Sometimes changes in currents, or in the way water washes under the ice, can affect melting. Hall, Conway and colleagues made dozens of visits to the West Atlantic Ice Sheet to measure the rate of melting. Clues included deposits of penguin guano. If the penguins lived there, there couldn't have been any glacial ice at that site or offshore in the ocean. They also looked at radar imaging of subsurface ice structures and of ground-level ice.
Ice weighs land down, and the earth springs back when the ice melts.
Roosevelt Island in the Ross Sea provided other clues. We used radar to look at the layers of ice inside Roosevelt Island. In another study, Ian Joughin and colleagues at the Jet Propulsion Laboratory and California Institute of Technology in Pasadena found water is pouring off the ice sheet in ice streams that slide quickly to the ocean and break off into giant icebergs. Hall said she hoped the studies would not give ammunition to critics of the global warming theory who say human activity is having no effect whatsoever. She noted that collapse of the WAIS would not be the only effect of global warming.