In recent decades, glaciers in the Amundsen Sea
Embayment in West Antarctica have made the largest con-
tribution to mass loss from the entire Antarctic Ice Sheet.
Glacier retreat and acceleration have led to concerns about
the stability of the region and the effects of future climate
change. Coastal thinning and near-synchronous increases in
ice flux across neighbouring glaciers suggest that ocean-
driven melting is one of the main drivers of mass imbalance.
However, the response of individual glaciers to changes in
ocean conditions varies according to their local geometry.
One of the largest and fastest-flowing of these glaciers, Pine
Island Glacier (PIG), underwent a retreat from a subglacial
ridge in the 1940s following a period of unusually warm con-
ditions. Despite subsequent cooler periods, the glacier failed
to recover back to the ridge and continued retreating to its
present-day position. Here, we use the ice-flow model Úa to
investigate the sensitivity of this retreat to changes in basal
melting. We show that a short period of increased basal melt
was sufficient to force the glacier from its stable position on
the ridge and undergo an irreversible retreat to the next to-
pographic high. Once high melting begins upstream of the
ridge, only near-zero melt rates can stop the retreat, indicat-
ing a possible hysteresis in the system. Our results suggest
that unstable and irreversible responses to warm anomalies
are possible and can lead to substantial changes in ice flux
over relatively short periods of only a few decades