Marine Ice-Sheet Instability (MISI)

Marine Ice-Sheet Instability (MISI) is a glaciological concept describing a self-sustaining process of ice-sheet retreat that occurs when the grounding line of a marine-based ice sheet — one resting on bedrock below sea level — retreats into deeper water. This mechanism is particularly important for understanding the potential rapid loss of ice from the West Antarctic Ice Sheet (WAIS) and its implications for global sea-level rise. MISI has become a cornerstone of modern climate science, linking ice-sheet dynamics, oceanic processes, and long-term sea-level changes.

The Concept of MISI

The principle of Marine Ice-Sheet Instability arises from the relationship between ice thickness, grounding-line position, and ice flux (the rate at which ice flows across the grounding line). The grounding line marks the transition between the grounded ice sheet and the floating ice shelf. In a stable configuration, the ice flux through this line is balanced by the accumulation of ice inland.
However, if the grounding line retreats inland onto a retrograde bed slope — where the bedrock beneath the ice sheet deepens towards the interior — instability can develop. In such cases, as the grounding line moves into deeper water, the ice at the grounding line becomes thicker and flows more rapidly into the ocean. This increased ice flux enhances further retreat, creating a positive feedback loop. The process continues until a new stable position is reached, often much farther inland, or until most of the ice sheet in that region is lost.

Physical Mechanism

The MISI process can be summarised in the following steps:

1. Perturbation – External factors, such as oceanic warming, basal melting, or reduced buttressing by an ice shelf, cause the grounding line to retreat slightly.
2. Retrograde Bed Slope Effect – If the bedrock beneath slopes downward inland, the grounding line moves into deeper water.
3. Increased Ice Flux – The thicker ice at the new grounding-line position leads to a greater ice flux across the grounding line.
4. Self-Reinforcing Retreat – The enhanced ice discharge further thins the ice sheet near the coast, causing continued inland retreat.

This feedback continues until the ice retreats to a bedrock high point or stabilises on a prograde slope (where the bedrock rises inland).

Conditions for Instability

MISI only occurs under specific physical and geometric conditions:

• The ice sheet must rest on bedrock below sea level (marine-based).
• The bed topography must slope downward towards the interior of the continent (retrograde slope).
• Buttressing ice shelves — floating extensions of the ice sheet — must be weakened or lost, allowing faster ice flow.
• External forcing, such as oceanic or atmospheric warming, must initiate the initial retreat.

When these conditions align, the ice sheet’s grounding line can retreat unstably, potentially leading to substantial and rapid ice loss over centuries.

Relevance to the West Antarctic Ice Sheet

The West Antarctic Ice Sheet (WAIS) is the most significant example of a region vulnerable to MISI. Much of the WAIS rests on bedrock that lies well below sea level and deepens inland, creating conditions conducive to instability.
Particularly susceptible areas include:

• Thwaites Glacier and Pine Island Glacier in the Amundsen Sea sector.
• These glaciers act as gateways for ice flowing from the interior of West Antarctica into the ocean.
• Observations over recent decades show grounding-line retreat, ice thinning, and accelerated flow — key indicators of ongoing MISI processes.

Numerical ice-sheet models suggest that if grounding-line retreat in this sector continues unchecked, it could lead to the eventual collapse of large portions of the WAIS, contributing several metres to global sea-level rise over the next few centuries.

Role of Ice Shelves and Ocean Interaction

Ice shelves play a critical role in moderating or delaying MISI. They exert a buttressing effect, resisting the seaward flow of ice and helping to stabilise the grounding line. However, when warm ocean currents intrude beneath ice shelves, basal melting can thin them significantly, reducing their buttressing capacity.
In the Amundsen Sea, for example, warm Circumpolar Deep Water (CDW) has been observed eroding the underside of floating ice shelves, particularly those connected to Thwaites and Pine Island glaciers. The resulting thinning weakens the mechanical support provided to the grounded ice sheet, triggering grounding-line retreat and activating MISI feedbacks.

Observational and Modelling Evidence

Satellite observations, airborne radar surveys, and numerical simulations have provided strong evidence supporting the existence and operation of MISI in Antarctica.

• Satellite altimetry has recorded substantial thinning of ice in the Amundsen and Bellingshausen Sea sectors.
• Grounding-line mapping has revealed continuous inland retreat over recent decades.
• Ice-flow models, beginning with studies in the 1970s and refined through the 21st century, have reproduced the instability mechanism under realistic Antarctic conditions.

These data collectively indicate that sections of the WAIS may already be in an irreversible retreat phase driven by MISI, though the timescales remain uncertain.

Timescales and Sea-Level Implications

The full manifestation of MISI-driven retreat can occur over centuries to millennia, depending on local conditions and feedback strength. However, once initiated, the process is difficult to halt because the underlying feedback is internal to the ice-sheet system.
If the entire WAIS were to collapse through MISI, it could contribute up to 3.3 to 5 metres to global mean sea level. Even partial retreat of key glaciers could raise sea level by tens of centimetres to over a metre within the next few hundred years, posing severe challenges for coastal infrastructure and ecosystems worldwide.

Related Concepts: Marine Ice-Cliff Instability (MICI)

Closely linked to MISI is the concept of Marine Ice-Cliff Instability (MICI). While MISI focuses on grounding-line retreat over retrograde beds, MICI involves the structural collapse of tall, unstable ice cliffs exposed above sea level following ice-shelf loss. Together, MISI and MICI represent complementary mechanisms that may accelerate ice-sheet collapse under future warming scenarios.

Scientific and Policy Significance

Understanding MISI is crucial for improving projections of future sea-level rise in global climate models. Current assessments by the Intergovernmental Panel on Climate Change (IPCC) incorporate MISI-based mechanisms to refine estimates of Antarctic ice-sheet contributions to sea-level change.
From a policy perspective, MISI underscores the urgency of mitigating climate change, particularly by reducing greenhouse gas emissions that drive oceanic and atmospheric warming. Even modest temperature increases can enhance basal melting rates and destabilise marine-based ice sheets.