Antarctica is undergoing dramatic changes as a result of global warming. One of the most worrying phenomena is the breaking up of ice shelves under the weight of melting lakes. Recent studies provide concrete evidence of how the cycle of melting and draining weakens ice shelves, threatening their integrity and facilitating the flow of glaciers into the sea. This process increases the risk of sudden collapses, such as Larsen B in 2002, and contributes to sea level rise, posing a major threat to coastal regions.
by Laurie Henry
Antarctica’s ice shelves play a crucial role in regulating the flow of ice and ensuring the stability of the ice sheet, more commonly known as the ice cap. As a result of global warming, rising temperatures are causing melting lakes to form on the surface of these ice shelves, with immense consequences.
The fracture process illustrated by the collapse of the Larsen B platform
The rupture mechanism caused by melt lakes was highlighted by the spectacular collapse of the Larsen B ice shelf in 2002. Before its collapse, Larsen B, located on the east coast of the Antarctic Peninsula along the Weddell Sea, was dotted with thousands of meltwater lakes covering an area of several hundred square kilometres.
Satellite observations showed that these lakes were draining rapidly, often in a matter of days, leading to the formation of large cracks in the ice. Predictive models had suggested that the combined weight of these lakes and their rapid drainage would exert sufficient pressure to fracture the platform. For example, a meltwater lake 10 metres deep can exert a hydrostatic pressure of about 98 kPa (kilopascals). Hydrostatic pressure is the pressure exerted by a fluid in equilibrium with gravity, and increases with depth.
In fact, when hundreds of these lakes form and empty simultaneously, the cumulative pressure on the platform can reach several hundred kilopascals, exceeding the ice’s ability to withstand it.
This intense pressure creates stresses that propagate as fractures in the ice structure. Data show that meltwater lakes add tens of metres to the water column. In January 2002, satellite imagery documented that the rapidly emptying meltwater lakes on Larsen B were creating radial and circular fractures that would eventually fragment the entire platform. In just a few weeks, an area of 3,250 km2 of ice disintegrated, dramatically demonstrating the potential impact of meltwater lakes on the stability of ice shelves.
Detailed study of the George VI platform
Recent work on the George VI Ice Shelf, located between Alexander Island and Palmer Land in Antarctica, has confirmed fracture hypotheses through detailed and quantified field observations.
A team of researchers led by Alison Banwell from the Cooperative Institute for Research in Environmental Sciences (CIRES) installed GPS sensors and self-triggering cameras to monitor changes in the ice surface during the 2019/2020 melt season. The GPS sensors measured vertical and horizontal displacements, recording data every 15 seconds. This allowed the ice movement to be tracked in real time.
South Doline on the George VI North Ice Shelf. (a) WorldView-2 image of the South Doline (18 January 2020) with instrument positions indicated. The dotted lines show the field of view of the camera in time lapse. A melt lake is visible in the centre. (b) Interpretation of image A with morphological features and meltwater collection area. (c) Photo looking north from GPS02 in November 2019, showing GPS01 and most of the basin. (d) Photo looking south from GPS01 in November 2021 showing GPS02 and the time-lapse camera. © A. Banwell et al, 2024 © A. Banwell et al., 2024
At the same time, self-triggering cameras installed around the lakes took pictures every 30 minutes, providing a detailed visual record of the formation, expansion and drainage of the lakes. The researchers analysed these images to identify morphological changes and ice movement.
(a) Difference in the vertical elevation of the South Sinkhole ice on the George VI Ice Shelf (centre minus edge) over 100 days compared to models 1-3 (red lines). The black line represents the 5-day moving average. (b) Maximum tensile stress in the upper 10 m of the South Sinkhole ice for model 3 (red line) and observed change in horizontal separation (black line). The break on 25 January is associated with a stress of 75 kPa. © A. Banwell et al., 2024
Hydrostatic pressure responsible for breaking the ice
GPS data showed that the ice at the centre of the melting lakes had sunk about 60 centimetres under the weight of the accumulated water, far more than the 30 centimetres previously estimated, confirming the significant pressure exerted by the lakes. In addition, the horizontal distance between the edge and centre of the lake basin had increased by more than 70 centimetres, indicating that the cracks were widening.
The sensors recorded periods of rapid drainage of the lakes, with water levels dropping by several metres in a few days, corresponding to sudden increases in the horizontal distance measured by GPS. These rapid changes led to the formation of circular fractures around the lakes, also known as ‘ring fractures’, which can be seen in the time-lapse images.
These cracks form around melting lakes as the weight of the water exerts a continuous and increasing pressure on the surrounding ice. The cracking process starts with small cracks about 300 metres from the centre of the lake and gradually spreads radially under the effect of hydrostatic pressure, measured at up to 75 kPa. These images also show how these cracks spread radially from the centre of the lakes.
(a) Oblique view of the sinkhole on the George VI ice shelf at noon on 17 January 2020. (b) Image of the sinkhole basin generated from the oblique photo in (a). (c) Digitised areas of the meltwater lakes (red outline). © A. Banwell et al., 2024
Improvement of numerical modelling and forecasting
This growing instability of the ice shelf is a cause for concern. As the cracks widen, the structural integrity of the platform is increasingly compromised, increasing the risk of a sudden collapse, similar to the Larsen B platform in 2002. Such a collapse would release large amounts of ice into the ocean, contributing significantly to sea level rise.
The observations in this study will therefore improve predictive models and provide a better understanding of the processes involved in the destabilisation of ice shelves. These improved models are essential for anticipating the future effects of global warming and developing appropriate adaptation strategies in the face of these rapid and potentially devastating environmental changes.
For example, the Thwaites and Brunt Ice Shelves in West Antarctica and the Weddell Sea respectively have already been identified as vulnerable due to their geography and local climate conditions. The Thwaites Ice Shelf, often referred to as the ‘Doomsday Glacier’, covers an area of approximately 192,000 km2 and is one of the largest potential contributors to sea level rise. Studies show that the retreat of the glacier’s anchor line, combined with the intrusion of warm ocean water, is accelerating its melting.
Meanwhile, the Brunt Ice Shelf has experienced major fractures in recent years, most notably with the appearance of the Halloween crack in 2016 and the rapidly widening Chasm 1 crack. These fractures have led to the formation of massive icebergs, one of the most recent of which, A-74, broke off in February 2021 and measured around 1,270 square kilometres. These events show that the stability of ice shelves can be compromised much more quickly than expected, particularly under the combined effect of pressure from melting lakes and rising oceanic and atmospheric temperatures.
Source : Banwell AF, Willis IC, Stevens LA, Dell RL, MacAyeal DR., “Observed meltwater-induced flexure and fracture at a doline on George VI Ice Shelf, Antarctica”. Journal of Glaciology, 2024
Cover image: Meltwater spreading across the George VI Ice Shelf in Antarctica. Lauren Dauphin/NASA Earth Observatory