Sea ice is sticking to Alaska's northern coast for less time each year, according to 27 years of data analyzed by University of Alaska Fairbanks scientists.
Such landfast ice, which stays attached to the shoreline instead of drifting with winds and currents, also has covered less total area in recent winters.
The work led by research professor Andrew Mahoney of the University of Alaska Fairbanks Geophysical Institute was published in January in the Journal of Geophysical Research: Oceans . Former UAF graduate student Andrew Einhorn is a co-author.
The new assessment extends the timeframe of a 2014 study by Mahoney that covered 1996-2008. It focuses on the Chukchi and Beaufort seas.
Landfast sea ice has been declining in the Chukchi Sea for decades. The new analysis found that the extent of Beaufort Sea landfast sea ice has also begun to decline in recent years after remaining relatively stable between the 1970s and early 2000s.
"Landfast ice is the ice that is used by people," Mahoney said. "It has a much more immediate connection with humans."
Residents travel across the stable ice to reach hunting and fishing areas. The oil and gas industry uses the frozen surface to build seasonal ice roads that connect to nearshore facilities. By remaining fixed in place, landfast ice also helps shield the shoreline from strong waves and allows river water to spread farther offshore.
"The shortening of the landfast ice season may matter even more for coastal communities than any loss of ice area during that season," Mahoney said, "because it leaves shorelines more exposed to waves and makes hunting conditions much more uncertain."
The landfast ice season has shrunk mostly because the ice is forming later in the year. Even after air temperatures drop below freezing in the fall, the ocean is staying warm longer, so it takes more time for solid ice to develop along the coast.
From 1996-2023, the landfast season has shortened by 57 days in the Chukchi Sea and 39 days in the Beaufort Sea. In the Chukchi, that's due to later ice attachment and earlier ice detachment. In the Beaufort, it's due to later ice attachment only.
Sea ice can attach to land in several ways. Newly formed sea ice can freeze directly to the coastline, anchor to a shallow seafloor or bond with grounded ice ridges. These ridges are jumbles of sea ice blocks pushed to the coast, where they pile up and become thick enough to sit on the seafloor.
"Landfast ice is diminishing with the rest of the ice in the Arctic," Mahoney said. "In some ways it is following the same trends as we see in the rest of the Arctic, but we are also seeing some new changes."
The decline in Beaufort Sea landfast ice is reflected in the percentage of total landfast sea ice on the U.S. Outer Continental Shelf. The total decreased from 3.8% in the first nine years of Mahoney and Einhorn's 27-year record to 2% in the final nine years, 2014–2023.
The two scientists found that the Beaufort's landfast sea ice was not extending as far from shore in recent years. It previously could reach waters near 20 meters deep annually, distinguishing the Beaufort Sea from other regions of the Arctic where landfast ice retreat had already been observed.
They speculate the recent decline is related to the overall thinning of Arctic sea ice, which results in the creation of fewer ice ridges with bottoms deep enough to become grounded on the seafloor and anchor the ice.
"We are seeing evidence that grounded ridges are not forming where they used to," Mahoney said.
Additional research is needed to better understand why, Mahoney said.
"This is where the chicken and egg part of it comes in," he said, "because once a ridge becomes grounded, it acts like a traffic jam; additional ice piles up into it and it becomes larger and larger."
"But we don't yet know whether the action that starts the ridge just isn't happening or whether the traffic jam afterward isn't happening," he said. "For one reason or another, we don't see evidence of grounded ridges where they had been forming, and that's the outcome you would expect if the ice is getting thinner."
The new extended work uses data from the National Ice Center and the National Weather Service Alaska Sea Ice Program.
