Modeling ice sheets from the bottom up

Three facts should guide ice-sheet modeling. (I) Ice height above the bed is controlled by the strength of ice-bed coupling, reducing ice thickness by some 90 percent when coupling vanishes. (2) Ice-bed coupling vanishes along ice streams that end as floating ice shelves and drain up to 90 percent of an ice sheet. (3) Because of(]) and (2), ice sheets can rapidly collapse and disintegrate, thereby removing ice sheets from Earth's climate system and forcing abrupt climate change. The first model of ice-sheet dynamics was developed in Australia and applied to the present Antarctic Ice Sheet in 1970. It treated slow sheet flow, which prevails over some 90 percent of the ice sheet, but is the least dynamic component. The model made top-down calculations of ice velocities and temperatures, based on known surface conditions and an assumed basal geothermal heat flux. In 1972, Joseph Fletcher proposed a six-step research strategy for studying dynamic systems. The first step was identifying the most dynamic components, which for Antarctica are fast ice streams that discharge up to 90 percent of the ice. Ice-sheet models developed at the University of Maine in the 1970s were based on the Fletcher strategy and focused on ice streams, including calving dynamics when ice streams end in water. These models calculated the elevation of ice sheets based in the strength of ice-bed coupling. This was a bottom-up approach that lowered ice elevations some 90 percent when ice-bed coupling vanished. Top-down modeling is able to simulate changes in the size and shape of ice sheets through a whole glaciation cycle, provided the mass balance is treated correctly. Bottom-up modeling is able to produce accurate changes in ice elevations based on changes in ice-bed coupling, provided the force balance is treated correctly. Truly holistic ice-sheet models should synthesize top-down and bottom-up approaches by combining the mass balance with the force balance in ways that merge abrupt changes in stream flow with slow changes in sheet flow. Then discharging 90 percent of the ice by ice streams mobilizes 90 percent of the area so ice sheets can self-destruct, and thereby terminate a glaciation cycle. (C) 2009 Elsevier Ltd. All rights reserved.