Arctic Coasts in Motion: 64 Years of Shoreline Change in Two Alaska Parks

The ice-free season along the Chukchi Sea coast of two Alaska parks has been growing by about 10 days per decade, which adds up to more than a month of extra exposure to waves and storms. A team from the University of Alaska Fairbanks and the Alaska Division of Geological and Geophysical Surveys (Louise Farquharson, Richard Buzard, Daniel Mann and Benjamin Jones) asked what that has done to the coastline of Bering Land Bridge National Preserve and Cape Krusenstern National Monument. Their article appeared in the National Park Service's Alaska Park Science.

A coast in constant flux

The parks' coasts run over 300 miles (480 km) and include ice-rich permafrost lowlands and bluffs, deltas, lagoons, sand dunes, spits and barrier islands. Sea ice, Arctic storm tracks, tides, waves and wind act on them at many scales of time and space, so the shoreline is always shifting. Eroding permafrost bluffs feed sand spits; barrier islands and beach ridges shift as sea level rises and storm surges move large volumes of sediment. That complexity makes future change hard for land managers to predict.

Permafrost is ground that stays frozen year-round. Much of the coast is bluffs of alternating frozen silt and massive bodies of ice, and when the ice melts the surface sinks and the sediment loses its cement. Three things are pushing in that direction:

• Warming ground. Mean annual ground temperatures in the preserve are projected to rise by up to 3°C by 2050, which should speed thaw of the bluffs.
• Warming sea. The Chukchi Sea surface has warmed by 0.5°C per decade since 1982, and warmer water thaws bluffs faster, a process called thermoerosion.
• Less sea ice. Ice protects bluffs from waves and blocks sediment movement along the shore. Arctic sea ice extent and thickness have declined by more than 10% per decade since satellite observations began, and in the southern Chukchi Sea the time that ice stays frozen to shore has shortened by about a week per decade over 40 years.

How they measured it

The team traced the edge of coastal bluffs on aligned aerial photographs from 1950, 1980 and 2003 and on high-resolution satellite images from 2014, a 64-year record, and compared the lines with the Digital Shoreline Analysis System. They also compared elevation data sets from 2003 to 2016 and checked the digital results with field surveys at long-term monitoring sites, measuring the distance from permanent markers to the edge of a bluff, beach ridge or foredune. Satellite records from the National Snow and Ice Data Center gave the number of ice-free days and the days when waves could travel more than 50 km before reaching land, because waves gain energy over longer distances.

What they found

The ice-free season has lengthened by about 10 days per decade since satellite observations began, because ice breaks up earlier in spring and forms later in fall. That is on average one extra day of exposure every year, and also less sea ice in the summer and fall storm season, so waves travel farther and hit harder for longer.

Along permafrost bluffs, warm air and water melt the binding ice and sediment slumps onto beaches. Thaw of ice wedges from the top down forms gullies, which worsen the erosion; measurements between 2003 and 2016 suggested erosion was faster where gullies were present, and the gullies were nearly 2 m deep. Offshore, coarser sediment such as sand and pebbles moves along the shore while finer material is drawn into deeper water. At barrier islands the processes differ: sediment carried along the shore builds spits and feeds barrier islands downdrift, and in storms sand washes across the islands into lagoons as overwash fans.

Erosion has dominated, though some places gained sediment.

Negative numbers mean the shoreline moved inland. In the preserve, the fastest-changing coast was the low-lying barrier islands, with mean rates up to -5 feet/year (-1.53 m/yr) between 2003 and 2014, while ice-rich bluffs changed more slowly, at a maximum of -1.7 feet/year (-0.51 m/yr) between 1950 and 1980. In both parks, the spread between the fastest and slowest rates was greatest between 2003 and 2014. That variability makes it difficult to predict how continuing sea-ice decline will affect overall rates.

An uncertain future

The coasts have become more dynamic over six decades, probably in part because of the longer open-water season. They show no sign of being suddenly transformed. But as the open-water season keeps lengthening and bluff permafrost warms, erosional shores may erode faster, and shores now gaining sediment may gain faster or turn to erosion. The authors say that knowing how the coast changed, along with continued monitoring, can help manage coastal habitats and ecosystems, oil-spill response, marine debris collection and the preservation of cultural artifacts in the two parks, which also hold archaeological sites, records of the Bering Land Bridge's past and subsistence resources for local communities.

Sources

• L. M. Farquharson, R. M. Buzard, D. H. Mann and B. M. Jones, Coastal Dynamics in Bering Land Bridge National Preserve and Cape Krusenstern National Monument, Alaska Park Science 18(1), National Park Service; rewritten in hubnx's own words.