Sea-Level Riddle

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.”

Work is under way to improve and test the existing ice-sheet models. I know of no plausible scenarios under which an ice sheet would be lost over the next few decades, but the ongoing work does suggest that in the next decades, warming may initiate substantial change, perhaps crossing a threshold leading to much greater shrinkage or loss over centuries.

Ice sheets spread like pancake batter, on a greased griddle in some places but on a bumpy waffle iron in others, with islands blocking floating ice shelves that restrain the ice behind them. Warming’s most immediate impact may be to cause melting beneath those ice shelves, but in some places we don’t know the water depth well enough to build models. Despite heroic efforts, the waffle-iron and greased-griddle characteristics of the substrate are only partially mapped. The ability of surface meltwater to penetrate the ice and enhance lubrication is poorly understood. Recent changes were observed by satellites and other platforms, some of which may be lost to inadequate funding. And in global-climate models, the ice sheets remain inert white lumps, uncoupled from their surroundings. Small, mostly academic groups are working on ice-flow models, but the big, primarily government­-run centers that guide policy makers do not have the funding for ice-sheet modeling that they do for atmospheric, ocean, and land-surface modeling. Having played a small role in the 2007 IPCC, I believe that the assessment of the ice sheets was done well and that lack of a “best estimate or upper bound for sea level rise” reflects the science. But policy makers seeking economically and ecologically wise ways to avoid dangerous anthropogenic climate interference surely deserve additional guidance. That will not come soon without additional focus on the ice sheets.

Richard Alley, a professor of geosciences at Pennsylvania State University, was lead author of the chapter on Earth’s cryosphere in the most recent IPCC report.