Complex dynamics will govern future Arctic sea-ice loss, new study suggests

Two distinct bursts of warm Atlantic water reached the Fram Strait between Greenland and Iceland more than three million years ago, when the world’s climate was even hotter than today. The warm water helped melt sea ice, but was then trapped below a fresher surface layer, a new study has found. 

The study’s findings have implications for climate modelling, ecosystem management, shipping, and fisheries.

 Led by Sina Panitz from the Department of Geosciences at iC3’s host university, researchers studied sediments from the seabed north of Svalbard, where warm Atlantic water currently enters the Arctic Ocean. 

Within the sediment, her team looked at tiny fossil remains from marine plankton. These fossils, called dinoflagellate cysts, can show whether past surface waters were warm and Atlantic, or cold and polar.

The researchers focused on a period between about 3.4 and 3.0 million years ago, which includes a warm interval that may be able to tell us about Arctic conditions in a future, hotter world. 

Most of the time, the surface ocean at the site was cold and polar. But twice, at around 3.19 and 3.10 million years ago, the team found signs of warmer Atlantic surface water.

The findings may change how scientists think about future Arctic sea-ice loss.

The result was not simple warming from top to bottom. Instead, the water column became more strongly layered following the bursts of Atlantic water. Warm Atlantic water sat below a fresher surface layer linked to sea-ice melt. This surface layer may have acted like a lid.

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Fossil clues from the seafloor

The team compared these new results with earlier chemical evidence from the same drill site. That earlier work showed changes in deeper water, sea ice and ocean layering.

 “What surprised us was the order of events,” Sina said. “The Atlantic water first appeared at the surface. Then, later, stronger Atlantic water inflow showed up deeper down.”

Together, the records point to a three-step process:

First, polar waters and sea ice dominated. 

Then, Atlantic surface waters arrived and helped melt sea ice. 

Finally, Atlantic water increased deeper down, while the surface became fresher and more strongly separated from the depths. 

“This gives us a rare view of how the Arctic Ocean can reorganise itself,” said Jochen Knies from the iC3 Polar Research Hub, senior author of the study. “It shows that more Atlantic heat does not always mean a direct loss of sea ice at the surface.”

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Why this matters now

Today, as the Arctic is rapidly warming, Atlantic water is also becoming more important in parts of the Arctic Ocean.

This process is often linked to sea-ice decline, because warmer and saltier water can increase mixing. That can bring heat up towards the surface.

 The new study shows another possible outcome over longer timescales. Sea-ice melt can freshen the surface ocean. That freshwater can strengthen layering and help keep deeper heat away from the atmosphere and sea ice.

This does not mean future sea-ice loss will stop. But it does suggest that Arctic change may not follow a straight line.

“Models need to capture both sides of this story,” said Sina. “Atlantic heat can speed change, but freshwater from melting ice can also change how that heat moves through the ocean.”

 “Our findings have implications beyond climate projections. They are also relevant for ecosystem management, shipping, and fisheries.”

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A wider iC3 puzzle

 The study connects closely to earlier iC3 work on past Arctic climates, which has looked at whether the Arctic was ice-free during past warm periods, and examined how glaciers transformed Arctic ocean currents. The iC3-affiliated Into the Blue project will spend several years collecting additional records of past Arctic climates and analysing them.

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Paper and researchers

The paper, “Late Pliocene Atlantic surface water pulses precede enhanced deeper water inflow and stratification in the Fram Strait”, is available open access in the journal Geology.

Sina Longman (née Panitz) is a postdoc at the Department of Geosciences of UiT The Arctic University of Norway, located in Tromsø. Jochen Knies leads the iC3 research unit exploring how past changes in ice affected the global carbon cycle and marine ecosystems. He additionally co-leads the Into the Blue project.