Glacier Mass Balance: equilibrium or disequilibrium response?

Figure 1. CRYSYS (CRYospheric SYStem in Canada) mass balance program (M. Demuth) monitors the Place and Helm Glacier in the British Columbia, Coast Range north of Vancouver. The USGS benchmark program monitors South Cascade Glacier, North Cascades, WA (R. Krimmel), Wolverine Glacier, Kenai Mountains, AK and Gulkana Glacier, Alaska Range, AK (R. March). The Juneau Icefield Research Program monitors the mass balance of Lemon Creek Glacier, Coast Range, AK (M. Miller and M. Pelto). The North Cascade Glacier Climate Project monitors the mass balance of Columbia, Daniels, Easton, Foss, Ice Worm, Lower Curtis, Lynch, Rainbow, Sholes, and Yawning Glacier, North Cascades, WA (M. Pelto).

Figure 2. The annual mass balance signal is remarkably coherent from glacier to glacier.

Annual mass balance measurements over at least 20 years are available on 16 glaciers in North America which are submitted to the WGMS. The annual balance and cumulative balance records of these glaciers illustrate the adjustment of these glaciers to climate change from 1984-2005 (Figure 1-2). The annual balance record, in meters of water equivalent, 1984-2005 indicates that despite continued significant retreat of all 16 glaciers, mass balances are declining. The mean annual mass balance has declined from -0.35 m/a 1984-1990, to -0.59 m/a in 1991-1997, to -0.91 m/a from 1998-2005. Instead of approaching equilibrium after several decades of dominantly negative annual balance, and significant retreat, mass balance is becoming more negative. This indicates the glaciers are getting further away from equilibrium, not approaching it. The consistency of the cumulative balance trends further indicates the glaciers mass balance record is dominated by large scale regional to global climate change.

Which glaciers are experiencing an equilibrium response? Glacier thinning in the accumulation zone is limited on Lemon Creek (Miller and Pelto, 1999), Gulkana and Wolverine (Sapiano, et. al, 1998), and Easton, Lynch and Rainbow Glacier (Pelto, 2006). Each appears able to retreat to a new point of equilibrium with the current climate. On Columbia, Daniels, Foss, Ice Worm, Lower Curtis, Sholes, and Yawning Glacier thinning is not notably less in the accumulation zone than the ablation zone, indicating disequilibrium (Pelto, 2006). Two North Cascade glaciers where mass balance measurements were conducted the Spider and the Lewis Glacier disappeared during this interval (Pelto, 2006). On Helm and Place Glacier the loss of more than 20% of their entire volume, since 1984 suggest they too are in disequilibrium. South Cascade Glacier has lost 70% of its volume since 1890 (Josberger et al., 2006), and is still experiencing large negative balances indicating disequilibrium. In Glacier National Park, North Cascades, Helm Glacier and Place Glacier frequent loss of the entire snowcover by the end of the ablation season has become commonplace (WGMS, 2005). The result is in net ablation throughout the accumulation area causing thinning of the glacier in the accumulation zone.

Are the glaciers responding primarily to climate changes of the last 30 years, or to the post Little Ice Age conditions? In the North Cascades Rainbow, Yawning, Easton and Lower Curtis Glacier all advanced during the 1950-1975. Sholes, Columbia, Ice Worm, did not advance or retreat significantly during this period. Thus, each of these glaciers is definitively responding to the climate of the last 30 years.

Figure 3. Lower Curtis Glacier in 2003 and in 1908 Photograph by namesake Asahel Curtis

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