Rising ocean temperatures threaten Greenland ice sheet

Scientists at the University of California, Irvine and NASA’s Jet Propulsion Laboratory have for the first time quantified the impact of warming coastal waters on individual glaciers in Greenland’s fjords. Their work is the subject of a study recently published in Scientific advances.

Working under the auspices of the Oceans Melting Greenland mission for the past five years, researchers have used ships and planes to study 226 glaciers in all areas of one of Earth’s largest islands. They found that 74 glaciers located in deep, steep-walled valleys accounted for almost half of the total ice loss in Greenland between 1992 and 2017.

These fjord-bound glaciers were found to be the most prone to clearing, a process by which hot, salty water at the bottom of canyons melts ice from below, causing masses to break up faster than usual. In contrast, the team found that 51 glaciers positioned in shallower ravines suffered less clearing and contributed only about 15% of the total ice loss.

“I was surprised how unbalanced he was. The largest and deepest glaciers are carved out much faster than the smaller glaciers in shallow fjords, ”said senior author Michael Wood, postdoctoral researcher at NASA’s Jet Propulsion Laboratory in Southern California, said research as a doctoral student at the UCI. “In other words, the largest glaciers are the most sensitive to warming waters and they are the ones that are really causing the loss of ice in Greenland.”

The study highlighted the dynamics that deeper fjords allow the intrusion of warmer ocean waters than shallow waters, accelerating the undercutting process with some of Greenland’s largest glaciers.

Greenland is home to one of only two ice caps on Earth, the larger of which is Antarctica. The ice in Greenland is more than three kilometers thick in places. On the outskirts of the land mass, vast glaciers stretching from the ice cap slowly traverse the valleys like icy conveyor belts, which enter fjords and then melt or break off into icebergs. The ice is replenished by falling snow which is compressed over time in the pack ice.

If the ice cap were in equilibrium, the amount of snow accumulated at the top would roughly equal the ice lost through melting, evaporation, and calving – pieces breaking free from anchored masses and floating in the ocean.

But the ice cap has been out of balance since the 1990s. Melting has accelerated and calving has increased, causing glaciers to retreat from the sea to land. Together, they are the result of the shrinking of the ice cap.

According to the research team, the accumulation of warm salt water at the bottom of the fjords has been accelerated by the rise in temperatures during the summer months, which heat the surfaces of the glaciers, creating puddles of meltwater. . This liquid escapes through cracks in the ice to form underground freshwater rivers that flow into the sea where it interacts with the salt water below the fjords.

Melt water from glaciers is salt-free, so it is lighter than seawater and rises to the surface as a plume, dragging the hot water into contact with the bottom of the glaciers. The depth of the fjord is a fairly unchanging factor, but other factors such as seawater temperature and the amount of meltwater from glacial surfaces are greatly affected by global warming. The three factors combine to cause accelerated deterioration of the Greenland ice sheet, the researchers said.

As the water temperature around the Greenland coastline is predicted to continue to increase in the future, these results suggest that some climate models may underestimate glacial ice loss by at least a factor of two s’ they do not take into account the undercut by a warm ocean.

The study also helps to understand why many glaciers in Greenland never recovered after a sudden warming of the oceans between 1998 and 2007, which caused the ocean temperature to rise by almost 2 degrees Celsius. Although ocean warming stopped between 2008 and 2017, glaciers had already suffered such extreme undercutting over the previous decade that they continued to retreat at an accelerating rate.

“We have known for more than a decade that the warmer ocean plays a major role in the evolution of the glaciers of Greenland”, said Eric Rignot, deputy principal researcher of the OMG, also of the JPL and the UCI . “But for the first time, we were able to quantify the undercutting effect and demonstrate its dominant impact on glacier retreat over the past 20 years.”