One glacier's retreat, a global warming does not make: An argument for systematic monitoring of North American glaciers

One predicted response of the Earth to global change is a warming in the northern latitudes, with an increase in the rate of glacier melting and retreat. Since 1986, the terminus of Mendenhall Glacier, a 15-kilometer-long valley glacier near Juneau, Alaska, has been retreating rapidly, at a rate more than ten times greater than its rate during the past few decades. At first glance, some might attribute this rapid retreat to global warming. In reality, the changes at Mendenhall Glacier have little or nothing to do with climatic warming or global change. Rather, ironically, they are the result of an unusual sequence of events, triggered by a small advance of the glacier. In 1984-85, a minor advance of Mendenhall Glacier dammed Nugget Creek, whose waters then overflowed on to the glacier and created a supra-glacial lake. Located a few hundred meters from the terminus, the lake expanded by ablation and iceberg production. During the summer of 1988, the expanding lake breached the remaining ice of the glacier terminus and merged with Mendenhall Lake. Today, the glacier's terminus has a high rate of iceberg production that contributes to the continuing rapid retreat of the glacier. In this instance, the authors were able to closely monitor the sequence of events and the changes at the glacier's terminus, and so determine that the Mendenhall Glacier responded to a sequence of non-climatic events. If they had only a pre-1985 observation and then were to see the Mendenhall Glacier in 1990, their interpretation of the events and changes at the Mendenhall terminus might have been far different, and incorrect. Unfortunately, with most other glaciers, there may be many years between observations. Glaciers may be sensitive indicators of global change, but we must be very cautious about attributing all future changes in glaciers to global change or climate warming