Antarctica loses 12,800 km² of coastline over 30 years

The variability in sea ice in the Southern Ocean around Antarctica over recent decades has followed a rather peculiar pattern. Up until 2016, a net increase in sea-ice extent was observed, whereas over the past three years an opposite trend towards substantially lower concentrations appears to have begun. This shift in behaviour occurred under conditions of sustained global warming, which proved intriguing for the scientific community over the past decade.

This work, published by Rignot and co-authors, is a highly welcome contribution to the field, as it addresses the quantification of changes in the region where ice emerges from the continent and enters the ocean (an area commonly referred to as the grounding line) using satellite data for the period 1992–2025. The authors find that 77 % of the Antarctic coastline has not experienced substantial changes in the grounding line or, equivalently, that only 23 % of the coastline has seen a reduction in area. These results are relevant for understanding the causes of such asymmetries and provide further evidence to advance our understanding of cryospheric dynamics and their potential future impacts on sea level and the global climate.

The study, published in Proceedings of the National Academy of Sciences (PNAS) and based on more than 30 years of satellite synthetic aperture radar (SAR) observations, highlights a marked retreat of Antarctica’s grounding line and a consequent reduction in the area of the floating ice shelves fringing parts of the Antarctic coastline. The grounding line is the point at which the ice sheet loses contact with the bedrock and begins to float, at which stage it is referred to as an ice shelf. The significance of the study lies in the fact that ice shelves, being laterally confined by the sides of large embayments of the Antarctic continent, tend to buttress the flow of ice from the continent’s interior. Consequently, if the grounding line retreats and the area of the ice shelves diminishes, the rate at which the Antarctic ice sheet discharges ice into the ocean in the form of icebergs increases.

These findings are consistent with a wide range of evidence showing that the so-called Amundsen Sea sector of Antarctica, together with the Antarctic Peninsula, are the two Antarctic regions that have lost the greatest mass in recent decades as a result of submarine melting and the acceleration of solid ice discharge into the ocean. The release of continental ice into the ocean contributes to sea-level rise. Recent estimates place sea-level rise at around 4 mm per year, but the rate has been increasing for almost a century and is expected to continue to do so in the coming decades. Accordingly, projections by the Intergovernmental Panel on Climate Change (IPCC) for sea-level rise by the end of the twenty-first century range from 40 to 80 cm under low greenhouse gas emission scenarios to between 60 cm and 1 metre under high-emission scenarios. Only about half of this rise is attributed to the loss of mass from glaciers and ice sheets; the other half is due to the thermal expansion of the oceans (as seawater warms, it expands).

Despite this, satellite gravimetry data over the past three years indicate that increased snowfall has led to net mass gains for Antarctica as a whole during that period, notwithstanding substantial regional losses in certain areas. This is not incompatible with climate warming; indeed, warming leads to greater evaporation over tropical oceans, which in turn results in increased snowfall in the polar regions.

In summary, this is a thorough and significant study. The use of SAR data acquired from different satellite platforms over a 33-year period means that sensor resolution has varied (and improved) over time, but the lower precision of the earlier data has been duly taken into account in the error estimates.

There are also certain sources of uncertainty for which insufficient data were available for full assessment, but conservative error bounds have been assigned in such cases. The study’s results are therefore entirely credible within the stated margins of error.

This comprehensive study, conducted from 1992 to the present, regards the boundary between continental ice and the ocean —the Grounding Line (GL)— as a sensitive indicator of the stability and mass balance of Antarctic glaciers. By integrating satellite data —specifically differential synthetic aperture radar interferometry (DInSAR — from a range of sources, the authors have been able to produce a map of GL variations over the past three decades. The results show that most (77%) of the coastline has not experienced any GL migration. However, certain sectors are highly vulnerable to the loss of large expanses of ice. These areas are located in West Antarctica, the Antarctic Peninsula and parts of East Antarctica, where the total loss of continental ice is estimated at 12,820 km².

In the study, and taking into account the effects of multiple tidal cycles, the transition between continental ice and floating ice is considered more appropriately represented as an interface or transition zone —the Ice Grounding Zone (IGZ)— which encompasses the GL. The size of the IGZ is related to ice melt, which is lower where the IGZ is narrower. Glacier retreat may be associated either with further development of the ice shelf or, conversely, with its regression. The authors emphasise the complexity of the interacting factors involved in this relationship. One of the most significant is the intrusion of warm waters from the Circumpolar Deep Water (CDW) mass, which leads to more rapid ice melt within the IGZ. This oceanic influence on glacier retreat is strongly conditioned by the morphology and bathymetry of the continental shelf. However, in parts of East Antarctica, glacier retreat appears to be linked to other, as yet unidentified factors rather than to CDW.

The authors conclude that, in light of their findings, inferring GL positions and their changes from altimetric data is not as robust as previously thought. Moreover, GL migration and the mass balance of the ice sheet are closely interrelated, with sectors showing GL retreat also exhibiting a deficit in glacial mass.

The observations and conclusions drawn from this continent-wide study provide an important benchmark for the development of future models of ice-sheet formation and loss based on the IGZ, and more broadly for modelling the evolution of Antarctic glacier masses.