Melting ice sheets in North America added 30 feet to global sea levels

For decades, the standard story went like this: as the last ice age waned, Antarctica’s vast ice sheets were believed to be the main engine pushing global sea levels higher.

However, a new study turns that narrative upside down. Led by Tulane University, the work shows that retreating North American ice sheets were the dominant driver of sea level rise between 9,000 and 8,000 years ago. This added over 30 feet (about 10 meters) to the world’s oceans.

“[Such a discovery] requires a major revision of the ice melt history during this critical time interval,” said Torbjörn Törnqvist, a geology professor at Tulane and co-author of the study.

“The amount of freshwater that entered the North Atlantic Ocean was much larger than previously believed, which has several implications.”

Hidden marshes reveal the past

Reconstructing sea levels earlier than 8,000 years ago is notoriously difficult. The best records are often locked offshore, requiring costly and technically daunting drilling campaigns.

The Tulane-led team, however, found a rare opportunity on land. Former Tulane postdoctoral researcher Lael Vetter discovered deeply buried ancient marsh sediments on the west bank of the Mississippi River across from New Orleans.

Those layers – precisely dated with radiocarbon – record how the coastline rose and fell through time.

Building on that foundation, former Tulane Ph.D. student Udita Mukherjee assembled a global comparison. She combined the Mississippi Delta record with high-quality sea-level data from Europe and Southeast Asia.

These regions “tilted” differently as ice loads shifted, making them ideal for separating which ice sheets were melting – and when.

New data reshapes the ice melt story

The pattern that emerged was striking. The timing and rate differences between sites could only be explained if North America’s ice sheets contributed far more meltwater than previously assumed. At the same time, Antarctica’s role remained comparatively modest.

“This research provides a stark reminder of the complexities of our climate system and melting ice sheets,” said Mukherjee, now a postdoctoral fellow at the University of Hong Kong.

“Broadening our focus beyond North America and Europe to include valuable, high-quality data from Southeast Asia was critical for this study. By embracing a truly global perspective in climate studies, we can enhance our understanding and work together towards a sustainable future.”

Ice melt meets the Atlantic

The finding has immediate relevance for one of Earth’s most consequential climate gears: the Atlantic Meridional Overturning Circulation (AMOC), the system of currents that includes the Gulf Stream.

The AMOC ferries heat from the tropics toward the North Atlantic, helping keep Northwestern Europe milder than its latitude would suggest. It is sensitive to freshwater.

Large pulses of meltwater can freshen the ocean’s surface, making it lighter and less inclined to sink, potentially weakening the circulation.

By showing that the North Atlantic absorbed an even larger jolt of freshwater at the end of the last ice age than scientists had believed, the study stumbles upon a paradox. If the AMOC endured that onslaught without a lasting collapse, perhaps the system is more resilient than some recent modeling studies suggest.

The Gulf Stream’s uncertain future

The findings suggest that the AMOC was surprisingly resilient in the past. By contrast, recent studies have concluded that a weakening – or even a collapse – of the Gulf Stream could be imminent.

Törnqvist cautioned against simple conclusions: “Clearly, we don’t yet fully understand what drives this key component of the climate system.”

That nuance matters. The late-glacial world differed markedly from today’s. Ice sheets were larger, and coastlines and wind patterns were distinct. Moreover, freshwater was delivered in pulses tied to retreating North American ice.

Still, the new reconstruction challenges modelers to reconcile an ocean circulation that withstood a much larger freshwater shock with projections of 21st-century risk.

Clues from land and sea

At the heart of the findings lies a simple but powerful approach. Global sea level may be a single number, but Earth’s crust does not respond uniformly as ice melts. Regions once buried under thick ice rebound upward after the weight lifts, while far-field locations sink slightly.

Ocean water redistributes as well. That means tide gauges, marshes, corals, and other sea-level “recorders” rise and fall by different amounts at different times as ice sheets wax and wane.

By stitching together records from the Mississippi Delta, Europe, and Southeast Asia – each with distinct sensitivities to ice-loss patterns – the researchers tested which melt scenarios could reproduce the observed global mosaic.

Scenarios that relied heavily on Antarctic melt came up short. Only a substantially larger contribution from the waning Laurentide and Cordilleran ice sheets over North America could match the data.

The key insight, in other words, did not come from a single spectacular core but from a global jigsaw puzzle assembled with care.

From past ice melt to future risk

The study does not let Antarctica off the hook today. West Antarctica’s marine-based ice sheet remains a major long-term threat, and Greenland’s melt is already raising global sea levels.

What the new reconstruction does is sharpen the baseline for how Earth’s system behaved under a massive freshwater pulse. This approach corrects a long-standing misallocation of credit between hemispheres for a pivotal period of postglacial sea-level rise.

The study also highlights the value of unconventional archives. The ancient marshes buried beneath the modern New Orleans landscape proved to be as revealing as an offshore borehole.

As the Artemis era of lunar exploration reminds us of the power of well-chosen landing sites, the Tulane-led work shows the same truth in paleoclimate research: where you look matters.

With a better-resolved history of what melted when, scientists can refine models of ocean circulation, sea-level rise, and climate teleconnections.

Armed with this knowledge, policymakers can plan for a future where, once again, the question is not only how much ice melts – but where.

The research is published in the journal Nature Geoscience.

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