Global mean sea level (GMSL) is rising steadily, rising by approximately 20 cm from 1901 to 2018. The rate of change is accelerating significantly, from about 1.4 mm per year (1901–1990) to approximately 3.7 mm per year (2006–2018), and reached 4.5 mm per year in 2023. Although multiple processes contribute to sea level rise, the dominant source today is melting of glaciers and ice sheets, which added approximately 1.6 mm per year to GMSL between 2006 and 2018, outpacing the thermal expansion of the oceans. The contribution of ice sheets to sea level rise is now greater than that of smaller mountain glaciers and ice caps.

The main source of concern is that The loss of mass from the Greenland and Antarctic ice sheets has quadrupled since the 1990sThese massive bodies of ice on Earth, namely the Greenland Ice Sheet (GrIS), the West Antarctic Ice Sheet (WAIS), and the East Antarctic Ice Sheet (EAIS), contain the equivalent of approximately 65 meters GMSL. Even small changes in their volume can have catastrophic consequences for coastal populations worldwide. An estimated one billion people live in areas less than 10 meters above sea level, and about 230 million within 1 meter. Without adaptation, conservative estimates suggest that a 20 cm sea level rise by 2050 would lead to average global flood losses of US$1 trillion per year or more for the 136 largest coastal cities.

Warming of +1.5°C is too high for polar ice sheets. Current climate conditions, representing a warming of approximately +1.2°C above pre-industrial levels (1850–1900), are already causing significant ice sheet mass loss. This trend is in sharp contrast to reconstructions from the 1970s and 1980s, when ice sheets were close to equilibrium. Evidence from paleoclimate records from past warm periods, when global average temperatures were similar to or slightly higher than today, clearly shows that such conditions led to increase in GMSL by several metersFor example, during the last interglacial (approximately 129–116 thousand years ago), when global temperatures were +0.5 to +1.5 °C higher than pre-industrial, GMSL was probably several meters higher than today.

Ice sheets show strongly nonlinear responses to climate change and are sensitive to temperature thresholds that, once exceeded, lead to significantly higher rates of sea level rise. These rapid changes are driven by self-reinforcing feedback loopsKey ones include:

1. Feedback between surface elevation and melting: Reducing the surface area of an ice sheet exposes it to warmer air at lower altitudes, which increases the rate of melting and leads to further reduction in surface area.
2. Marine Ice Sheet Instability (MISI): It occurs when an ice sheet rests on bedrock below sea level that is deepening inland. The initial retreat leads to increased ice thickness and increased ice flow at the grounding line. It appears that WAIS may already be in this phase.
3. Marine Ice Reef Instability (MICI): Hypothesized mechanism initiated by rapid removal of supporting ice shelves, exposing mechanically unstable ice cliffs prone to collapse.

These feedbacks mean that relatively small warming can initiate a large and rapid ice sheet response that is essentially irreversible within the timescales of human civilization. The ice sheets would take tens of thousands of years to return to their original state, while retreat would take only centuries to millennia. There is also a likely lag in response, meaning that warming that has already occurred could lead to future instabilities, and the threshold may be crossed before we realize it.

Numerical modeling suggests that even under the most optimistic scenarios (such as SSP1-1.9, where temperatures stabilize around +1.4°C by 2100) they cannot stop sea level rise from ice sheets. A warming scenario of +1.5°C leads to an acceleration of sea level rise from Antarctica to more than 1.5 mm per year by 2100 and a total contribution of up to 1.03 m by 2300, with no signs of slowing down. Even the current forcing (approximately +1.2°C) could be sufficient to trigger instabilities generating high rates of GMSL rise (e.g. >10 mm per year) that would test the limits of adaptation.

Based on evidence from paleoclimatic records, contemporary observations, and numerical modeling, the authors of the article conclude that +1.5°C is too high and that a global average temperature lower than the current one is needed to maintain ice sheets in a broader equilibrium. They hypothesize that The “safe limit” for ice sheets is likely to be close to or even below +1.0°C above pre-industrial levels.This level of warming is similar to conditions in the 1980s, when ice sheets were generally in equilibrium.

Given the catastrophic consequences of a rapid collapse of one or more ice sheets leading to a multi-metre rise in sea levels, there is an urgent need to adopt the precautionary principle. This requires a return to colder conditions than the current onesto slow sea level rise from ice sheets and prevent rapid acceleration. Spring

Article published for nature.com

Glossary of key terms

• AIS (Antarctic Ice Sheet): Antarctic glacier, a vast mass of ice covering most of the Antarctic continent.
• ASE (Amundsen Sea Embayment): Amundsen Sea Gulf, a region in West Antarctica known for significant glacier loss.
• Barystatic SLR (Sea Level Rise): The rise in sea level caused by the addition of mass (water) to the oceans, especially the melting of glaciers and ice sheets.
• CDW (Circumpolar Deep Water): Circumpolar deep water, warm and salty ocean water that can contribute to melting at the bottom of ice shelves.
• Cryosphere: Cryosphere, the parts of the Earth's surface where water is in the solid state, including glaciers, ice sheets, snow, permafrost, and sea ice.
• Dynamic Topography: Dynamic topography, vertical movements of the Earth's surface caused by convection in the Earth's mantle, which can affect estimates of past sea level.
• EAIS (East Antarctic Ice Sheet): East Antarctic Ice Sheet, a larger part of the Antarctic Ice Sheet, usually considered more stable than the WAIS.
• GIA (Glacial-Isostatic Adjustment): Glacial isostatic adjustment, the uplift or subsidence of the Earth's crust in response to loading or unloading by a mass of ice.
• GMSL (Global Mean Sea Level): Global mean sea level, the average height of the ocean surface around the world.
• Grounding Line: The landward edge of a glacier, the line where an ice sheet loses contact with the ground and begins to float as an ice shelf.
• GrIS (Greenland Ice Sheet): Greenland Ice Sheet, a vast mass of ice covering most of Greenland.
• Hysteresis: Hysteresis, a phenomenon in which the state of a system depends on its past; in the context of glaciers, it means that returning to the previous state requires a greater drop in temperature than was needed to trigger the change.
• Hydrofracturing: Hydrofracturing, a process in which water from melting snow and ice flows into cracks in a glacier and widens them, leading to the breakup of the ice.
• IPCC (Intergovernmental Panel on Climate Change): Intergovernmental Panel on Climate Change, the main international body for assessing climate change.
• ISMIP (Ice Sheet Model Intercomparison Project): Glacier Model Intercomparison Project, an international initiative to compare and evaluate glacier simulations.
• Last Deglaciation: The last deglaciation, the period after the last glacial maximum when glaciers retreated and sea level rise was rapid.
• LIG (Last Interglacial): Last interglacial period, a warm period between the last two ice ages (~129–116 thousand years ago).
• Marine Ice Cliff Instability (MICI): Sea ice shelf instability, a hypothetical mechanism in which rapid removal of ice shelves exposes tall ice shelves at the Earth's edge, which collapse mechanically.
• Marine Ice Sheet Instability (MISI): Sea ice instability, a mechanism by which a glacier resting on bedrock below sea level that is deepening inland (retrograde slope) retreats and its landward edge moves into deeper water with thicker ice, leading to accelerated retreat and mass loss.
• Meltwater Pulse 1A: Meltwater pulse 1A, a period of rapid sea level rise during the last deglaciation.
• MIS (Marine Isotope Stage): Marine isotope phase, a division of geologic time based on the ratio of oxygen isotopes in marine sediments that reflects changes in ice volume.
• MPWP (Mid-Pliocene Warm Period): Mid-Pliocene Warm Period, a geological period (~3.3–2.9 million years ago) when atmospheric CO2 concentrations and global temperatures were similar to or slightly higher than today.
• Palaeo-records: Paleorecords, records of past climatic and environmental conditions, derived from geological materials such as ice cores, sediments, and fossils.
• Paris Climate Agreement: Paris Agreement, an international treaty aimed at limiting global warming.
• Pre-industrial: Pre-industrial, the period before the beginning of large-scale industrialization, usually the reference period (1850–1900) for comparing global temperatures.
• Retrograde Slope: Retrograde slope, the slope of the bed of a glacier that deepens inland.
• SAM (Southern Annular Mode): Southern Annular Mode, a pattern of atmospheric variability in the Southern Hemisphere.
• SLE (Sea Level Equivalent): Sea level equivalent, the height by which global sea level would rise if all the ice in a glacier or ice shelf melted.
• SLR (Sea Level Rise): Sea level rise.
• SSP (Shared Socio-economic Pathway): Common socio-economic scenario, scenarios of possible future developments of society and climate, used in climate modeling.
• Thermal Expansion: Thermal expansion, the increase in volume of ocean water due to an increase in temperature.
• Tipping Point: Tipping point, a critical threshold, beyond which irreversible and often rapid change in the system may occur.
• WAIS (West Antarctic Ice Sheet): The West Antarctic Ice Sheet, a smaller and more dynamic part of the Antarctic Ice Sheet, considered particularly vulnerable to climate change.