Future Projections of Petermann Glacier Under Ocean Warming Depend Strongly on Friction Law.

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°C and 5°C ocean warming scenarios, respectively. Plain Language Summary: Rocks, sediments, and water under glaciers can influence glacier speed and their response to climate change. How to best represent these effects in computer models is unclear, which magnifies uncertainties in our projections of future sea‐level rise. We, therefore, compared six ways to represent the under‐ice environment in a computer model, and simulate the future of Petermann Glacier, northern Greenland's largest and fastest glacier. We study how the glacier reacts to the 2°C–5°C warmer ocean expected, if carbon emissions continue, while not considering the effects of a future warmer atmosphere. The experiments indicate that Petermann's ice shelf, the glacier's floating extension, will likely breakup within the next decades. Our estimates of future sea‐level rise by year 2300 from Petermann Glacier also quadruple, depending on how the under‐ice conditions is treated in the model. This dramatically highlights the limitations of current ice‐sheet models and projections, and pinpoints the need to improve the representation of the under‐ice environment and marine‐based ice sheets in further research. Key Points: Breakup of Petermann's ice shelf in response to a warming ocean is likely in the coming decadesProjected sea‐level rise contributions are more sensitive to the friction law than to ocean warmingPresent‐day Petermann Glacier is best represented with a Budd or Schoof friction law [ABSTRACT FROM AUTHOR]