Grand challenges

There is a risk of pomposity and arrogance in even contemplating a list of grand challenges. Everyone will have their own list. However, in the belief that it is sometimes useful and humbling to highlight what we do not know, here is my list.

1 The West Antarctic Ice Sheet stands out as a potential threat in that if it approaches a threshold of collapse, it could raise global sea level by 6 m and change the ocean and atmospheric circulation in the Southern Hemisphere. The signs that it has been on a trajectory of decline throughout the Holocene and that other parts of the ice sheet are thinning today could be significant. Immediate questions are: How secure and widespread is the evidence of Holocene decline? Is the Weddell Sea sector involved? How has the rate of change varied during the Holocene? What evidence is available from nunataks and the immediate offshore zone to improve our level of understanding? Is there a threshold of collapse? All these questions are answerable by a combination of field work and modelling.

2 The North Atlantic thermohaline circulation plays a fundamental role in initiating and/or modulating climate change. It is possible that global warming will lead to continued warming of the North Atlantic. It is also possible that an increase in the surface water flux in the Arctic is sufficient to close off the circulation and lead to cooling and a further Ice Age. Crucial unknowns are the volume of fresh water supplied by the Greenland Ice Sheet and the sensitivity of the locations at which it is incorporated in the thermohaline circulation. Here is a clear glaciological task that can be accomplished by a concerted effort.

3 Ice-sheet models are a sophisticated way of representing the behaviour of ice sheets and could be used for prediction if we had faith in them. At present there seem two major impediments. One is the lack of an effective sliding theory. Some would argue that the parameter is so important and knowledge is so limited that the predictive power of any current model is questionable. Another major problem is the lack of data about the morphology of the bed of current ice sheets. There are parts of Antarctica the size of western Europe with only a handful of depth soundings. Bearing in mind the importance of topography in determining the flow and thermal regime of an ice sheet, it is not easy to have faith in model predictions based on such sparse information, however sophisticated the glaciological components of the model.

4 There is a wealth of data on past glacier behaviour at the time the world changed from a glacial to interglacial mode between the onset of deglaciation ca. 17,500yr ago and the start of the Holocene ca. 11,500yr ago. High-resolution dating for this time span is hindered by the error margins associated with radiocarbon and surface-exposure age dating. In particular it is difficult to obtain firm evidence on which to test rival theories about leads, lags and synchrony of glacier fluctuations. Higher resolution dating over this period would firm up our understanding of the mechanisms of abrupt climate change.

5 The interdisciplinary approach that brought together glacio-logical theory and empirical evidence has proven its value over the past decades. Is it possible to detect a weakening of the link, perhaps because we are in danger of taking it for granted? There are currently two areas where some in the field community are using arguments that are controversial among glaciologists, namely the importance of thrusting in creating marginal moraines and the assumption that lateral meltwater flow indicates a cold-based ice margin. One suspects that discussion among the wider cross-disciplinary community would lead to deeper understanding on such issues.

6 Finally, are we ready to contribute fully to the interpretation of remote sensing data obtained by new extraterrestrial probes? Already it is clear that there is an exciting and challenging range of new glacial forms and processes. Not only are they fascinating in their own right, but they add a humbling perspective to our earth-bound glaciers.

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