Modeling the Arctic's deep freeze: what the data reveals

As you can see from satellite images of the Arctic today, the boundary between ice and water shifts significantly: the sea freezes in winter, retreats in summer, and follows this cycle year after year. But has this seasonal pattern always existed? Or was the Arctic once buried beneath a vast, permanent ice cover more than a kilometer thick?

A study published in Science Advances by researchers from the Geological Survey of Norway and UiT, The Arctic University of Norway, Plymouth University, the University of Exeter, the Alfred Wegener Institute in Germany, and GEOMAR Helmholtz Centre for Ocean Research, suggests that for at least the past 750,000 years, Arctic Ocean ice has been seasonal, even during the coldest ice ages. This challenges a long-standing idea that the Arctic was once sealed beneath a continuous ice shelf, effectively cut off from the atmosphere and ocean circulation.

The research team combined chemical signals preserved in ocean sediments with advanced climate modeling and reconstructed ancient sea-ice conditions. Their findings reshape how scientists understand the role of the Arctic in Earth's climate system over nearly a million years of glacial cycles.

The idea of a pan-Arctic ice shelf was first proposed in the 1970s. At the time, scientists envisioned a kilometer-thick sheet of ice spanning the Arctic Ocean during peak glacial periods, heavy enough to scour the seafloor. Geological features such as grooves etched into underwater ridges appeared to support this interpretation.

However, these features are difficult to interpret. They could have been created by grounded ice shelves—or by large icebergs scraping the seabed as they drifted. As analytical techniques improved, so did disagreement. Some researchers maintained that the Arctic froze solid at times, while others argued for a more variable ice cover.

The researchers approached the question using a different kind of evidence: organic compounds produced by microscopic Arctic organisms.

When sea ice forms, it supports communities of tiny algae known as diatoms. One molecule produced by these organisms, IP25, serves as a reliable indicator of seasonal sea ice. When the ice melts in spring, the algae sink to the seafloor, where the molecule can remain preserved in sediment for hundreds of thousands of years.

By measuring IP25 alongside two open-water biomarkers—brassicasterol and dinosterol, produced by plankton—the researchers could determine whether past conditions favored year-round ice, seasonal ice, or ice-free waters.

They examined sediment samples from two locations, the ODP Site 910A on the Yermak Plateau north of Svalbard, a key gateway between the Arctic Ocean and the Atlantic, and Core MD99-2277 in the central Nordic Seas, between Greenland and Norway

These sediments form layered records of climate history, with each layer representing thousands of years. Using isotopic data, microfossils, and magnetic markers, the team assembled a continuous record covering the last 750,000 years, spanning multiple glacial and interglacial periods.

The results were consistent across the record. IP25 appeared in every sample, indicating that sea ice melted seasonally even during the most severe ice ages. This seasonal melting allowed sunlight to reach surface waters and supported phytoplankton growth each summer.

In other words, the Arctic was not a permanently frozen, biologically inactive environment. Open-water indicators reinforced this conclusion, showing that marine life persisted even during glacial extremes.

There was one exception. Around 670,000 years ago, during Marine Isotope Stage 16, biological productivity declined sharply. For a brief interval—perhaps one or two thousand years—sea ice likely expanded farther south, approaching near-permanent coverage. Even so, any continuous ice shelf appears to have been short-lived.

To test whether these chemical records aligned with physical climate processes, the team used the AWI Earth System Model (AWI-ESM2) developed at the Alfred Wegener Institute. They simulated Arctic conditions during several climate states: the pre-industrial period, the Last Glacial Maximum (21,000 years ago), and the Penultimate Glacial Period (140,000 years ago).

The simulations showed that even during extreme cold phases, the Nordic Seas likely retained seasonal open water. Warm Atlantic currents continued to flow northward, limiting ice expansion and sustaining marine ecosystems. This behavior closely matched what the sediment data indicated. The evidence suggests that the Arctic was never a static, frozen cap. Instead, it remained a seasonally active environment, even during the coldest phases of the ice ages. Periods of open water allowed ecosystems to persist and oceans to continue interacting with the atmosphere.

So, the Arctic has always had seasons, even in its coldest times. What’s different now is the speed of change. The findings also affect how scientists think about ice-sheet growth and ocean circulation during ice ages. If the Arctic Ocean remained at least partly open, ocean heat transport may have played a larger role in regulating global climate than previously assumed.

Reconstructing past Arctic ice conditions helps scientists place modern changes in context. The Arctic is now warming roughly four times faster than the global average, with summer sea ice declining rapidly. Comparing these changes to long-term natural variability helps clarify how unusual current trends are, and how quickly they are unfolding.

Nowadays, the planet is entering a warming phase driven by human activity rather than natural cycles. Therefore, understanding this long-term behavior provides important perspective. The Arctic has proven resilient to dramatic climatic swings, but the pace and cause of the changes we are witnessing today are without precedent.

If you want to learn more, read the original article titled "Seasonal sea ice characterized the glacial Arctic-Atlantic gateway over the past 750 000 years" on Science Advances at http://dx.doi.org/10.1126/sciadv.adu7681.