Basal friction heavily controls the dynamics of fast-flowing glaciers. However, the best approach to modeling friction is unclear, increasing uncertainties in projections of future mass loss and sea-level rise. Here, we compare six friction laws and evaluate them for Petermann Glacier in northern Greenland, using a higher order three-dimensional ice-sheet model. We model glacier retreat and mass loss under an ocean-only warming until year 2300, while not considering the effects of a future warmer atmosphere. Regardless of the friction law, we find that breakup of Petermann's ice shelf is likely to occur within the next decades. However, future grounding-line retreat differs by 10s of km and estimates of sea-level rise may quadruple, depending on the friction law employed. A bedrock ridge halts the retreat for four of the laws, and Petermann retreats furthest when applying a Budd or a Coulomb-type till law. Depending on the friction law, sea-level contributions differ by 133% and 282% by 2300 for 2 degrees C and 5 degrees C ocean warming scenarios, respectively.