. That's 811,000 square miles below the average, tying with both 2007 and 2016 for second-lowest level on record, the data center said.

“This year’s minimum sea ice extent shows that there is no sign that the sea ice cover is rebounding,” Claire Parkinson, a climate change senior scientist at NASA’s Goddard Space Flight Center, said in a statement.

September is the month Arctic ice reaches its lowest "extent" of the year, toward the end of the Northern Hemisphere's summer.

According to NASA, it was a very warm summer in the Arctic, with average temperatures 7 to 9 degrees Fahrenheit above normal.

"The frequency of these bad ice years will make it impossible for polar bears to survive until another good year comes along," said Thea Bechshoft, a staff scientist at Polar Bears International. "That means that polar bears may disappear from some subpopulations, even before sea ice is gone every year,"

Sea ice is frozen ocean water that melts each summer, then refreezes each winter. The refreezing process has begun, the data center said. Arctic sea ice reaches its largest area in March each year.

This year ranks behind only 2012, when the lowest level on record was measured. Arctic sea ice has been measured since 1979.

"The trends we’ve been seeing with the sea ice minimum have just been a decrease," Kurtz said. "So in the '70s since the modern record began, tracking to today, there is variation from year to year, but it’s really just a downward trend."

The amount of summer sea ice in the Arctic has been steadily shrinking over the past few decades because of man-made global warming, according to the National Oceanic and Atmospheric Administration and NASA.

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This article originally appeared on USA TODAY: Climate change: Arctic sea ice shrinks to 2nd-lowest mark on record