Long ago, when ice sheets spread across much of the Northern Hemisphere, the Atlantic Ocean existed in a world that looked nothing like today. Massive glaciers covered land. Sea levels were lower. Temperatures plunged.
It’s easy to imagine the oceans slowing down too – a cold world with cold seas and weak currents.
But the ocean does not always follow simple rules. New evidence shows that even during one of the coldest chapters in Earth’s history, the Atlantic Ocean kept doing its job.
The ocean kept moving heat and salt – and it continued to shape the climate in ways that still matter now.
Cold world, active Atlantic waters
The focus of this story sits far below the waves, in a layer of water called North Atlantic Deep Water.
Today, this water forms when salty surface water cools, becomes dense, and sinks thousands of feet into the ocean. That sinking motion helps power the Atlantic’s massive circulation system.
For years, many scientists assumed this deep water nearly froze during the last ice age. The thinking was logical. More ice should mean colder oceans and weaker circulation. Yet new findings tell a different story.
During the height of the last ice age, this deep Atlantic water was only about 1.8°C colder than today. It also filled nearly the same vertical space in the ocean, stretching from about 0.6 to 2.5 miles below the surface.
That means the Atlantic circulation did not stall. It stayed active, even under extreme cold.
Tiny fossils with a long memory
To uncover this history, researchers turned to the ocean floor. Mud samples collected near the Bahamas, Bermuda, South Carolina, and Iceland held tiny fossil shells called foraminifera.
These organisms once floated in seawater. As they grew, their shells locked in chemical signals from the water around them.
By studying those signals, scientists could tell how warm and salty the water was when the organisms lived, between about 19,000 and 23,000 years ago.
The shells showed that deep waters carried chemical traces linking them to surface waters from the subtropics and Nordic Seas.
That link matters. It means warm surface water still sank and fed the deep Atlantic, keeping heat moving north.
What surprised the scientists
“We were amazed to find that the deep Atlantic stayed relatively warm and salty during one of Earth’s coldest periods,” said Dr. Jack Wharton, an expert in the Department of Geography at University College London.
“Taken together, our data tell us the ocean’s circulation system kept running even under extreme conditions, which is crucial for understanding how our climate engine works.”
“The same climate models that correctly predicted this past behaviour also warn that these currents are vulnerable to weakening as the planet warms – and that could have dramatic consequences for future climate,”
That mix of surprise and concern runs through the study. The past offers reassurance that climate models can get big things right. It also raises alarms about what comes next.
A closer look across the Atlantic
The study did not rely on one location or one set of samples. By comparing fossil data from many parts of the North Atlantic, the team could see the full picture.
“The microfossils recovered from the ocean floor show that deep waters in the North Atlantic were far from freezing during the last ice age,” said Professor David Thornalley.
“By examining locations across the North Atlantic, we can show that warm, salty surface waters continued to sink and form North Atlantic Deep Water that reached similar depths to today.”
That sinking motion is key. It shows the deep ocean behaved much like it does now, even when ice ruled the planet.
Why this matters today
The Atlantic Meridional Overturning Circulation (AMOC) acts like a global heat mover.
The AMOC carries warmth from the tropics northward and helps keep Europe and parts of North Africa relatively mild. When this system weakens, regional climates can change fast.
The ice age findings line up closely with what climate models predicted for that period. That builds trust in those same models when they look forward.
Many of them suggest a warmer planet could weaken this circulation, not strengthen it. As surface waters warm, they become less dense and sink less easily. That slows the entire system.
“This research helps us better understand the mechanisms that drive ocean circulation and improves our ability to predict future climate change,” said Professor Mark Maslin.
“Many of our best climate models indicate that Atlantic circulation is likely to weaken under the type of warming we’re likely to face in the coming decades – it would have a tremendous, destabilising impact on the climate of Europe and North Africa.”
A colder future in a warmer world
If the AMOC were to shut down, the effects would be severe. Average annual temperatures in the UK could fall by as much as 7°C by the end of this century. Winters could become up to 15°C colder.
Sea ice could reach the shores of Scotland. Farmland across the UK and much of Europe would shrink, and African monsoon patterns could be thrown off balance.
The last ice age shows that the ocean can endure incredible stress. It also shows that change does not always move in obvious directions.
A warming planet does not guarantee a warmer future everywhere. Sometimes, the past warns us by doing the opposite of what we expect.
The full study was published in the journal Nature.
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