Holocene glacier fluctuations: is the current rate of retreat exceptional?

Most glaciers in the Northern Hemisphere reached their postglacial maximum in recent times, that is, after the medieval period. During the last 100 or 150 years a significant retreat has taken place, and there is little sign that this is coming to an end. The current worldwide shrinkage of glaciers is considered to be a strong indication of global warming. However, glacier retreat should be judged against the natural variability of glacier systems. Numerical glacier models can be used to quantify this variability. I have studied the natural variability of three glaciers for which long historic records of glacier length exist: Nigardsbreen, Norway; Rhonegletscher, Switzerland; and Franz Josef Glacier, New Zealand. Integrations for a 10 000 year period, driven by random forcing of a realistic strength, show that the current retreat cannot be explained from natural variability in glacier length and must be due to external forcing.