Ocean memory amplifies Antarctica's sea-level contribution after climate overshoot

The ice sheets of Antarctica are not static blocks of frozen water; they are dynamic, slow-moving rivers of ice that constantly interact with the air above and the ocean below. For centuries, scientists have understood that when the atmosphere warms, the ice melts. However, a more subtle and powerful force is at play beneath the floating edges of these ice sheets: the ocean. Warm seawater can melt the ice from below, thinning the floating shelves that hold back the massive glaciers on land. When these shelves weaken or break apart, the glaciers behind them speed up, dumping more ice into the sea and raising global water levels. This process is governed by a concept known as "ocean memory." Just as a hot cup of coffee takes a long time to cool down even after the room temperature drops, the deep ocean retains heat for decades or centuries after the air above it begins to cool. This lingering warmth means the ocean can continue to erode ice long after the atmosphere has started to recover.

This delay creates a critical uncertainty for the future of sea levels. If the world manages to lower greenhouse gas emissions and cool the atmosphere, will the ice sheets immediately stop losing mass, or will they continue to shrink because the ocean remains warm? A new study led by researchers from the University of Tasmania and international partners explores this question using advanced computer simulations. They asked a specific, urgent question: if the planet temporarily exceeds the temperature limits set by the Paris Agreement before cooling back down, how long does that period of high heat need to last to cause irreversible damage? More importantly, does the speed at which the ocean cools matter more than the speed at which the air cools? The researchers found that the duration of the heat exposure is important, but the timing of the ocean's recovery is the true game-changer. Even if the air cools quickly, a slow-cooling ocean can continue to drive ice loss for generations, amplifying the rise in sea levels far beyond what the atmosphere alone would suggest.

To investigate this, the team used a powerful computer model of the entire Antarctic ice sheet, a digital twin that simulates how ice flows, melts, and reacts to changing conditions. They started with a baseline scenario where the planet follows a path of low emissions, keeping warming relatively modest. Then, they created several "overshoot" scenarios. In these simulations, the climate was forced to follow a high-emissions path for a set period—20, 50, 75, or 100 years—before abruptly switching back to the low-emissions path. This allowed them to test how the length of the heat exposure affected the ice sheet's future. They also ran a separate set of experiments to see what happened if the air cooled down quickly while the ocean stayed warm for much longer, mimicking the real-world possibility that the deep ocean takes centuries to shed its heat.

The results revealed a stark difference between short and long periods of heat. When the high-emissions forcing lasted for only 20 or 50 years, the extra ice loss compared to the low-emissions path was relatively small, adding less than 3 millimeters to global sea levels by the year 2300. However, as the duration of the heat increased, the damage grew non-linearly. After 75 years of high forcing, the extra contribution jumped to 8.6 millimeters. After 100 years, it surged to 21.5 millimeters. This suggests that shortening the time the planet spends in a high-heat state offers a significant benefit, but the relationship is not a simple straight line; the longer the heat persists, the more severe the consequences become. The study also showed that the location of the ice loss shifts over time. In shorter scenarios, the East Antarctic ice sheet was the main contributor to the extra loss, but in the longest scenarios, the West Antarctic ice sheet became the dominant source of melting, a region known for its vulnerability to rapid collapse.

The most surprising finding emerged when the researchers separated the recovery of the air from the recovery of the ocean. In one scenario, they let the atmosphere cool down after 50 years of heat, but kept the ocean forcing high for another 50 years. In this case, the extra sea-level contribution skyrocketed to 44.1 millimeters by 2300, nearly double the amount seen when both the air and ocean cooled together after 50 years. This happened because of a delicate balance between snow and melt. When the atmosphere cools, it usually brings more snowfall, which can help replace lost ice. However, if the ocean stays warm, it continues to melt the ice from below, weakening the shelves and speeding up the flow of glaciers. The study found that the continued ocean melting eventually overwhelmed the benefits of the increased snowfall. In fact, when the ocean stayed warm while the air cooled, the ice sheet lost more mass than it would have if the high heat had persisted in both the air and ocean simultaneously. This is because the cooling air reduced the snowfall that would have otherwise helped offset the ocean-driven loss.

The researchers also tested how different assumptions about the ice sheet's behavior and the climate data affected these outcomes. They tried using different climate models and different ways of calculating how fast the ice slides over the bedrock. While the exact numbers varied depending on these settings, the main pattern held true: a delayed recovery of the ocean always led to greater sea-level rise. Whether they used a standard climate model or one that predicted more snowfall, the message remained consistent. If the ocean stays warm while the air cools, the ice sheet continues to lose mass at an accelerated rate. This confirms that the "memory" of the ocean is a dominant factor in the future of Antarctic ice.

These simulations do not predict a specific future, but they highlight a critical risk in our current climate trajectory. The study suggests that simply lowering global temperatures is not enough to stop the ice sheets from contributing to sea-level rise if the ocean remains warm. The recovery of the ice sheet depends heavily on how quickly the ocean sheds its heat. If the ocean takes decades or centuries to cool, the ice will continue to retreat, driven by the lingering warmth beneath the shelves. This means that the benefits of cutting emissions could be partially undone if the ocean's recovery is slow. The findings underscore that the fight against sea-level rise is not just about how hot the planet gets, but also about how long the heat lingers in the deep ocean. For policymakers and planners, this adds a new layer of complexity: the timing of climate recovery matters as much as the peak temperature itself. The study concludes that while shortening the duration of high emissions is a powerful tool to limit sea-level rise, its success depends entirely on how quickly the ocean follows suit. Without a rapid cooling of the ocean, the ice sheet will continue to pay the price of past warming for centuries to come.