Steep Glacier Bed Knickpoints Mitigate Inland Thinning in Greenland.

Greenland's outlet glaciers have been a leading source of mass loss and accompanying sea‐level rise from the Greenland Ice Sheet (GrIS) over the last 25 years. The dynamic component of outlet glacier mass loss depends on both the ice flux through the terminus and the inland extent of glacier thinning, initiated at the ice‐ocean interface. Here, we find limits to the inland spread of thinning that initiates at glacier termini for 141 ocean‐terminating outlet glaciers around the GrIS. Inland diffusion of thinning is limited by steep reaches of bed topography that we call "knickpoints." We show that knickpoints exist beneath the majority of outlet glaciers but they are less steep in regions of gentle bed topography, giving glaciers in gentle bed topography the potential to contribute to ongoing and future mass loss from the GrIS by allowing the diffusion of thinning far into the ice sheet interior. Plain Language Summary: Fast‐flowing outlet glaciers around the edge of the Greenland Ice Sheet transport ice from the ice sheet interior to the ocean. In response to a warming climate, many of these glaciers have retreated, causing thinning at the edge of the ice sheet that spreads into the interior. Depending on the shape of each outlet glacier, thinning can either spread far into the interior or stall where the glacier flows over particularly steep bedrock beneath the glacier. By investigating the shapes of 141 outlet glaciers around Greenland, we find that steep bedrock features, which we call "knickpoints," can effectively stall the inland spread of thinning in regions where the bedrock beneath the ice sheet is mountainous. On the other hand, in regions of more gentle topography, these knickpoints are either not present or are less steep and cannot stall thinning from spreading far into the ice sheet interior. This means that numerous small glaciers flowing over the gentle topography of Northwest Greenland may allow thinning to spread to the center of the ice sheet. Because of this, these smaller glaciers may play as big of a role in future sea‐level rise as the more well‐known and well‐studied larger glaciers. Key Points: Greenland glaciers with high potential to contribute to mass loss identified using novel metricThe majority of thinning limits are set by a knickpoint, a steep drop in bed topographyGlaciers that transmit thinning far inland are preferentially in regions with gentle bed topography [ABSTRACT FROM AUTHOR]