Sir,

By illustrating the importance of correctly posing instability problems, Reference FowlerFowler (1980) has made a useful contribution. The possible existence of thermal instabilities in glaciers remains an unsettled question and we agree that further study with more realistic models is warranted. As Fowler has pointed out, there are obvious shortcomings in our work (Reference Clarke, Clarke, Nitsan and PatersonClarke and others, 1977) and that of Reference Yuen and SchubertYuen and Schubert (1979); however we do not accept some of his suggestions.

In our work and that of Yuen and Schubert, ice thickness is kept constant and it is shown that multiple solutions for ice flux can exist. Fowler correctly questions the assumption of holding thickness constant and shows that the instability may possibly disappear if different assumptions are made. According to his interpretation of Yuen and Schubert’s figure 3, the instability does not exist if ice flux is held constant and thickness allowed to vary.

Which starting point is of greater glaciological interest? We think that prescribing flux is dubious advice to follow since it immediately eliminates the one class of flow instability that we know occurs, namely glacier surges. The striking thing about surges is surely the ten- to hundred-fold increase in ice flux which is typical, not the 10% variation in ice thickness. As Fowler states, the flux must be compatible with mass conservation but, taking surging glaciers as our guide, we think his interpretation of this requirement is unduly restrictive. These glaciers display low- and high-flux states; the effect of mass conservation is not to fix the flux but to prevent the high-flux state from being sustained indefinitely; surges stop.

In summary, there are difficulties associated both with Fowler’s assumptions and our own. If neither constant thickness nor constant flux are acceptable starting points, it may prove impossible to make models both simple and realistic.