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Polar Ice Sheets and Global Sea Level:
How Well Can We Predict the Future?

Volume 5, Number 30: 24 July 2002

The query imbedded in the title of this week's Editorial is borrowed from a provocative new paper that suggests humanity should have some idea of the answer to this question before we rush headlong into adopting expensive and economy-wrenching measures to battle what could well turn out to be an imaginary enemy.  The author - C.J. van der Veen of the Byrd Polar Research Center at Ohio State University - puts it this way: "for purposes of formulating policies, some of which could be unpopular or costly, it is imperative that probability density functions be derived for predicted values such as sea level rise."  Hence, the stated objective of his study was "to evaluate the applicability of accumulation and ablation models on which predicted ice-sheet contributions to global sea level are based, and to assess the level of uncertainty in these predictions arising from uncertain model parameters."

After lengthy analyses of various topics related to these objectives, van der Veen arrives at some important conclusions that the general public has not only a right, but a need, to know.  In this regard, for example, the polar scientist notes that with "greater societal relevance comes increased responsibility for geophysical modelers to demonstrate convincingly the veracity of their models to accurately predict future evolution of the earth's natural system or particular components thereof."  In stepping forward to perform this task for glaciological modelers, however, he is forced to conclude that "the validity of the parameterizations used by [various] glaciological modeling studies to estimate changes in surface accumulation and ablation under changing climate conditions has not been convincingly demonstrated."

Some of the problems associated with model testing, of course, are observational, i.e., there must be a documented history capable of being simulated.  With respect to the mass balance of the Greenland Ice Sheet, for example, van der Veen notes that "it is currently not well known whether or not the ice sheet is growing or shrinking, although most studies agree that the whole of Greenland is not far out of balance in either direction."  Hence, if what is "known" is really not all that certain, there is little opportunity to assess model performance.  Furthermore, even if a model prediction turns out to be consistent with present or past observations, van der Veen notes that "there is no guarantee that the model will perform equally well when used to predict the future," especially if one of the model parameters extends into a range that is beyond the range within which the model was tested.

Admittedly, these observations appear to suggest that it is essentially impossible for a model to ever be "proven" to be a valid tool for assessing the likelihood of future events; and that perspective is correct.  At best, says van der Veen, models can only be confirmed "by matching observational data that were not used to calibrate model parameters."  But even then, considering the observations of the preceding paragraph, it really becomes a matter of faith as to how well one believes a model that has successfully replicated the past will predict the future.

Laying these considerations aside - but remembering they imply that whatever follows may be even less well defined than what is suggested by the numbers - van der Veen calculates that within the context of greenhouse-warming-induced sea level change, uncertainties in model parameters are sufficiently great to yield a 95% confidence range of projected contributions from Greenland and Antarctica that encompass global sea-level lowering as well as rise by 2100 A.D. for low, middle and high warming scenarios based on surface mass balance calculations.  Hence, even for the worst of the global warming projections - which could well be way off base itself, as we personally believe it is - there could be little to no change in mean global sea level due to the ongoing rise in the air's CO2 content.

In view of these findings, van der Veen concludes that the confidence level that can be placed in current ice sheet mass balance models "is quite low."  Paraphrasing an earlier assessment of the subject, in fact, he says that today's best models "currently reside on the lower rungs of the ladder of excellence."  Hence, it is not surprising that he states that "considerable improvements are needed before accurate assessments of future sea-level change can be made."

Clearly, the results of van der Veen's eye-opening study should be trumpeted in the ears of the public at large, as well as those of all world leaders.  Via this small essay, we are doing our part.  It is now up to you to do yours.

Dr. Sherwood B. Idso
President
Dr. Keith E. Idso
Vice President

Reference
van der Veen, C.J.  2002.  Polar ice sheets and global sea level: how well can we predict the future?  Global and Planetary Change 32: 165-194.