For decades, global climate models have predicted that the ocean surrounding Antarctica should be warming due to climate change. However, real-world data tells a different story—these waters have actually cooled over most of the past forty years. This apparent contradiction has puzzled scientists and policymakers alike, raising fundamental questions about how accurately climate models represent Earth’s changing climate.
Now, researchers from Stanford University have uncovered the primary culprit behind this discrepancy: missing meltwater and underestimated rainfall. Their findings, published in Geophysical Research Letters, suggest that Antarctic cooling is not an anomaly but rather a direct response to global warming itself. The discovery has major implications for future climate projections, particularly regarding sea level rise, ocean circulation, and extreme weather patterns across the globe.
Why Are Antarctic Waters Cooling?
At first glance, the idea that global warming could trigger ocean cooling seems counterintuitive. However, the mechanism behind this paradox is relatively straightforward. As rising global temperatures melt Antarctica’s vast ice sheets, the resulting influx of freshwater into the ocean creates a layer of less salty (fresher) water at the surface.
This fresh surface layer is much less dense than the saltier water below it, making it harder for the two layers to mix. Normally, deep ocean currents bring warmer waters from the depths up to the surface, where they release heat into the atmosphere. But with a growing barrier of freshwater at the top, this natural mixing process is severely weakened.
“The fresher you make that surface layer, the harder it is to mix warm water up,” explained Earle Wilson, assistant professor of Earth system science at the Stanford Doerr School of Sustainability and senior author of the study. As a result, heat remains trapped in deeper ocean layers, while the surface cools—creating the cooling trend that has puzzled scientists for decades.
The Missing Factor in Climate Models
One of the biggest reasons climate models fail to capture this cooling trend is that they don’t fully account for the impact of meltwater on ocean circulation. Wilson emphasized this flaw, noting, “The impact of glacial meltwater on ocean circulation is completely missing from most climate models.”
This oversight has created a major source of uncertainty in projections of future sea level rise and regional climate patterns. The missing freshwater not only alters sea surface temperatures but also affects the formation of sea ice, the movement of ocean currents, and even the behavior of large-scale climate systems like El Niño and La Niña.
Climate models are crucial tools for policymakers attempting to prepare for future climate impacts. If they continue to underestimate the effects of freshwater input, our understanding of near-term climate changes—especially in regions influenced by Southern Ocean dynamics—may be significantly off-target.
Reconciling Global Climate Discrepancies
The cooling of the Southern Ocean is just one piece of a larger puzzle. Many global climate models also struggle to accurately simulate temperature trends in other parts of the world.
For example, these models tend to underestimate the cooling observed in the eastern Pacific and the Southern Ocean while overestimating the warming in the Indian and western Pacific Oceans. Another well-known discrepancy involves the frequency of La Niña events, which are characterized by cooler-than-average sea surface temperatures in the eastern Pacific.
Understanding why these mismatches occur is critical, as ocean temperature patterns strongly influence weather systems worldwide. The Southern Ocean, in particular, plays a vital role in regulating global climate.
“Oceans globally have absorbed more than a quarter of the carbon dioxide emitted by human activities and more than 90% of the excess heat trapped in our climate system by greenhouse gases,” said Zachary Kaufman, the study’s lead author and a postdoctoral scholar in Earth system science. “The Southern Ocean is one of the primary places that happens.”
Because the Southern Ocean is responsible for absorbing such vast amounts of heat and carbon, its surface temperatures affect everything from global sea level rise to rainfall patterns as far away as California. If scientists fail to properly account for its dynamics, future climate predictions may miss critical details that could shape climate policy and disaster preparedness strategies worldwide.
A Surprising Discovery About Meltwater’s Role
To better understand how freshening affects the Southern Ocean, Wilson and Kaufman set out to quantify how much of the observed cooling in climate simulations was due to freshwater inputs.
Their initial assumption was that the distribution of meltwater wouldn’t significantly impact the results. “We naively figured it wouldn’t matter exactly where you put the freshwater,” Wilson admitted.
But the researchers were surprised to find that surface temperatures were much more sensitive to meltwater concentrated along the Antarctic coast than to rainwater falling more broadly over the ocean.
“Applying freshwater near the Antarctic margin has a bigger influence on sea ice formation and the seasonal cycle of sea ice extent, which then has downstream impacts on sea surface temperature,” Wilson explained.
This discovery suggests that localized meltwater input plays a far greater role in shaping the Southern Ocean’s temperature and circulation patterns than previously thought. Future climate models will need to incorporate these effects more accurately to improve their predictive power.
Quantifying the Effect of Missing Meltwater
In previous studies, researchers have attempted to assess the impact of Antarctic meltwater on global climate by adding a fixed amount of freshwater to a single climate model simulation—an approach known as a “hosing” experiment.
However, this method often produces inconsistent results. “You get very divergent results because people set up their experiments slightly differently, and the models are a little different,” Wilson noted. “It’s unclear if these are really apples-to-apples comparisons.”
To avoid this problem, Wilson and his team took a new approach. They worked with multiple simulations, using data from 17 different climate models. This allowed them to isolate the role of missing freshwater with far greater accuracy.
Their analysis revealed that the missing meltwater explains up to 60% of the discrepancy between observed and predicted Southern Ocean surface temperatures from 1990 to 2021.
“There’s been some debate over whether that meltwater is enough over the historical period to really matter,” Kaufman said. “We show that it does.”
Implications for Future Climate Research and Policy
The study provides compelling evidence that the melting of Antarctica’s ice sheets is already altering ocean dynamics in ways that climate models have yet to fully capture. These changes are not just theoretical—they are actively reshaping the global climate system.
By improving how climate models account for meltwater input, scientists can refine their predictions of sea level rise, storm intensity, and global weather patterns.
Wilson emphasized the broader significance of the findings: “We’ve known for some time that ice sheet melting will impact ocean circulation over the next century and beyond. Our results provide new evidence that these meltwater trends are already altering ocean dynamics and possibly the global climate.”
This research serves as a crucial step toward bridging the gap between climate models and real-world observations. But it also underscores the urgency of addressing climate change. If Antarctica’s melting ice is already disrupting ocean circulation today, the consequences for future generations could be even more profound than current models suggest.
As scientists continue to refine climate projections, one thing is clear—the Southern Ocean is not an isolated system. What happens in the icy waters around Antarctica has ripple effects across the entire planet.
Reference: The Impact of Underestimated Southern Ocean Freshening on Simulated Historical Sea Surface Temperature Trends, Geophysical Research Letters (2025). DOI: 10.1029/2024GL112639. agupubs.onlinelibrary.wiley.co … 10.1029/2024GL112639