Wimpy ice extent in Bering Sea part of new normal under climate change
“This is a thing that I never expected to see,” said Rick Thoman as he was looking at a picture of a sunrise taken in Nome during this past winter solstice, December 21, without a single chunk of ice floating on the water.
Thoman, an Alaska climate specialist at UAF’s International Arctic Research Center, gave a Strait Science talk in person in Nome last week. During his lecture, he discussed the 2022/2023 winter’s weather. But he also explained the patterns observed in sea ice coverage and how those fit in with larger climate trends.
Researchers at the National Weather Service’s Alaska Sea Ice Program map the concentration of sea ice throughout the region. Their data showed that the Chukchi Sea still had very low concentrations of ice even into mid-December.
“It took until right around Christmas for the Chukchi Sea to completely ice over,” Thoman said. “That is historically impossibly late. But in the last 10 years, that’s been come pretty typical.”
Because of the warm November and early December, sea ice was also slower to build out from the Alaska coast. Sea ice got within maybe about 50 miles of St. Paul, Thoman said, but it did not ever reach there. On February 17, the sea ice reached its maximum extent in the Bering Sea, according to data from the National Snow and Ice Data Center (NSIDC) based in Colorado. This was the same date the ice reached its maximum last year.
Using satellite imagery, NSIDC researchers have a record of ice extent that goes back to 1979. In earlier years, the sea ice maximum often occurred in mid-March or early April. But the sea ice tends to shrink earlier in more recent years.
“I think at this point, we need to recognize that the peak extent of sea ice in the Bering Sea maybe used to be in mid-March, but it’s pretty clear that’s not the case anymore,” Thoman said.
The size of the extent is also shrinking. Thoman produced a graph that showed that the median sea ice extent hovered around 330,000 square miles in the Bering Sea in the 1980s, 1990s and early 2000s. But in the last decade, that median has dropped to about 270,000 square miles.
“What’s happened in the last 11 years, this is clearly a different regime,” Thoman said. “Even last year—when the ice not only got to St. Paul but actually for a few days also got to St. George in the Pribilof—was, by historic standards, pretty wimpy. Our sea ice extent maximum this year is pretty much right in line with this new normal that we live in.”
For the Arctic as a whole, sea ice maximum extent is also trending earlier. In their most recent analysis, NSIDC researchers found that Arctic sea ice likely reached its maximum extent for the year on March 6. That date is six days earlier than the March 12 average observed between 1981 and 2010.
With an extent of 5.64 million square miles, the 2023 Arctic-wide
maximum was the fifth lowest in the 45-year record of such measurements. The four years with lower sea ice maximums were 2017, 2018, 2016 and 2015, according to NSIDC data. The top 10 years all occurred since 2007.
When the sea ice reached its maximum this year, its extent was not only below average in the Bering Sea but also in the Sea of Okhotsk, the Barents Sea and the Labrador Sea. The sea ice extent was well below average in the Gulf of St. Lawrence. The Greenland Sea was the rare area that experienced above average sea ice extent, according to NSIDC.
It’s not only important for researchers to understand the extent of
the sea ice but the quality of the ice, too. Until recently, scientists had a hard time assessing ice thickness in the Bering Sea with satellite data.
“It’s been possible since the early 2000s to get an estimate of the thickness of thick sea ice from satellite data, and that’s worked reasonably well over the high Arctic net,” Thoman said in his talk. “For our part of the world, for ice less than about three feet thick, what’s called radar altimetry does not work very well. It works good for that thick ice. It does not work so well for the thin ice.”
But recently scientists at the Alfred Wegener Institute for Polar and Marine Research in Germany were able to merge sets of data on both
thick ice and the thin ice.
“That’s been a real advance,” Thoman said.
The data on ice thickness, however, only extends back about a decade, making it difficult for scientists to compare current conditions with those in the past. At least compared to the average of this last decade, sea ice was thicker in mid-March in certain spots throughout the Bering Sea, such as along the south shores of St. Lawrence Island Bay and Norton Sound. Thoman attributed this to the repeated storms and south winds that pushed up ice in these areas.