When Arctic sea ice reached its summer minimum on Sept. 9, it crept back to the second lowest point since the record low set in September of 2007, according to the National Snow and Ice Data Center in the United States. An early estimate, by a German group using different measurements, went further, calling this year the record lowest extent for Arctic sea ice.
This year's summer minimum, reached on Sept. 9, measured at 1.67 million square miles (4.33 million square kilometers), fell short of the record on file at the NSIDC for Sept. 16, 2007, at 1.59 million square miles (4.13 million square km).
The sea ice undergoes a natural seasonal cycle, growing across the Arctic waters during the cold of winter and then retreating in response to the warmth of summer. Historically, the ice — which provides important habitat for walrus and polar bears — reaches its minimum extent between the first week of September and around the end of the third week of the month, according to Walt Meier, a research scientist at NSIDC.
Even though different groups' measurements may not agree, the trend is clear: The Arctic sea ice is shrinking. The last five years have all ranked as the five lowest years on record since continuous record-keeping began in 1979, Meier said.
The German group, at the University of Bremen, reported a minimum of 1.64 million square miles (4.24 million square km) on Sept. 8, lower than the record they had cited for 2007. They use a satellite sensor that can detect ice cover at a higher resolution than that used by NSIDC. The two groups probably came up with different results because this year ice was more dispersed in the water, and the Bremen group was able to pick up on details, leading to more variability between the two sets of measurements, Meier said.
The NSIDC makes its call for the annual minimum by watching changes in the ice extent.
"When you get a few days in a row of it heading back up and some pretty good increases from day to day, then we start to feel pretty confident," Meier said. It's possible the ice extent could dip back down again, possibly because winds consolidate the ice, but it is unlikely to go any lower than the extent measured on Sept. 9, he said.
Scientists fault a combination of natural weather variations, such as wind patterns, as well as warming caused by the greenhouse gases humans emit for the shrinking ice. The loss of ice not only has implications for wildlife — once ice is lost, it becomes more difficult to replace, and the loss of ice can alter weather patterns elsewhere in the world.
While it's impossible to predict what will happen to sea-ice extent in the next few years, the long-term trend is clear: "Eventually, we will continue going downward as temperatures rise," he said.