The high rates of bottom melting observed here strongly suggest that the Dotson and Crosson sub-ice-shelf cavities experienced a larger influx of ocean heat sometime between 2002 and 2009. Such influx accords with the increase in mass loss from all of the ASE major glaciers in the mid-2000s, including the tributaries of the Dotson and Crosson ice shelves1,11,16. It also accords with the higher observed thinning rates of ASE ice shelves outside their grounding zones, including Crosson and PIG, during the mid-2000s compared with earlier years14. A regional scale, simultaneous change is most likely the result of increased temperature and/or flow of warm waters in the sub-ice-shelf cavities. Warm modified CDW was observed to flow southward22,23 in a trough in the continental shelf off the Dotson Ice Shelf24. It is not yet known whether the trough continues to the grounding zone unhindered by ridges, or whether warm waters could also reach the Dotson–Crosson grounding zones through the Crosson sub-ice-shelf cavity. A large number of ice rises in the Dotson Ice Shelf, especially between the Kohler Range and Bear Peninsula (Fig. 1a), suggests that its cavity away from the grounding zone might be shallow and/or traversed by a ridge there hence limiting the access of deep warm waters. The increases in ice flow speeds and volume discharge of the three glaciers, and in mass loss from the ASE region as a whole, slowed by 2009 or 2010 (refs 1, 11 and 16). Again, the regional scale of the slowing suggests widespread changes in oceanic conditions on the continental shelf leading to slower increases in the influx of ocean heat. Such changes in ocean conditions in the vicinity of PIG between 2010 and 2012 were associated with a decrease in its bottom melting7, and our results suggest similar changes in the Dotson–Crosson area. Thus, the measured bottom ice loss rates in the period 2009–2014 are either similar to, or lower than, those in 2002–2009 (Fig. 1a; Supplementary Fig. 2; Supplementary Table 1). These rates indicate that the influx of ocean heat might have started to stabilize around 2009, and that the observed changes in the areas of Dotson–Crosson and PIG were part of a widespread event.

Oceanic conditions and melting rates beneath the Dotson–Crosson ice shelves driven by CDW variability could be modulated by changes in the extent of a large winter polynya, which opens off the Dotson Ice Shelf and Bear Peninsula25. Dense polynya water at the surface melting temperature, resulting from sea ice formation and the accompanying brine rejection and loss of buoyancy, was hypothesized to influence submarine melting rates by mixing and modifying water properties in sub-ice-shelf cavities26,27. A decrease in the extent of winter polynyas could therefore reduce the volume of the dense, cold water available to mix with warm CDW at depth hence contributing to enhanced bottom melting and vice versa.

SG experienced the highest melting rates in 2002–2009 and the farthest grounding line retreat by 2011. Unlike the other two glaciers, its rapid melting persisted in 2009–2014, and it continued to retreat between 2011 and 2014, albeit at a slightly slower pace. We explain these differences by a combination of cavity oceanic conditions and bed topography. SG has retreated enough to reach a 2,000-m deep trench28 after having been grounded at 500–800 m depth in 1996 (ref. 13). This retreat has created a topographic feature having the shape of an elongated bowl that is more than a kilometre deep from rim to bottom (Fig. 3f in Rignot et al.13). We propose that the deepening and the bed topography uncovered by the retreat are favourable to giving access to warm dense water in the SG grounding zone contributing to its rapid melting and retreat, in a manner similar to the hypothesized flooding of the Jakobshavn cavity with warm water that passed over a sill at the mouth of its fjord29. Available evidence suggests that the temperature of the denser water reaching the deepening SG grounding zone would be higher. Thus, water temperature and salinity profiles from the vicinity of PIG show that the densest water at 1,000 m depth is ∼1 °C warmer than that at 500 m (refs 6 and 7). Furthermore, the relationship between deeper grounding lines and higher bottom melting was previously noted9,28,30. The three glaciers have in common the great depths of their grounding lines (Figs 2c, 3b and 4b), but the deepest by far is SG which in 2014 is grounded at nearly 2,100 m below sea level. As the melting point of seawater decreases with depth at a rate of 0.75 °C km−1 (ref. 31), the SG grounding line corresponds to an additional ∼0.8 °C of ocean thermal forcing compared with PIG, of which the grounding line is at 1,000 m depth5. We further hypothesize that the rapid retreat of the SG grounding line could explain at least part of the observed high bottom melting rates. Such fast retreat transforms the basal boundary condition of grounded ice from contact with bed to exposure to ocean water and much higher melting32. Rapid thinning will hence result if the glacier cannot adjust quickly enough to compensate mass lost to the ocean at the newly exposed bottom surface. SG retreat will probably slow given that its bed upstream from the 2014 grounding line rises inland over ∼10 km, and its upper surface starts rising above the flotation level (Fig. 2c).

The grounding lines of Kohler and Pope glaciers did not retreat as deeply (Figs 3b and 4b), hence their grounding zones were not as susceptible to the increased inflow of warmer waters as that of SG. KG is the glacier with the smallest grounding line retreat between 1996 and 2014 and relatively the lowest bottom melting between 2002 and 2009. Its bed rises steeply by 300 m over <1.5 km directly upstream from the 2009 grounding line and its upper surface rises well above floatation level (Fig. 4b). Later, under the hypothesized change in oceanic conditions around 2009, such bed and surface geometry contributed to the grounding line actually readvancing between 2009 and 2011 and again between 2011 and 2014. PG has the second farthest grounding line retreat by 2014 and second largest bottom melting between 2002 and 2009. Its bed rises inland, but more gently than KG, and its 2009 grounding line is still ∼5 km downstream of the first prominent bed topographic rise located upstream from kilometre −16 (Fig. 3b). It also exhibits an ice plain (characterized by an upper surface having low slope and at or only slightly above floatation level33) extending over several kilometres. These geometric factors probably contributed to the 2014 PG grounding line maintaining its 2009 location, or regaining it after an intervening retreat, under the modified oceanic conditions.

Observations such as those presented here can contribute to more rigorous explorations of the differences in behaviour among the rapidly changing glaciers of Antarctica and their possible evolutions, and the geometric and dynamic factors involved5,10,34. This requires knowledge of ocean forcing changes and 3D modelling of the coupled glacier/ice-shelf/ocean system, including accurate sub-ice shelf bathymetry, an evolving ice-shelf bottom morphology, cavity circulation and bed topography35,36,37. Concerning the ASE more specifically, the observations are a compelling illustration of the changes that the ASE region is experiencing as it continues to be the largest source of Antarctic mass loss to the ocean. Radar sounding offers an unprecedented view of the actual bottom ice loss over wide areas in the critical grounding zones of ice shelves. Measuring ice thinning is difficult in grounding zones experiencing not only rapid melting but also changing geometry and location as grounding lines migrate. Migration reconfigures continuously the floatation conditions in and around a grounding zone, and changes the location and extent of grounding on ridges and ice rises, hence affecting the accuracy of bottom melting estimates based on ice surface heights. At the same time, submarine ice losses in these rapidly changing areas are so large that they can now be measured directly with airborne radar sounding. This makes possible a new approach for the monitoring of ice-shelf thickness changes on regional or continental scales as they respond to thermal forcing variability of the ocean and more generally to climate change.