Glaciers are a significant resource of mountain ranges in Alaska and a dominant feature on Mount McKinley. The glaciers of Denali National Park and Preserve, Alaska, are vast, covering 3,779 km² (1,459 mi²), approximately one-sixth of the park’s area. Each year, these glaciers gain mass whenever snowfall accumulates at the surface, and they lose mass primarily through surface melting (ablation) during the summer. A glacier’s mass balance is the difference between accumulation and ablation and describes the overall health of the glacier. Generally, the average summer season temperature (May to September) drives total ablation for any given year and total winter snowfall drives accumulation. When deviating mass balance trends persist for many years they can result in significant landscape changes that can alter park visitors’ experience. These trends have a direct influence on a wide range of hydrologic, ecologic, and geologic systems. For example, glaciers provide a steady base flow of freshwater discharge upon which many ecosystems thrive, and when glacier meltwater and sediment discharge change because of a change in mass balance, these ecosystems are altered. Glaciers also advance and retreat in response to changes in mass balance, which can create new or destroy existing habitat and contribute to changes in the climate of alpine regions. Glacier behavior has a large influence on the braided river systems that are a common feature of the mountain landscapes of Alaska. On a global scale, long-term trends in glacier mass balance can make significant contributions to changes in sea level.
Each of the world’s glaciers is a unique entity and has a different set of physical characteristics that dictate how it responds to changes in mass balance and hence climate. Corresponding to the wide variety of mountain shapes and sizes in the Alaska Range is a large array of shapes, sizes, and behaviors among glaciers (Molnia 2008). Important characteristics that determine glacier behavior include size, elevation range, aspect, slope, number and arrangement of tributaries, the area-altitude distribution (hypsometry), and the underlying and surrounding geology of the glacier’s basin. Areas with readily erodable bedrock tend to have large areas of surface ice covered by rock debris, which can insulate the ice on the lower glacier, retarding melt rates, changing flow rates, and masking areal retreat. Many glaciers in the Alaska Range exhibit surge-type behavior, which is a periodic acceleration of all or part of the glacier to speeds of 10 to 100 times the normal quiescent speed. Surge-type glaciers may advance in a seemingly unpredictable way when other glaciers are retreating. With such a variety of glaciers it is imperative to have a thorough characterization of the glacier population and of which glaciers can represent the population as study glaciers.
The National Park Service has monitored the mass balance of two glaciers in Denali National Park since 1991. These field measurements give detailed information on mass variations at specific locations, but they have limited spatial coverage. Recently, satellite and airborne technologies have begun to provide information on glacier variations that span broader geographic regions. This article presents long-term field and recent remote sensing data sets that we combine in order to assess the following questions: What are the spatial patterns of ice loss and gain? What are the variability and trends of ice loss and gain through time? What glaciers are representative of and can serve as indexes of the changes taking place among the larger population of glaciers? Do all Denali glaciers exhibit a melting trend attributable to global or regional climate change?