Climatic Effects in Unstable Glacier Response

Climate can trigger two different kinds of unstable response in ice masses, the first a change in the flow regime, and the second a rapid growth or decay without significant change in flow. In the first category, a comparison of the twentieth century surge history of Variegated Glacier with mass balance information, some of it obtained by correlation with long term climate data, shows that surges occur when the cumulative balance at a representative point in the accumulation area reaches a critical level. This provides a basis for understanding the surge intervals, which vary from 12 to about 18 years. In the second category, it has been known for many years that glaciers and ice sheets with low surface slope are particularly sensitive to climate because a small climate warming, for example, can increase the equilibrium line elevation enough to cause a large reduction in the accumulation area. This would cause a large ultimate reduction in the size of the ice mass or even its disappearance. Consideration of the effect of the elevation of the terminus on the mass balance rate there suggests, surprisingly, that steep glaciers may exhibit a similar large or unstable response. The effect is a non-linear one and does not show up in the traditional linear theory. It suggests that some (perhaps many) glaciers are so close to instability that their future behavior under present climate conditions is essentially unpredictable with any model.