Warm waters from the Atlantic near Greenland are now heating the deep layers of the Arctic Ocean, an area once considered relatively insulated from climate change.

The Arctic Ocean has seen a reduction of about 40% in its sea ice cover over the past 40 years, primarily due to the impact of atmospheric warming on sea levels. Researchers at the Ocean University of China evaluated the latest data collected by icebreakers to assess the temperature increase of the ocean floor.

In the Eurasian Basin, which is one of the ocean’s two principal sections, temperatures at depths ranging from 1500 meters to 2600 meters have increased by 0.074 degrees Celsius since 1990.

While this temperature rise may seem minor, it equates to nearly 500 trillion megajoules of energy. Such energy could potentially melt up to one-third of the least extensive sea ice area.

“The deep ocean is more dynamic than previously assumed,” states Chen Xianyao, one of the research team members. “We suspected that the deep ocean was warming, but not at this pace.”

An underwater ridge separating Greenland and Siberia divides the Arctic Ocean into two basins. The Amerasian Basin is primarily cut off from the Pacific Ocean by the shallow Bering Strait. However, warm Atlantic waters can still flow north along the Scandinavian coast into the upper Eurasian Basin through an extension of the Atlantic Meridional Overturning Circulation (AMOC). During winter, when seawater freezes, the salts are released, resulting in denser water that sinks and drags some warmer Atlantic water down with it.

Geothermal heat from the Earth warms the deep waters of the Eurasian Basin.

Previously, these warming trends were balanced by cold water flowing down from a neighboring basin east of Greenland. Yet, as the Greenland ice sheet continues to melt, more freshwater is entering the Greenland Basin. This influx has slowed the downward movement of cold, salty water, raising the temperature of deep waters in the Greenland Basin from -1.1°C to -0.7°C—a significantly rapid increase. Consequently, the influx of cold Greenland waters is no longer counteracting the heat from geothermal sources or the warm Atlantic waters sinking into the Arctic.

“The rising temperatures in the Greenland Basin are now reaching the Arctic,” says Son Louise, another research team member.

This research uncovers new warming mechanisms deep within the Arctic Ocean, “indicating a broader trend of global warming,” according to James McWilliams from UCLA.

The ongoing warming might eventually contribute to the melting of both sea ice and permafrost found on the ocean floor, which contains ice-like structures known as clathrates. If disturbed, these can release methane into the atmosphere, a phenomenon believed to have contributed to the Permian mass extinction.