The last remaining Northern Greenland ice shelves bracing the region's vast ice sheet have lost a third of their volume in the last four decades, researchers said on Tuesday, warning of the risk of "dramatic" sea level rise. 

An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. The floating shelves play a crucial role in regulating the flow of ice into the ocean from glaciers hosting enough ice to ultimately raise sea levels by 2.1 meters. 

These ice shelves have lost more than 35 percent of their total volume since 1978, with three of them collapsing completely, the study found. 

With continued global warming driven by fossil fuel pollution expected to further heat ocean waters, the ice shelves are "extremely vulnerable" to further retreat and even collapse, according to the study published in Nature Communications.

"This could have dramatic consequences in terms of SLR (sea level rise)," the authors said. 

They added that this is the area of Greenland with the greatest potential to raise ocean levels, possibly over centuries.

The melting of the ice shelves themselves does not contribute to sea level rise but they exist as "dams" regulating the discharge of ice into the ocean from the ice sheet.

If these natural barriers disintegrate, it may cause the glaciers to dump more ice into the oceans, the study found.

Glaciers in this region had previously been considered stable by scientists, unlike other parts of Greenland's ice sheet that began to weaken in the middle of the 1980s.

But the authors found that the glaciers have started to discharge ice in response to weakening ice shelves, which have been melting from below by warming oceans.  

"We have identified a very significant increase in melting since the 2000s which clearly corresponds to a rise in ocean temperatures in this area during that time," said the lead author Romain Millan, a researcher at the French National Center for Scientific Research (CNRS). 

The researchers based in Denmark, France and the United States used thousands of satellite images combined with field measurements and climate models to reconstruct the nature of these buoyant glacier extensions.

Greenland's northern glaciers have only started to destabilize in the last 20 years, meaning more ice has been lost than gained.

"The glacier Zachariae Isstrom, for example, which broke loose in 2003 almost doubled the amount of ice it discharged into the ocean," said Millan. 

The Greenland ice sheet is a major contributor to global sea level rise, accounting for some 17 percent of the observed rise in water levels between 2006 and 2018.

"What happens to the poles and sea levels in the future depends on the decisions taken by politicians to reduce greenhouse gas emissions," said Millan. 

World leaders and climate negotiators will gather in Dubai from November 30 for the latest United Nations summit (COP28) as record-shattering temperatures, rising wildfires and worsening natural disasters heighten alarm about the fate of the planet.

(With input from AFP)