When we think of a warming Arctic, most of us imagine melting sea ice on the surface and rising ocean levels. These changes are dramatic and visible. But deep beneath the ice sheet, in the freezing darkness thousands of meters below, Below the surface, another, lesser-known but equally disturbing drama is taking place. The deep waters of the Arctic Ocean are also warming, but the process is far from uniform.

The Arctic Ocean is geologically divided into two main parts: the Eurasian Basin and the Amerasian Basin. Recent measurements have revealed a startling fact – the deep waters in the Eurasian Basin are warming dramatically faster than in its neighboring Amerasian part.

Why does this huge difference exist? Why is one half of the planet's northernmost ocean warming significantly faster than the other? The answer revealed by a new research, points to a surprising culprit located far beyond the Arctic Ocean itself, and to a massive undersea barrier that is deciding the fate of the entire region.

First clue: Almost sevenfold difference in temperature and the dead ends of science

The data from the last decades is clear. At depths of 2000 to 2600 meters, the Eurasian Basin (EB) is warming at a rate of approximately 0.020°C per decade. At the same depth, however, the neighboring Amerasia Basin (AB) is warming only at a rate of 0.003°C per decade. That's almost a seven-fold difference.

Traditional explanations, such as heat rising from the Earth's interior (geothermal heat), are not sufficient to explain this phenomenon. Although geothermal flow contributes to the overall ocean heat budget, the observed warming rate in the Eurasian Basin is almost five times greater than even the strongest geothermal flow in the region could produce.

Another theory was so-called slope convection – a process in which dense, salty water created when ice freezes in shallow waters sinks to the depths, dragging warmer water with it. But modelling showed that even this mechanism could not explain such a massive and localized increase in temperature in the Eurasian Basin. The real cause had to lie elsewhere. So scientists had to turn their attention from the ocean floor to its edges – and it was there, in the neighbouring Atlantic, that they found a key piece of the puzzle.

The Culprit Revealed: Atlantic Warm Current in Disguise

The search led scientists to a key player in this climate drama: the deep Greenland Basin (GB), which borders the Arctic Ocean. Historically, this basin acted as a source of very cold, dense water that flowed into the Eurasian Basin, keeping it cool. However, this cooling mechanism has been failing in recent decades.

The Greenland Basin is experiencing one of the fastest warming trends observed in the global deep ocean. Between 1990 and 2022, its deep waters warmed at an astonishing rate. 0.103°C per decade.

As the water in the Greenland Basin itself has warmed dramatically, its cooling effect on the Eurasian Basin has weakened significantly. „Less cold“ water is now flowing into the Arctic, which ultimately causes its accelerated warming. The key turning point occurred around 2018. At that time, the temperature in the deep Greenland Basin reached a level comparable to the Eurasian Basin. At this point, the deep water of the Greenland Basin not only stopped cooling the Eurasian Basin, but began to actively contribute to its warming.

Climate divide: The fate of the Arctic is determined by a huge undersea mountain range

But the question remains: why isn't the Amerasian Basin also feeling this warming effect? The answer lies in the topography of the seafloor—specifically, the Lomonosov Ridge (LR). It is a massive submarine mountain range—essentially a giant mountain range hidden beneath the ocean—with an average depth of about 1,500 meters, which physically separates the Eurasian and Amerasian Basins.

This ridge acts as a gigantic underwater barrier. It blocks the flow of warming water from the Greenland Basin and prevents it from advancing further into the Amerasian Basin. This warming signal remains "trapped" in the Eurasian Basin.

The result is a clearly divided fate. The Eurasian Basin is directly exposed to the warming influence from the Atlantic, while the Amerasian Basin is shielded from this influence. This is the main reason why we see such a dramatic difference in their warming rates.

The mystery of the uneven warming of the Arctic depths is thus solved. It is not primary heat from the Earth's interior, but a chain reaction triggered in the Atlantic Ocean. Rapid warming of the deep Greenland Basin sends "less cold" water into the Eurasian Basin, while the massive Lomonosov Ridge acts as a climatic dividing line, protecting the other half of the Arctic.

This discovery shows how even the most remote and deepest parts of our planet are interconnected in a complex climate system. But a key question remains: As this hidden warming in the depths continues, what other unforeseen chain reactions might be triggered in the planet's most sensitive ecosystems? JRi

The study was published in the journal Science Advances