Coastline Changes from Melting Ice Sheets

In their article “Satellite-Observed Changes in the Arctic” in the August 2004 issue of Physics Today (page 38 ), Josefino Comiso and Claire Parkinson state, “The [Greenland] ice sheet is 1.7 km thick on average, with a total volume of ice that, if entirely melted, would increase Earth’s sea level by about 7.2 m” (p. 40). That statement is incorrect. It would be correct to say that as a result of that imaginary melting of Greenland’s ice sheet, all ocean waters would get 7.2 m deeper.

However, when the ocean waters get 7.2 m deeper, every square meter on all ocean floors covering 71% of Earth’s surface would be subject to an additional downward pressure from 7.2 metric tons. To preserve Earth’s volume, some land areas would have to rise correspondingly according to Archimedes’ principle, so that the isostatic equilibrium between continents and oceans would remain within reasonable limits.

For example, during the last ice age, Scandinavia’s ice sheet, which was up to 3 km thick, pressed Earth’s crust down by as much as 700 m into the underlying mantle. The pressure of the ice sheet thus forced some of the mantle material to flow outward under the crust in the surrounding areas and raise those areas, both ground and seafloor, by smaller amounts. At the ice age maximum, the ground under the ice sheet was pressed down by an extra pressure of up to 300 metric tons per square meter.

The Scandinavian ice sheets melted some 10 000 to 8 000 years ago, and the mantle material started to flow back and raise Earth’s crust in the depressed areas toward its pre–ice age elevations. That backflow and the resulting land rise were rather rapid originally, but the land uplift has slowed to just under 1 cm per year, now that most of the mantle material’s backflow has stopped.

As a second example of seafloor movements under variable loads, consider that even 1-m-high ocean tides at some shores tilt the seafloor and the neighboring shores twice a day by easily measurable amounts.

If the Greenland ice sheet melts, it will do so over centuries, and Earth will have plenty of time to adjust toward its isostatic equilibrium. There certainly will not be anywhere near a 7.2-m rise in the mean (average) sea level.