Ice shelves are an important part of the Antarctic ice sheet, controlling the flow speed into the sea of outlet glaciers. Their thickness is a crucial factor in calculations of ice sheet contributions to sea-level rise. The area of a glacier’s flux gate (where the ice leaves solid ground and goes afloat), multiplied by the area of this gate, yields the amount of ice discharged into the sea by that glacier. Combined with upglacier accumulation rates, this yields most of a glacier’s mass balance.
The accuracy of ice shelf thickness measurements is currently one of the major uncertainties in both mass balance and sea-level calculations. Earlier methods such as radio-echo sounding, while accurate, are limited by their poor spatial coverage. Ice2sea scientists Jennifer Griggs and Jonathan Bamber from Bristol University have produced a map of ice thickness for all Antarctic ice shelves from satellite altimetry data from the European Remote-sensing Satellite ERS-1 and NASA’s ICESat. Knowing the surface height of the ice shelf allows them to calculate the thickness of the ice, assuming it floats in the sea in equilibrium. Besides improving spatial resolution, they also address the other main source of uncertainty for ice-shelf thickness: They correct for firn layers on the ice surface which affect the shelf’s density and mass, and estimates of its thickness. This is because a denser, heavier shelf would float lower in the water, with a greater proportion submerged, and vice versa.
Biases are still present, for instance due to the presence of marine ice (seawater frozen to the bottom of the ice shelf) which has a different density than ice originating inland. While there is therefore still scope for improved accuracy, the researchers have managed to produce a dataset in close agreement with existing ones but with improved resolution.
Ice2sea Work Package: WP3.2
Publication: Griggs,J. & J.L. Bamber (2011), Antarctic ice-shelf thickness from satellite radar altimetry. Journal of Glaciology, 57, 485-498.