Response of glaciers to changes in mass balance

As we discussed in Chapter 3, climatic change may involve changes in precipitation, in temperature, in radiation balance, or most likely in all three of these variables. From the perspective of a glacier, however, the net effect is to change the amount and the spatial distribution of accumulation and melt. This leads to discrepancies between the specific net balance and the local emergence or submergence velocity, and hence to changes in glacier geometry (see pp. 90-92).

Were the climate of a region to remain constant for a long time, several decades or even centuries, the geometry of non-surging glaciers in that region would adjust so that the specific net balance was everywhere equal to the local emergence or submergence velocity, and in addition (or as a consequence) the integral of the specific net balance over the glacier, Bn = f bndA, would be zero. The glacier would then be said to be in a steady state, an ideal that may occasionally be approached but rarely, if ever, reached.

The principal adjustment that takes place is, of course, a change in length or size. A positive mass balance, maintained over a period of years, will result in an advance. As the glacier expands to lower elevations or more southerly latitudes, the summer balance becomes more negative until it becomes equal (in magnitude) to the winter balance, and the net balance returns to zero, and conversely.

The goal of this chapter is to study the details of the adjustment process. In particular, we will see that changes in mass balance lead to changes in thickness which influence the speed of the glacier, and hence the rate at which ice is transferred from the accumulation area to the ablation area. The changes in thickness propagate and diffuse down the glacier. Thus, the propagation and diffusion processes control the way in which the profile adjusts to the new mass balance conditions. In addition, the total time required for the adjustment depends on the volume of ice that must be gained or lost in order to reach a new steady state. Thus, years can elapse before the terminus gets the message that something has happened higher on the glacier, and decades may pass before it adjusts to the changes.

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