Recent observations show that the rate at which the Antarctic ice sheet (AIS) is contributing to sea level rise is increasing. Increases in ice-ocean heat exchange have the potential to induce substantial mass loss through the melting of its ice shelves. Lack of data and limitations in modeling, however, has made it challenging to quantify the importance of ocean-induced changes in ice shelf thickness as a driver for ongoing mass loss. Here, we use a numerical ice sheet model in combination with satellite observations of ice shelf thinning from 1994 to 2017 to quantify instantaneous changes in ice flow across all AIS grounding lines, resulting from changes in ice shelf buttressing alone. Our process-based predictions are in good agreement with observed spatial patterns of ice loss, providing support for the notion that a significant portion of the current ice loss of the AIS is ocean driven and caused by a reduction in ice shelf buttressing.