Bear glacier retreat – From a Glaciers Perspective

**Change in terminus position of coastal glaciers in the Sewqrd region. On this 2023 Landsat image yellow dots are ther 2023 terminus and red dots the 1986 terminus.**

Turn east along the coast from Seward Alaska and you are offshore of the Sargent Icefield, turn west and you are offshore of the Harding Icefield. Each has several large glaciers that were either tidewater or end in lakes separated from the ocean by a narrow coastal zone. Here we compare the response to climate change of seven of these glaciers from 1986-2023.

**Change in terminus position of coastal glaciers in the Sewqrd region. On this 1986 Landsat image-yellow dots are ther 2023 terminus and red dots the 1986 terminus.**

Travelling south down Resurrection Bay and turning east you enter into Day Harbor, at the head of which is the outlet for Ellsworth Glacier. The glacier terminates in an expanding lake that was 3.5 km long in 1986 and in 2023 is 8 km long. The 4000 m retreat generated some large icebers still in the lake.

**Elsworth Glacier retreat from 2016-2020 leading to quick lake expansion in Landsat Images**

Travelling east from Day Harbor 15 km brings you into Johnstone Bay. Excelsior Glacier termiates just inland from the coast is a rapidly expanding lake. The lake was 5 km long in 1986, and by 2023 is 10.5 km long, a retreat of 5500 m. The glacier has separated with the Roan Glacier terminus to the east retreating from the lake. The Excelsior Glacier terminus currently is steep and nearing the north end of the lake.

**Rapid expansion of Big Johnstone lake due to Excelsior Glacier retreat. Map from 1950, Landsat images 2001 and 2013.**

Heading south down Resurection Bay on the western shore is Bear Glacier. From 1950-1986 the glacier retreated 160 m, and by 1980 the terminus was calving small icebergs into an ice-marginal lake that was beginning to develop. As thinning continued, much of the terminus became afloat by 2000. Bruce Molnia, USGS observed that passive calving, characterized by the release of large tabular icebergs from Bear’s low gradient, floating terminus became frequent. Between 2000 and 2007, the terminus retreated about 3.5 km, yielding large icebergs that floated in the lake. The amount of calving has declined from the period of more rapid retreat from 2002-2008. Black et al (2022) reported Bear Glacier retreating 5170 m. losing 17.28 km²of area from 1984-2021. From 1986 to 2024 the glacier has retreated 6200 m, leading to a lake with an area of km2.

The retreat of Bear Glacier from 1986 to 2024 leading to growth of the lake to 22 km², in Landsat images.

Continuing around Aialik Peninsula and traveling north into Aialik Bay, along the western shore is Pedersen Glacier. The glacier drops quickly from the plateau of the icefield through a pair of icefalls terminating in a lake at 25 meters above sea level. Giffen et al (2014) observed that Pedersen Glacier retreated slow but steady from 1951-1986 at 706 m (20 m/a) and 434 m (23 m/year) from 1986-2005. Black et al (2022) reported a retreat of 3170 m and loss of 4.25 km² from 1984-2021. We note a 3500 m retreat from 1986-2023.

**Pederse Glacier retreat in Landsat images, leading to rapid lake expasion to 4 km².**

At the head of Aialik Bay is the tidewater Aialik Glacier. This glacier advanced 180 m from 1950-1986.From 1986-2006 the glacier retreated 290 m. Black et al (2022) observed the terminus was stable from 2000-2021 despite ongoing receession of the eastern margin of the glacier.

The Holgate Arm extends off of Aialik Bay on the west shore just south of Pedersen glacier. This tidewater glacier retreated 250 m from 1950-1986. The glacier has had several small periods of advance and retreat since 1986. Black et al (2022) note a period of advance from 2010-2021, the terminus has begun a small retreat from 2021-23 and is now just behind the 1986 position.

Northwestern Glacier is at the head of Northwestern Fjord off of Harris Bay, the next Bay west of Aialik Bay. This tidewater retreated 5200 m from 1950-1986, and an additional 1600 m since 1986. This retreat has led to a separation into two primary arms of the glacier. The rate of retreat has slowed since 2000 and the glaciers tidewater connection is limited, and will likely cease with even further minor retreat.

The Harding and Sargent icefield both have limited accumulation area above 1500 m. This means that they are prone to complete loss with a limited rise in snowline elevation. This is similar to the Juenau Icefield situation, where our research has indicated accelerated losses (APNews).

Bear Glacier in the Kenai Mountains of Alaska is a 25 km long outlet glacier of the Harding Icefield. When first mapped in 1909 it had a large piedmont lobe filling a basin and terminated 400 m from the edge of the forest, though it appeared the retreat had been quite recent within 25 years. By 1950 the glacier had retreated another 400 m. Austin Post observed in the 1960’s the drainage of a lake seven kilometers upglacier from the terminus indicating that slow thinning was occurring in the entire lower section of the glacier. The piedmont lobe slowly downwasted through the 20th century preconditioning the glacier for a rapid retreat. The repeat photographs of Bruce Molnia, USGS portray this spectacular change. By 1980 the terminus had retreated 1 km, thinned by 150 m, and was calving small icebergs into an ice-marginal lake that was beginning to develop in the still narrow basin. As thinning continued, much of the terminus became afloat by 2000. Bruce Molnia, USGS observed that passive calving, characterized by the release of large tabular icebergs from Bear’s low gradient, floating terminus became frequent. Between 2000 and 2007, the terminus retreated about 3.5 km, yielding large icebergs that floated in the lake. The first image is from Google Earth the second taken by Bruce Molnia in 2002 followed by GoogleEarth images from 2005 and 2011. The terminus position in 2005 is in green and 2011 in red, retreat was 400 m during this six year period. The last image is a Landsat image from 8/3/2012 and indicates continued retreat along the east margin and expansion of the length of the calving front. The amount of calving has declined from the period of more rapid retreat from 2002-2008. An examination of the expanding lake identified locations with depths exceeding 75 meters. Retreat indicates the response of the terminus region of a glacier to climate in this case long term downwasting, but does not reflect how healthy the higher regions of the glacier are. How does the large 150-200 m of thinning in the terminus region compare to further upglacier? The U of Alaska-Fairbanks established a program of laser altimetry examining thickness changes along the Bear Glacier that have been reported by A.Arendt, K.Echelmeyer and C.Larson. The graphs of the last decade of change indicate that little thinning has occurred above meters since 2007. In a satellite image of the region of the glacier from 2500-3500 feet, a small rock island (nunatak) generates a consistent and long term lateral moraine. in the lower center of the image belowThe size of the moraine and the size of this nunatak have not changed appreciably in the last decade, suggesting limited thinning in the region above 2700 feet. The accumulation zone of this glacier remains sizable, but insufficient to supply a large piedmont lobe-now gone, or a calving terminus. This is not unlike the retreat of the Gilkey Glacier, Norris Glacier or Field Glacier of the Juneau Icefield. In the St. Elias range Yakutat Glacier is another example.