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Projections of sea‐level rise from ice‐sheet shrinkage in a warming world have large uncertainties, linked to limited knowledge of changes at the ocean‐ice sheet interface. This interface most typically is modeled as a grounding line, across which still‐connected ice flows into the ocean to float as an ice shelf, or where icebergs calve from a cliff before the ice begins to float. But, extensive and rapidly increasing evidence shows that this is really a grounding zone, and that processes in this grounding zone omitted from many models could exert major controls on sea‐level rise. Plain Language Summary: Marine‐terminating glaciers flow into the ocean across extensive grounding zones. These kilometers‐long and glacier‐wide zones represent the last broad region of glacier contact with the rock and sediments below before the ice enters the ocean as a floating ice shelf or calved icebergs. Loss of this basal drag along with enhanced basal melting caused by tidally driven seawater intrusion leads to faster outflow and rapid thinning of the overlying ice. As a result of the local thinning, grounding zones retreat inland and sea level rises with more loss of previously grounded ice. Most ice‐sheet models used in sea‐level projections do not include grounding‐zone processes, but rather stop their ice‐ocean interactions at a grounding line. They are thereby omitting important dynamic feedbacks and underestimating future sea‐level contributions from the marine‐based sectors of the Greenland and Antarctic ice sheets. Key Points: Tidally modulated seawater intrusion leads to loss of ice‐bed contact as well as significant (maximal) basal melting within grounding zonesThe future dynamics of marine outlet glaciers are ultimately controlled by coupled processes operating within and through grounding zonesDespite the importance of grounding zones to ice‐sheet dynamics, most ice‐sheet models used in sea‐level projections do not include them [ABSTRACT FROM AUTHOR]

Open ocean areas surrounded by sea ice and maintained by ocean heat, or sensible‐heat polynyas, are linked to key ice‐sheet processes, such as ice‐shelf basal melt and ice‐shelf fracture, when they occur near ice‐shelf fronts. However, the lack of detailed multi‐year records of polynya variability prevent assessing coupling between polynya and frontal dynamics. Here, we present the first multi‐decadal polynya area record (2000–2022) at Pine Island Glacier (PIG), West Antarctica, from thermal and optical satellite imagery. We found substantial interannual variability in polynya area, with consistencies in the timing of polynya opening, maximal extent, and closing. Furthermore, the largest polynya in our record (269 km2) occurred at PIG's western margin just 68 days before iceberg B‐27 calved, suggesting that polynya size and position may influence rifting dynamics. Our new data set provides a pathway to assess coevolving polynya and frontal dynamics, demonstrating the importance of building long‐term, year‐round polynya variability records. Plain Language Summary: Persistent sensible‐heat polynyas are areas of open ocean surrounded by sea ice maintained by ocean heat. These surface features occur near ice‐shelf fronts at the coastal margins of Antarctica and therefore have the potential to impact ice‐shelf stability through heat transfer processes. However, our understanding of long‐term polynya variability remains limited due to the lack of multi‐year records documenting polynya evolution. Here, we use satellite imagery to measure polynya area near Pine Island Glacier (PIG) and build the first multi‐decadal record in Antarctica. We observed a large amount of year‐to‐year area variability from 2000 to 2022, with the largest polynya in our record (269 km2) occurring at the western edge of PIG just 68 days before a large iceberg calved from PIG. This correspondence suggests that polynya size and position may influence iceberg calving. Our new data set provides a pathway to assess potentially coupled ice and ocean processes, demonstrating the importance of building long‐term, year‐round polynya variability records. Key Points: We generated a 22 years record of polynya area at Pine Island Glacier from satellite thermal and optical imageryOur data set shows high variability in sensible‐heat polynya area (0–322 km2) from 2000 to 2022Large, marginal, and persistent sensible‐heat polynyas may reduce ice‐shelf buttressing and contribute to rift initiation and propagation [ABSTRACT FROM AUTHOR]

Thwaites Ice Shelf (TWIS), the floating extension of Thwaites Glacier, West Antarctica, is changing rapidly and may completely disintegrate in the near future. Any buttressing that the ice shelf provides to the upstream grounded Thwaites glacier will then be lost. Previously, it has been argued that this could lead to onset of dynamical instability and the rapid demise of the entire glacier. Here we provide the first systematic quantitative assessment of how strongly the upstream ice is buttressed by TWIS and how its collapse affects future projections. By modeling the stresses acting along the current grounding line, we show that they deviate insignificantly from the stresses after ice shelf collapse. Using three ice‐flow models, we furthermore model the transient evolution of Thwaites Glacier and find that a complete disintegration of the ice shelf will not substantially impact future mass loss over the next 50 years. Plain Language Summary: As ice is lost, global sea levels rise. Currently glaciers and ice sheets contribute about half of observed sea level rise (SLR). Some ice is lost through surface melting, but sea levels can also be impacted by ice that flows directly into the ocean. This loss of ice, from the grounded interior into the surrounding ocean, is particularly important for the Antarctic Ice Sheet. There the grounded sections of the ice sheet are sometimes surrounded by floating seaward extensions referred to as ice shelves. These ice shelves can restrict, that is, buttress, upstream flow. Here we estimate the buttressing provided to one of the largest and most dynamical glaciers of the Antarctic Ice Sheet, the Thwaites Glacier. The glacier is currently abutted by an ice shelf, which has become progressively smaller and weaker over the last few decades. It has been suggested that a disintegration of the ice shelf might lead to greatly enhanced ice flow. However, we find that the ice shelf does not significantly buttress upstream flow, and removing the ice shelf has little effect on SLR over the next 50 years. Key Points: Thwaites Ice Shelf provides limited buttressing to the grounded upstream Thwaites GlacierRemoval of the ice shelf changes the contribution of the West Antarctic Ice Sheet to sea level rise by at most a few millimeters over 50 yearsObservations and modeling suggest that Thwaites Glacier is currently unsteady and stable [ABSTRACT FROM AUTHOR]