The thunder of collapsing ice
In Kenai Fjords National Park, the sound of "white thunder" echoes across the water as massive blocks of ice shear off the faces of tidewater glaciers. This process, known as glacial calving, is a constant and dramatic display of natural forces. The park's glaciers originate in the Harding Icefield, a vast expanse of ice covering over 700 square miles (1,813 km2). Nearly 40 glaciers flow from this icefield, slowly grinding their way through mountain valleys toward the sea.
When these rivers of ice reach the ocean at the end of a fjord, they become tidewater glaciers. The interaction with the comparatively warm, buoyant seawater destabilizes the glacier's front, or terminus. This leads to the fracturing and collapse of ice, creating icebergs. The immense weight of the falling ice—sometimes many tons at once—crashing into the bay generates significant seismic waves. These "icequakes" are recorded by the Alaska Earthquake Center and can be equivalent to a magnitude 2 or 3 earthquake. The most active calving glaciers in the park include Aialik Glacier and Holgate Glacier.
The intense blue color visible in the freshly calved ice is a result of its density. The ice has been compressed over centuries, forcing out air bubbles that would otherwise scatter light. This dense ice absorbs all other colors of the light spectrum, reflecting only the blue wavelengths.
An ecosystem fueled by meltwater
Glacial calving is a fundamental process that shapes the local marine ecosystem. The introduction of large quantities of freshwater and ice into the salty fjord has deep effects. The violent turbulence caused by calving stirs up the water column, bringing deep, nutrient-rich water to the surface. This upwelling, combined with the nutrients released from the melting freshwater, fuels massive blooms of phytoplankton.
These microscopic organisms form the base of the food web in the fjords. Fish thrive on this abundance, which in turn supports a large population of seabirds, such as black-legged kittiwakes and puffins. The newly formed icebergs also have a direct purpose for wildlife. Harbor seals (Phoca vitulina) use the floating ice floes as "haul-outs"—safe platforms to rest, give birth, and nurse their pups away from predators like orcas and bears. A newborn harbor seal pup weighs around 25 pounds and can gain a pound and a half daily while nursing on these icy refuges.
A measure of a changing climate
While calving is a natural part of a tidewater glacier's life cycle, its rate shows environmental change. A 2022 study by the University of Washington and the National Park Service analyzed 38 years of satellite imagery for 19 of the park's major glaciers. The study found that 13 of the glaciers showed substantial retreat.
Lake-terminating glaciers, those ending in freshwater lakes, are retreating the fastest. Bear Glacier, for instance, retreated 5 kilometers (3 miles) between 1984 and 2021. Pedersen Glacier retreated 3.2 kilometers (2 miles) in the same period. Overall, from 1984 to 2021, the 19 glaciers studied lost a cumulative 42 square kilometers of ice. This acceleration in ice loss is linked to warming air and ocean temperatures, which increase melting and destabilize the glacier fronts, leading to more frequent calving events.