The Great Divide: An Introduction to Earth’s Frozen Giants

Antarctica holds approximately 60% of the planet’s fresh water in its vast ice sheets, making it the dominant factor in understanding global sea level rise. The continent is divided by the Transantarctic Mountains into two fundamentally distinct cryospheric systems: the East Antarctic Ice Sheet (EAIS) and the West Antarctic Ice Sheet (WAIS). While they share a common latitude and extreme climate, their underlying geology, size, and physical dynamics are so different that they are best understood as two separate giants.

The East Antarctic Ice Sheet is a massive, high-altitude terrestrial ice cap resting primarily on bedrock above sea level. In contrast, the West Antarctic Ice Sheet is classified as a marine ice sheet, with its base grounded well below sea level. This single geological distinction drives a profound divergence in stability, vulnerability to warming oceans, and contribution to potential sea level rise. The EAIS contains enough ice to raise global sea levels by roughly 58 meters if completely melted, making it the single largest potential reservoir of sea level rise on Earth. The WAIS holds a more immediate vulnerability, contributing a potential 3.3 meters of sea level rise, but with a much higher risk of rapid, dynamic collapse over the coming centuries.

Geographic Extent and Ice Volume

The Vast Interior: East Antarctica

Covering approximately 14 million square kilometers, the EAIS is the largest single mass of ice on Earth. It averages over 2.2 kilometers in thickness, with maximum thicknesses exceeding 4.8 kilometers at Dome Argus (Dome A). This immense volume is roughly equivalent to 26.5 million cubic kilometers of ice. The EAIS is characterized by several high-elevation domes—Dome A, Dome C (Concordia Station), and Dome F—which are among the coldest and driest places on Earth. The surface elevation at Dome A exceeds 4,000 meters above sea level. The vast interior is relatively isolated from oceanic influences, preserving some of the oldest continuous ice core climate records, with recent drilling efforts targeting ice as old as 1.5 million years. The sheer size and coldness of the EAIS give it a slow-moving, stable character over short time scales, but it is not immune to change.

The Marine Basin: West Antarctica

The WAIS is significantly smaller, covering roughly 2.2 million square kilometers with an ice volume of about 3.4 million cubic kilometers. It lies within the Ross Sea and Weddell Sea embayments, bound by the Transantarctic Mountains and the Antarctic Peninsula. Unlike its eastern counterpart, the bedrock beneath much of the WAIS is depressed by the weight of the overlying ice, plunging as deep as 2.5 kilometers below sea level. This creates a unique subglacial landscape of deep basins and islands. The ice sheet is drained by fast-moving ice streams—rivers of ice moving hundreds of meters per year—that flow into large floating ice shelves, notably the Ross Ice Shelf, the Ronne-Filchner Ice Shelf, and the smaller but critically important Thwaites and Pine Island ice shelves.

The Bedrock Below: A Tale of Two Topographies

The stability of an ice sheet is fundamentally controlled by the shape and elevation of the ground beneath it. The difference in subglacial topography between East and West Antarctica is stark and explains much of their contrasting behaviors.

The EAIS mostly sits on a high continental landmass. Although the ice is thick, the bedrock under much of the interior is near or above sea level. This high-standing bedrock acts as a natural brake, providing friction that slows the ice flow and makes the sheet mechanically stable. Even if the climate warms significantly, a large portion of the EAIS’s interior ice cannot easily collapse because it is physically grounded above sea level. There are exceptions, such as the Aurora Subglacial Basin and the Wilkes Basin, which lie below sea level. These regions represent potential avenues for faster ice loss and marine ice sheet instability, but they contain a relatively smaller fraction of the total EAIS volume compared to the high interior.

In West Antarctica, the topography is reversed. The bedrock is not only below sea level but also deepens inland. This retrograde slope is a critical destabilizing factor. When warm ocean water erodes the ice shelves and causes the grounding line (the point where the ice sheet lifts off the bedrock and begins to float) to retreat, it moves backward and downward into deeper water. Deeper water means the ice there is thicker, which further increases the rate of ice discharge into the ocean. This self-reinforcing feedback is known as Marine Ice Sheet Instability (MISI). The WAIS is almost entirely susceptible to this mechanism.

Stability and the Mechanisms of Change

Marine Ice Sheet Instability in West Antarctica

The WAIS is currently the most rapidly changing ice mass on the continent. Its stability is primarily threatened by the intrusion of warm Circumpolar Deep Water (CDW) onto the continental shelf. This water, which is several degrees above the in-situ freezing point of the ice, flows into sub-ice-shelf cavities and melts the ice shelves from below. As these floating ice shelves thin, their ability to buttress and hold back the grounded ice streams behind them diminishes.

The Thwaites Glacier, often called the “Doomsday Glacier” by the media, serves as the prime example of this instability. The International Thwaites Glacier Collaboration (ITGC) has documented that the glacier is both thinning and accelerating. The grounding line of Thwaites has retreated approximately 14 kilometers since the 1990s. The glacier is currently responsible for roughly 4% of global sea level rise. Scientists have identified a natural geological ridge beneath the glacier that could slow its retreat temporarily, but once the grounding line moves past this ridge, retreat into the deep interior basin could accelerate dramatically. Similar dynamics are observed at the Pine Island Glacier, which has also experienced significant thinning and acceleration.

The loss of the entire WAIS is not a question of if, but of when and how fast, in the context of sustained global warming. While a complete collapse is unlikely in the 21st century, recent modelling suggests that the rate of ice loss could push sea level rise contributions from Antarctica well beyond 1 meter by 2100 under high-emission scenarios, with the WAIS providing the lion’s share.

The Perceived Stability of East Antarctica

For decades, the EAIS was considered the “stable” ice sheet—a cold, inert giant largely unresponsive to rapid climate change. While this holds true for the vast, high-altitude interior, recent research has challenged the perception of complete stability at the margins.

The Totten Glacier, the largest outlet glacier in East Antarctica, drains a region of the Aurora Subglacial Basin that sits below sea level. Satellite observations have revealed that Totten Glacier is losing mass and that the ice shelf is experiencing significant basal melting driven by warm ocean water accessing the cavity. This suggests that the same marine ice sheet instability mechanisms threatening West Antarctica are also at work in certain sectors of East Antarctica. The Wilkes Basin, another major subglacial basin in EAIS, is also vulnerable, containing enough ice to raise sea levels by 3 to 4 meters. The current consensus is that the EAIS is near a state of mass balance (slight net loss or slight gain depending on the region), but the potential for rapid dynamic change lies in these marine-based basins.

Atmospheric vs. Oceanic Drivers

The mechanisms driving change in Antarctica combine atmospheric forcing and oceanic forcing. In West Antarctica, the primary driver is oceanic: the transport of warm CDW onto the shelf is modulated by winds, which are themselves influenced by the Southern Annular Mode (SAM) and stratospheric ozone depletion. In East Antarctica, the dominant question is how the ice sheet responds to changes in surface mass balance. A warmer atmosphere can hold more moisture, potentially leading to increased snowfall over the interior. This could partially offset dynamic losses at the margins. However, a warming atmosphere also increases the risk of surface melting on ice shelves, particularly on the Antarctic Peninsula and potentially on the large Ross and Ronne-Filchner Ice Shelves in the future. The collapse of the Larsen B Ice Shelf on the Peninsula in 2002 is a stark reminder that atmospheric warming can rapidly disintegrate ice shelves through hydrofracturing.

Contribution to Global Sea Level Rise

Observing the mass balance of these ice sheets is a high priority for the climate science community, relying heavily on satellite missions such as NASA’s GRACE (Gravity Recovery and Climate Experiment) and ICESat-2, as well as ESA’s CryoSat-2 and the Copernicus Sentinel program.

  • West Antarctic Ice Sheet: The WAIS is currently the largest contributor to sea level rise from Antarctica. It is losing approximately 150 to 200 billion tons of ice per year. This loss is not evenly distributed but is concentrated in the Amundsen Sea Embayment, home to the Thwaites and Pine Island Glaciers. The rate of ice loss from the WAIS has been accelerating over the past two decades.
  • East Antarctic Ice Sheet: The EAIS is currently close to a neutral contribution to sea level, though the margin of uncertainty is wide. Some regions, particularly in the Wilkes Land and Victoria Land coasts, are losing mass, while the interior is potentially gaining mass due to increased snowfall. Overall, the EAIS may be contributing a slight net loss (tens of billions of tons per year), but it is not currently the primary driver of Antarctic contribution to sea level rise.
  • Antarctic Peninsula: Though often grouped with WAIS in simplified discussions, the Antarctic Peninsula has warmed dramatically over the 20th century. It has contributed a significant amount of ice loss through glacier acceleration following ice shelf collapses, though its total volume is small relative to the main ice sheets.

The IPCC’s Sixth Assessment Report (AR6) assigns a very high confidence to the statement that the WAIS will continue to lose mass in the coming decades. The total sea level contribution from Antarctica by 2100 is projected to be between 0.1 and 0.5 meters under a low-emission scenario, and potentially exceeding 1 meter under a high-emission scenario, predominantly driven by WAIS dynamics. Beyond 2100, the picture becomes more alarming, with multi-meter sea level rise locked in by current emissions.

Implications for Climate Policy and Coastal Adaptation

Understanding the bifurcated nature of Antarctic ice sheet stability is essential for effective climate adaptation. The relatively slow, predictable melting of a terrestrial ice sheet (like the EAIS interior) can be modelled in a linear fashion in response to temperature increases. However, the marine ice sheet instability (MISI) and potential marine ice cliff instability (MICI) processes at work in the WAIS introduce non-linear, threshold-driven behavior.

This means that sea level rise projections are inherently uncertain. A collapse of the WAIS over several centuries is a realistic possibility, even under moderate warming. For coastal communities, this requires a paradigm shift in planning for resilience and adaptation, moving away from static flood risk assessments towards dynamic, scenario-based planning that accounts for the possibility of abrupt sea level rise.

Paleoclimate records provide a critical reference point. During the Pliocene epoch, approximately 3 to 5 million years ago, atmospheric CO2 levels were similar to today’s (around 400-450 ppm), and global temperatures were 2-4 degrees Celsius higher. Geological evidence strongly suggests that the WAIS was significantly smaller or completely absent during this period, contributing several meters to sea level. This provides a stark historical analogue for what may be in store under continued emissions.

Conclusion: Two Ice Sheets, One Shared Future

The East and West Antarctic Ice Sheets represent two ends of a spectrum of cryospheric stability. The EAIS is a massive, high-standing reservoir of ice that is largely insulated from rapid oceanic-driven change, though its deep subglacial basins hold a latent vulnerability. The WAIS is a much smaller but critically unstable marine ice sheet directly exposed to warming oceans and susceptible to rapid, irreversible collapse through the Marine Ice Sheet Instability mechanism.

Current monitoring efforts by institutions such as the National Snow and Ice Data Center and research led by the British Antarctic Survey continue to refine our understanding of these systems. The fate of the WAIS, particularly the Thwaites Glacier, will be a primary driver of global sea level within our lifetimes and the centuries to follow. While the EAIS may appear stable today, its long-term contribution cannot be ignored, as highlighted in the IPCC AR6 Working Group I report. The geological boundary of the Transantarctic Mountains separates two very different ice sheets, but their response to a warming world is deeply interconnected, underscoring the urgency of continued observation, research, and climate action to mitigate the most severe impacts.