Force-perturbation analysis of Pine Island Glacier, Antarctica, suggests cause for recent acceleration

Pine Island Glacier, flowing into the Amundsen Sea from West Antarctica, thinned substantially during the 1990s, its grounding line receded by several km, and its velocity increased by >10% to values approaching 3 km a(-1). Here, we use these observations, together with estimates of ice thickness and surface strain rates, to estimate the perturbation in forces resisting ice flow compatible with the observations. The analysis assumes that such perturbations are transmitted far upstream from where they originate, and that creep response to the perturbations can be described by equations similar to those that govern ice-shelf creep. It indicates that observed acceleration between 1996 and 2000 could have been caused by progressive ungrounding within the most seaward 25 km 'ice plain' of the grounded glacier. Earlier retreat and thinning of the glacier's floating ice shelf may have provided the conditions that initiated ungrounding of the ice plain. Our analysis indicates that continued ice-plain thinning at the current rate of about 2 m a(-1) will result in a velocity increase by 1 km a(-1) within the next 11 years as the ice plain becomes totally ungrounded.