Glacier Trends and Response to Climate in Denali National Park and Preserve

36 PARK SCIENCE • VOLUME 28 • NUMBER 2 • SUMMER 2011 Glacier trends and response to climate in Denali NPS/GUY ADEMA National Park and Preserve By Rob Burrows, Samuel Herreid, Guy Adema, Anthony Arendt, and Chris Larsen Figure 1. Researcher Adam Bucki records data from test pits and an ablation stake at the index measurement site on Kahiltna Glacier. LACIERS ARE A SIGNIFICANT ter park visitors’ experience. These trends Each of the world’s glaciers is a unique resource of mountain ranges in have a direct infl uence on a wide range of entity and has a diff erent set of physical GAlaska and a dominant feature on hydrologic, ecologic, and geologic systems. characteristics that dictate how it responds Mount McKinley. The glaciers of De- For example, glaciers provide a steady base to changes in mass balance and hence nali National Park and Preserve, Alaska, fl ow of freshwater discharge upon which climate. Corresponding to the wide variety are vast, covering 3,779 km2 (1,459 mi2), many ecosystems thrive, and when glacier of mountain shapes and sizes in the Alaska approximately one-sixth of the park’s meltwater and sediment discharge change Range is a large array of shapes, sizes, and area. Each year, these glaciers gain mass because of a change in mass balance, behaviors among glaciers (Molnia 2008). whenever snowfall accumulates at the these ecosystems are altered. Glaciers also Important characteristics that determine surface, and they lose mass primarily advance and retreat in response to changes glacier behavior include size, elevation through surface melting (ablation) during in mass balance, which can create new or range, aspect, slope, number and arrange- the summer. A glacier’s mass balance is destroy existing habitat and contribute to ment of tributaries, the area-altitude distri- the diff erence between accumulation and changes in the climate of alpine regions. bution (hypsometry), and the underlying ablation and describes the overall health of Glacier behavior has a large infl uence on and surrounding geology of the glacier’s the glacier. Generally, the average summer the braided river systems that are a com- basin. Areas with readily erodable bedrock season temperature (May to September) mon feature of the mountain landscapes tend to have large areas of surface ice drives total ablation for any given year and of Alaska. On a global scale, long-term covered by rock debris, which can insulate total winter snowfall drives accumula- trends in glacier mass balance can make the ice on the lower glacier, retarding melt tion. When deviating mass balance trends signifi cant contributions to changes in sea rates, changing fl ow rates, and masking persist for many years they can result in level. areal retreat. Many glaciers in the Alaska signifi cant landscape changes that can al- Range exhibit surge-type behavior, which FEATURES 37 values based on knowledge of the density Abstract of snow or ice that has accumulated or Glaciers cover approximately one-sixth of Denali National Park and Preserve in Alaska. ablated at that place. Snow and ice ablated They are not only enjoyed by visitors for their scenic and recreational values but are also an during the summer is termed summer bal- important driver of Denali’s diverse ecosystems by defi ning the hydrologic regime, generating ance. The units for water equivalent values landscape-scale braided river systems, and shaping the landforms of the Alaska Range. Scientists from the National Park Service and University of Alaska–Fairbanks have been are meters water equivalent (m w.e.). The researching glacier dynamics and monitoring glacier trends for more than 20 years using sum of the winter and summer balances glacier outlining from satellite imagery, index mass balance measurements, longitudinal is the net balance, which is a measure of elevation profi les, repeat photography, analysis of regional gravity changes, and other the annual gain or loss. Researchers then localized research. Mass balance measurements on the Traleika and Kahiltna glaciers showed extrapolate the point observations to the a cumulative net gain of mass from 1991 to 2003. Since 2003 the mass balance data and entire glacier surface, based on measured satellite-based gravimetric analysis show a net loss of ice mass. Airborne laser and lidar elevation profi les corroborate the mass balance measurements and also show localized or assumed gradients in the various mass changes because of glacier dynamics. Analysis of glacier extent reveals an 8% loss in area balance terms, and calculate the mass since 1950; however, whereas most glaciers lost area, a few surge-type glaciers gained area. balance of the entire glacier. Data from Repeated photographs from historical images help refi ne trends seen in other methods and that glacier may be used to represent other include dramatic examples of smaller glaciers decreasing in size and a surge-type glacier unmeasured glaciers in a region. Glaciers that has not changed as noticeably. Relations to climate trends are complicated and clearly demonstrate the importance of local infl uences on glacier behavior and trends. Losses of chosen to represent broader regions in ice from smaller glaciers and dramatic changes of other glaciers in Alaska suggest that this way are known as “index glaciers,” global climatic change may be overwhelming the dominant infl uence of large-scale climate and are usually chosen based on their oscillations on glacier change in Denali. location, physical characteristics, and ease of access. Key words climate change, Denali, glacier, Kahiltna Glacier, mass balance, Traleika Glacier In Denali National Park, Larry Mayo (formerly of the U.S. Geological Survey), and Keith Echelmeyer (formerly of the is a periodic acceleration of all or part are the variability and trends of ice loss University of Alaska–Fairbanks) chose the of the glacier to speeds of 10 to 100 times and gain through time? What glaciers are Kahiltna and Traleika glaciers as appropri- the normal quiescent speed. Surge-type representative of and can serve as indexes ate indexes because they capture a broad glaciers may advance in a seemingly of the changes taking place among the range of elevation and climate gradients unpredictable way when other glaciers are larger population of glaciers? Do all Denali spanning from the south to north side retreating. With such a variety of glaciers it glaciers exhibit a melting trend attribut- of the Alaska Range (Mayo 2001). These is imperative to have a thorough charac- able to global or regional climate change? researchers established a single observa- terization of the glacier population and of tion site on each glacier near the long-term which glaciers can represent the popula- equilibrium line altitude (ELA), the ap- tion as study glaciers. Mass balance proximate line at which the average glacier measurements mass balance is zero, and they determined The National Park Service has monitored mass balance gradients that are used to the mass balance of two glaciers in De- Direct fi eld measurements are a vital calculate the entire glacier mass balance nali National Park since 1991. These fi eld component of long-term glacier monitor- from this location. measurements give detailed information ing programs. At the end of the winter on mass variations at specifi c locations, accumulation season, glaciologists probe Index data collected since 1991 tell the but they have limited spatial coverage. snow depths, dig pits, and measure snow story of variability and change in space Recently, satellite and airborne technolo- density to determine the mass gained on and time (fi g. 2, next page). The aver- gies have begun to provide information the glacier over winter (winter balance). age 1991–2010 net balance at the Kahiltna on glacier variations that span broader To track ablation these scientists install Glacier site (shown in fi g. 1) was 0.19 ± 0.27 geographic regions. This article presents vertically oriented stakes in the snow and m w.e., showing that it is just above the long-term fi eld and recent remote sensing ice surface and revisit these locations to long-term ELA, while the average 1991– data sets that we combine in order to as- determine the change of the surface rela- 2010 net balance at the Traleika site was sess the following questions: What are the tive to the height of the stake (fi g. 1). These −0.62 ± 0.32, showing that it is below the spatial patterns of ice loss and gain? What changes are converted to water-equivalent long-term ELA (fi g. 2A). The net balance 38 PARK SCIENCE • VOLUME 28 • NUMBER 2 • SUMMER 2011 Net Balance at the Long-term ELA Cumulative Balance at the Long-term ELA with Five-year Moving Averages with Trend Line 1.5 3.5 Kahiltna Moving Avg Kahiltna KahiltnaKahiltna 3.0 Traleika Moving Avg 1.0 Traleika Traleika Traleika 2.5 0.5 2.0 1.5 0.0 1.0 −0.5 0.5 Net Balance (m w.e.) 0.0 −1.0 Cumulative Balance (m w.e.) −0.5 −1.5 −1.0 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 AB Balance Year Balance Year Figure 2. Net and cumulative mass balances on the Kahiltna and Traleika glaciers derived from index stake measurements show relative trends of two of Denali’s largest glaciers. Graph A (left) shows the yearly net balance data adjusted to the long-term equilibrium line altitude, along with the fi ve-year moving averages (solid lines). Graph B (right) shows the cumulative net balance and more correctly shows changes in balance that will drive advance or retreat.

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