Are the Greenland and Antarctic ice sheets our friends, which will moderate sea-level rise over the next century as polar snowfall increases? Or are they ticking bombs, soon to unleash floods on the world's coasts? The uncomfortable fact is that while the ice is looking less and less friendly (see "Measuring the Polar Meltdown"), we're really not sure. The United States has joined almost 200 other countries in seeking "stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system" under Article Two of the U.N. Framework Convention on Climate Change. Exactly what constitutes "dangerous interference" can be debated, but substantial ice-sheet shrinkage causing meters of sea-level rise is a strong candidate.

In 2001, the U.N. Intergovernmental Panel on Climate Change (IPCC) described the great difficulties in predicting ice-sheet changes but projected slight net growth over the next century. By 2007, ice-flow instabilities had occurred in Greenland and Antarctica, apparently from warming, and the ice sheets were contributing slightly to sea-level rise. The IPCC noted that whole-ice-sheet models had not anticipated and could not reproduce the changes, and so could not adequately project future changes. Although our understanding of most factors affecting sea-level rise had improved, 2007 projections by the IPCC excluded "future rapid dynamical changes in ice flow" because "understanding ... is too limited to provide a best estimate or an upper bound for sea level rise."