Rapid Coastal Erosion Correlated to Diminishing Sea Ice

Retreating sea ice leaves the Alaskan coast vulnerable to the full force of the ocean. Photo: Benjamin Jones, USGS

Rapid erosion of the northern coastline of Alaska midway between Point Barrow and Prudhoe Bay is accelerating at a steady rate of 30 to 45 feet a year, according to CPS-supported scientists presenting a study at the annual American Geophysical Union meeting this week in San Francisco. As the coast erodes, frozen blocks of silt and peat that contain 50 to 80 percent ice topple from bluffs into the Beaufort Sea during the summer.

The acceleration is caused by a combination of large waves pounding the shoreline and warm seawater melting the base of the bluffs, said CU-Boulder Associate Professor Robert Anderson, a co-author on the study. Once the blocks fall they melt within days and sweep silt material out to sea.

Anderson, along with collaborators Cameron Wobus of Stratus Consulting and Irina Overeem of CU’s Institute of Arctic and Alpine Research (INSTAAR) have studied the coastline for the past two summers with Office of Naval Research support. Equipped with two meteorology stations, a weather station, time-lapse cameras, detailed GPS and wave sensors outfitted with temperature loggers, they documented the summer ocean/shore dynamic.

Triple Whammy

Declining sea ice, warming sea water, and increased waves create a “triple whammy” that expedites erosion. For the majority of the year, the Beaufort Sea is covered with sea ice that disconnects from the coast during the summer. These ice-free summer conditions are lasting for longer periods of time, allowing warmer ocean water to lap the coast and weaken the frozen ground. And the longer that sea ice is not connected to the coastline, the further the distance grows between the ice and the shore.  This open-ocean distance between the sea ice and the shore, known as “fetch,” increases both the energy of waves crashing into the coast and the height to which warm seawater can come into contact with the frozen bluffs, said Anderson.

The shoreline bluffs are made up of contiguous, polygon-shaped blocks, primarily made of permafrost and measuring roughly 70 to 100 feet across. Ice “wedges” (created by seeping summer surface water that annually freezes and thaws) are driven deep into the cracks between individual blocks each year. The blocks closest to the sea are undermined as warm seawater melts their base, and eventually split apart from neighboring blocks and topple during stormy conditions, said Anderson.

Impacts of Erosion

As the coastline submits to the ocean, old whaling stations, military and oil related infrastructure and entire towns threaten to fall into the sea. In addition, the loss of sea ice alters ocean systems and diminishes habitat for creatures like the polar bear.

According to a 2009 CU-Boulder study, Arctic sea ice during the annual September minimum is now declining at a rate of 11.2 percent per decade. This year, only 19 percent of the ice cover was more than two years old — the least ever recorded in the satellite record and far below the 1981-2000 summer average of 48 percent.