Climate heating has reached even deepest parts of the Arctic Ocean

Warmer Atlantic water from near Greenland is heating up the depths of the Arctic Ocean, which was previously thought of as one of the few places not significantly affected by climate change.

The sea ice on top of the Arctic Ocean has shrunk by about 40 per cent in four decades, due largely to the effect that atmospheric warming has on the surface of the ocean. Now researchers from the Ocean University of China have analysed the latest measurements taken using icebreaker ships to estimate warming at the bottom of the ocean.

In one of the ocean’s two major basins – the Eurasian basin – the waters between 1500 and 2600 metres deep have warmed by 0.074°C since 1990.

While that doesn’t sound like much, it represents the transfer of almost 500 trillion megajoules of energy. If that amount of energy were present at the surface, it could melt as much as a third of the minimum sea ice extent.

“The deep ocean is much more active than what we thought,” says Xianyao Chen, a member of the research team. “I thought the deep ocean could be warming, but not so fast.”

An underwater mountain range running between Greenland and Siberia divides the Arctic Ocean into two basins. While the Amerasian basin is largely walled off from the Pacific by the shallow Bering Strait, an extension of the Atlantic Meridional Overturning Circulation, or AMOC, brings warm Atlantic water northwards along the coast of Scandinavia and into the upper layers of the Eurasian basin. As seawater freezes in the winter, the salt in it is ejected from the crystals. This forms dense water that sinks to the depths, taking some of the warm water from the Atlantic with it.

The geothermal heat of Earth also warms deep water in the Eurasian basin.

Previously, these warming processes have been offset by an influx of cold deep water from the basin immediately to the east of Greenland. But as the Greenland ice sheet melts, more freshwater has entered the Greenland basin. This has slowed the sinking of cold, salty water to the deep and helped raise the deep water temperature in the Greenland basin from -1.1°C to -0.7°C – one of the fastest warming rates in the deep ocean. As a result, the movement of Greenlandic deep water into the Arctic Ocean no longer cancels out the geothermal bottom heat and sinking of warm Atlantic water.

“The warming of the Greenland basin has extended to the Arctic,” says Ruizhe Song, part of the research team.

This research has shown a new heating process in the deep Arctic Ocean, “implicating global warming in yet another location”, says James McWilliams at the University of California, Los Angeles.

This warming could eventually begin to contribute to sea ice melt or even thaw sub-sea permafrost, he adds. The permafrost includes ice-like deposits known as clathrates that could release methane into the atmosphere if disturbed, a process that has been hypothesised as a cause of the Permian mass extinction.