📄 earth_science

West Antarctic Ice Sheet collapse and North Atlantic meltwater reconcile Last Interglacial climate records

This study uses Earth system model simulations to demonstrate that a collapsed West Antarctic Ice Sheet combined with North Atlantic meltwater release during the Last Interglacial explains anomalous climate records and suggests the Antarctic ice sheet could contribute up to ~4 meters to global sea level.

Original authors: Joseph Schnaubelt, Clay Tabor, Ran Feng, Austin Carter, Sarah Aarons, Johannes Sutter, Nicholas Golledge, Eric Steig

Published 2026-07-21

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Joseph Schnaubelt, Clay Tabor, Ran Feng, Austin Carter, Sarah Aarons, Johannes Sutter, Nicholas Golledge, Eric Steig

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Earth as a giant, complex house where the thermostat is set by the sun, but the walls and floors are made of ice. Sometimes, the sun gets a little brighter in certain seasons, and the whole house warms up. Scientists study a time called the "Last Interglacial," which happened roughly between 131,000 and 118,000 years ago. Think of this as Earth's last "summer vacation" before the current era. During this time, the planet was noticeably warmer than today, and the oceans were about 5 to 10 meters higher. That's like the water level rising enough to flood the first floor of a two-story house.

The big mystery is: where did all that extra water come from? We know the ice sheets at the North Pole (Greenland) and the South Pole (Antarctica) melted, but it's like trying to figure out who ate the last cookie when two kids were in the kitchen. Did the Greenland ice sheet melt first? Did the Antarctic one collapse? Or did they both melt at the same time? Understanding this is crucial because if we can figure out how the ice sheets reacted to warmth in the past, we might get a better clue about how they will react to our warming world today. The key players here are "ice sheets" (massive rivers of frozen water), "meltwater" (the fresh water that flows into the ocean when ice melts), and "isotopes" (which are like tiny chemical fingerprints in the ice that tell us how warm or cold it was when the snow fell).

Now, let's dive into what this new study did to solve the cookie mystery.

The researchers acted like climate detectives, using a super-powerful computer model to run a series of "what-if" scenarios. They wanted to see which combination of events could explain the weird signals found in ancient ice cores and ocean sediments from the Last Interglacial. Specifically, they were looking at two main suspects: a massive collapse of the West Antarctic Ice Sheet (the big chunk of ice in the southern part of Antarctica) and a sudden flood of fresh water from melting ice in the North Atlantic (caused by an event called Heinrich Event 11).

Think of the Earth's ocean circulation like a giant conveyor belt that moves heat around the planet. The study suggests that when a huge amount of fresh water dumped into the North Atlantic, it jammed this conveyor belt. This jam stopped warm water from heading north and instead forced it to pile up in the Southern Ocean, around Antarctica. It's like blocking a drain in a bathtub; the water has to go somewhere else, so it rises up in a different corner.

The team ran simulations with different versions of the Antarctic ice sheet: some where it was normal, some where the floating ice shelves were gone, and some where the whole West Antarctic Ice Sheet had completely collapsed, leaving an open ocean gateway. They also added different amounts of "hosing" (a scientific term for dumping fresh water) into the North Atlantic and the Southern Ocean.

Here is what they found: The only scenario that perfectly matched the real-world clues was a combination of two things. First, the West Antarctic Ice Sheet had to be significantly reduced, or even collapsed, creating a large gap of open ocean. Second, there had to be a massive influx of fresh water into the North Atlantic. When they combined these two factors, the model produced exactly the kind of warm Southern Ocean temperatures and reduced sea ice that we see in the geological records. It also explained why the ice cores from Antarctica show a specific chemical "fingerprint" (called enriched $\delta^{18}O$ values) that indicates the air was warmer and the moisture sources had changed.

The study suggests that the fresh water from the North Atlantic acted like a trigger. It warmed up the Southern Ocean, which then attacked the base of the West Antarctic Ice Sheet, causing it to collapse. This collapse, in turn, exposed more ocean to the sun, which melted even more ice and warmed the water further—a runaway effect. The researchers estimate that this specific collapse could have contributed up to about 4 meters to the global sea level rise during that time.

Crucially, the paper argues against the idea that the Antarctic ice sheet collapsed just because the air got warmer. The simulations showed that without the North Atlantic fresh water flood, the Southern Ocean didn't get warm enough to melt the ice sheet to the extent seen in the records. It also suggests that just melting the ice sheet wasn't enough to explain the warm ocean temperatures; you needed the North Atlantic event to push the heat south.

So, the story the paper tells is a chain reaction: A flood in the North Atlantic jammed the ocean's heat conveyor belt, pushing warmth toward Antarctica. This warmth helped melt the West Antarctic Ice Sheet, which opened up the ocean, which made the water even warmer, leading to a massive sea level rise. The authors are careful to say their results are based on computer simulations that match the data, suggesting this is a very likely explanation, but it's a reconstruction of the past, not a direct observation. However, the match between their "what-if" world and the real ancient clues is so strong that it gives scientists a new, clearer picture of how fragile these giant ice sheets can be when the ocean gets too warm.

Technical Summary: West Antarctic Ice Sheet collapse and North Atlantic meltwater reconcile Last Interglacial climate records

Problem Statement
The Last Interglacial (LIG; 131,000–118,000 years ago) is characterized by a global mean sea level (GMSL) approximately 5–10 meters higher than preindustrial levels. While this implies substantial melt contributions from both the Greenland and Antarctic ice sheets, the specific partitioning of this melt and the configuration of the Antarctic Ice Sheet (AIS) during this period remain unclear. Geological records, including marine diatoms and ice core data, suggest a significantly reduced West Antarctic Ice Sheet (WAIS) and anomalous isotopic enrichment ( $\delta^{18}O$ ) in Antarctic ice cores. However, existing explanations for these signals are conflicting: some attribute the enrichment to regional warming and sea ice retreat driven by Southern Ocean temperatures, while others suggest it results from changes in ice sheet topography and moisture transport due to WAIS collapse. Furthermore, the role of North Atlantic freshwater forcing (associated with Heinrich Event 11, ~135,000–130,000 years ago) in driving Southern Ocean warming and subsequent AIS retreat remains a critical but unquantified variable in reconciling these diverse proxy records.

Methodology
To address these uncertainties, the authors employed an ensemble of simulations using the water isotope-enabled version of the Community Earth System Model (iCESM). The study utilized four distinct Antarctic Ice Sheet topographic configurations based on previous ice-sheet modeling:

PI-like: Preindustrial configuration (Bedmap2) as a baseline.
LIG noShelves: Removal of Ross and Filchner-Ronne ice shelves (~2 m sea-level equivalent mass loss).
LIG noASE: Removal of the Amundsen Sea Embayment sector (~4 m sea-level equivalent mass loss).
LIG collapsed: Complete collapse of major ice shelves and interior WAIS, creating an open ocean gateway (~4 m sea-level equivalent mass loss).

These topographic scenarios were combined with various freshwater hosing experiments to simulate glacial meltwater fluxes:

North Atlantic Freshwater Flux (NAFWF): 0.2 Sv applied for 1,000 years between 50°N and 70°N, simulating meltwater release following Heinrich Event 11.
Southern Ocean Freshwater Flux (SOFWF): 0.2 Sv applied for 500 years poleward of 60°S, simulating local meltwater from AIS retreat.
Combined Scenarios: Simultaneous application of NAFWF and SOFWF.

The model outputs were rigorously compared against eight Antarctic ice core $\delta^{18}O$ records, a compilation of Southern Ocean summertime sea surface temperatures (SST $_{djf}$ ), and September sea ice concentrations (SICs). Model performance was assessed using Mean Squared Error (MSE) and bias minimization. Additionally, passive water tracers were employed to decompose the drivers of $\delta^{18}O$ changes into source vapor changes, rainout effects, and condensation fractionation.

Key Results

Oceanic Response: The inclusion of NAFWF significantly weakened the Atlantic Meridional Overturning Circulation (AMOC), reducing northward heat transport and accumulating heat in the Southern Ocean. This led to a 1–2°C warming across the entire Southern Ocean, even without AIS topographic reduction. In contrast, SOFWF alone cooled surface waters and expanded sea ice. WAIS collapse alone warmed the Southern Ocean south of ~50°S through ice-albedo feedbacks and exposure of the upper ocean to shortwave radiation.
Atmospheric and Isotopic Response: WAIS collapse induced heterogeneous surface temperature changes, with warming up to 6°C over stable West Antarctic regions and >10°C where ice was replaced by open ocean. Crucially, the collapse altered atmospheric circulation, strengthening easterly winds and inducing cyclonic anomalies that increased onshore moisture advection.
Reconciling Proxy Records: The simulation combining a fully collapsed WAIS and NAFWF (LIG collapsed NAFWF) provided the best fit for observed data, minimizing MSE and bias for both Antarctic ice core $\delta^{18}O$ $δ^{18} O$ and Southern Ocean SST/SIC proxies.
- $\delta^{18}O$ Enrichment: The study found that the dominant driver of $\delta^{18}O$ enrichment was "rainout" (preferential precipitation of heavy isotopes) due to warmer moisture sources and the removal of the topographic barrier by WAIS collapse, which allowed more efficient moisture advection inland. While NAFWF contributed to warming, the topographic change was the primary control on the isotopic signal.
- Sea Ice and SST: The best-performing simulations (involving NAFWF and reduced AIS) successfully reproduced the magnitude of Southern Ocean warming and sea ice retreat observed in proxy records. Simulations with only SOFWF failed to reproduce these conditions.
Decomposition of Drivers: Water tag analysis revealed that while source vapor depletion occurred in all scenarios, the rainout signal accounted for the majority of the $\delta^{18}O$ enrichment. The shift in moisture sources was primarily driven by the removal of the WAIS topographic barrier rather than solely by temperature changes.

Significance and Claims
The paper claims that the anomalous climate signals observed during the LIG—specifically the highly enriched $\delta^{18}O$ values in Antarctic ice, warm Southern Ocean temperatures, and reduced sea ice—are best explained by the combined effects of a substantially reduced WAIS and the release of glacial meltwater into the North Atlantic following Heinrich Event 11.

The authors conclude that:

Mechanism of Collapse: The North Atlantic meltwater pulse likely drove Southern Ocean warming early in the LIG, which forced the melt of marine-grounded portions of the WAIS. The resulting AIS retreat further amplified subsurface warming through local feedbacks, potentially triggering a full collapse of the WAIS.
Sea Level Contribution: This scenario suggests the Antarctic Ice Sheet may have contributed up to ~4 meters to the LIG global mean sea level rise, aligning with geological estimates when combined with contributions from Greenland and other sources.
Vulnerability: The results demonstrate that the Antarctic Ice Sheet is highly vulnerable to ocean warming, with a potential for rapid, ocean-forced collapse. The study highlights that subsurface warming, which is currently occurring in the Southern Ocean, could be a critical precursor to future ice sheet instability, even if surface warming rates appear modest.

The study emphasizes that while orbital forcing initiated the LIG, the specific configuration of the ice sheet and the interhemispheric coupling via meltwater fluxes were essential to reproduce the magnitude and pattern of the observed paleoclimate records.

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