After a cursory review of the science related to these two key questions, Oppenheimer and Alley say their review "leads to a multitude of questions [our italics] with respect to the basic science [our italics again] of the ice sheets," which we list below.  However, instead of listing them in their original question form, we post them in the form of statements that address what we do not know about the various sub-topics mentioned, which is obviously what prompts the questions in the first place and validates the content of the statements.

(1) We do not know if the apparent response of glaciers and ice streams to surface melting and melting at their termini (e.g., ice shelves) could occur more generally over the ice sheets.
(2) We do not know if dynamical responses are likely to continue for centuries and propagate further inland or if it is more likely that they will be damped over time.
(3) We do not know if surface melting could cause rapid collapse of the Ross or Filchner-Ronne ice shelves, as occurred for the smaller Larsen ice shelf.
(4) We do not know if ice sheets made a significant net contribution to sea level rise over the past several decades.
(5) We do not know what might be useful paleoclimate analogs for sea level and ice sheet behavior in a warmer world.
(6) We do not know the reliability of Antarctic and Southern Ocean temperatures (and polar amplification) that are projected by current GCMs, nor do we know why they differ so widely among models, nor how these differences might be resolved.
(7) We do not know the prospects for expanding measurements and improving models of ice sheets nor the timescales involved.
(8) We do not know if current uncertainties in future ice sheet behavior can be expressed quantitatively.
(9) We do not know what would be useful early warning signs of impending ice sheet disintegration nor when these might be detectable.
(10) We do not know, given current uncertainties, if our present understanding of the vulnerability of either the WAIS or GIS is potentially useful in defining "dangerous anthropogenic interference" with earth's climate system.
(11) We do not know if the concept of a threshold temperature is useful.
(12) We do not know if either ice sheet seems more vulnerable and thus may provide a more immediate measure of climate "danger" and a more pressing target for research.
(13) We do not know if any of the various temperatures proposed in the literature as demarking danger of disintegration for one or the other ice sheet are useful in contributing to a better understanding of "dangerous anthropogenic interference."
(14) We do not know on what timescale future learning might affect the answers to these questions.

In concluding their essay, Oppenheimer and Alley describe this list of deficiencies in our knowledge of things related to ice-sheet dynamics as "gaping holes in our understanding" that "will not be closed unless governments provide adequate resources for research," which seems a bit self-serving.  More importantly, however, they state that "if emissions of the greenhouse gases are not reduced while uncertainties are being resolved, there is a risk of making ice-sheet disintegration nearly inevitable."

Clearly, there is a risk - be it ever so small - that almost anything could occur.  But how probable are such high-risk phenomena?  To claim, as Oppenheimer and Alley do, that there is a risk of making ice-sheet disintegration nearly inevitable if emissions of greenhouse gases are not reduced while uncertainties are being resolved, is incredibly illogical, especially in light of what they say are "gaping holes in our understanding" of the subject, as enumerated in the list above.  In fact, given the degree of deficiency in our knowledge of the matter, it is perhaps as likely as not that a continuation of the planet's recovery from the relative cold of the Little Ice Age could actually lead to a buildup of polar ice; but there is no way we would ever say that outcome is "nearly inevitable."

Oppenheimer and Alley also do a disservice to logic in attempting to guide us in deciding what to do about anthropogenic CO2 emissions by considering only their projected deleterious climatic effects, when there are a host of highly-beneficial biological effects of elevated atmospheric CO2 that are much more well established.  As but one example, we note that the increased water use efficiency that is provided by the ongoing rise in the air's CO2 content will likely play a crucial role in helping humanity adequately feed our growing numbers just a few short decades from now, when without that help it will likely be a next-to-impossible task (Idso and Idso, 2000), which could well usurp all remaining pristine land and water in the process (Wallace, 2000) and thereby drive what little remains of the natural world to extinction (Tilman et al., 2001).

Let's examine all the issues related to CO2 and life on earth, based on what we really do know, as well as reasonable projections about earth's future, in lieu of those based on a lengthy laundry list of we-do-not-knows.

Sherwood, Keith and Craig Idso

References
Idso, C.D. and Idso, K.E.  2000.  Forecasting world food supplies: The impact of the rising atmospheric CO2 concentration.  Technology 7S: 33-56.

Oppenheimer, M. and Alley, R.B.  2005.  Ice sheets, global warming, and article 2 of the UNFCCC.  Climatic Change 68: 257-267.

Tilman, D., Fargione, J., Wolff, B., D'Antonio, C., Dobson, A., Howarth, R., Schindler, D., Schlesinger, W.H., Simberloff, D. and Swackhamer, D.  2001.  Forecasting agriculturally driven global environmental change.  Science 292: 281-284.

Wallace, J.S.  2000.  Increasing agricultural water use efficiency to meet future food production.  Agriculture, Ecosystems & Environment 82: 105-119.