The Key of Ice Loss

Nearly all the glaciers in regions with warm water had experienced retreat during the study period, but glaciers in areas dominated by cooler waters, had relative stability.

Glaciers on the Antarctic Peninsula have been retreating for decades, and some in the region have undergone particularly accelerated melting since the 1990s. Until recently, many scientists assumed that a steady increase in air temperature around the peninsula, the product of global warming, was the primary cause behind most of the ice loss. But new research looking at the western side of the peninsula suggests that this may not be the case after all. A study published Thursday in the journal Science suggests that warm ocean water may be the biggest driver of glacial retreat in that region - and it’s a problem that may not be getting enough attention.  

Alison Cook and her colleagues had found that the pattern of glacial retreat on the peninsula did not match the behavior they would have expected if atmospheric warming were the main driver. And in recent years, the idea that ocean water can have a significant effect on glaciers has been gaining interest among glaciologists and climate scientists.

Many glaciers around Antarctica, particularly in West Antarctica, end at the edge of the sea. Often, these glaciers terminate in what’s known as an ice shelf — a ledge of ice that’s connected to the glacier but not grounded to the bedrock, so it essentially floats on top of the sea. These ice shelves are instrumental in keeping glaciers stable and holding back the flow of ice behind them. If they weaken or break, glaciers can quickly begin pouring ice into the water.

But because these ice shelves aren’t grounded, all the space below them is filled with ocean water, which presses against the front of the glacier right up to the point where the ice becomes connected to the bedrock. As climate change is causing not only the air around the world to warm but also the ocean, scientists are concerned that rising water temperatures may be helping to melt many glaciers from the bottom up. This process can cause ice shelves to thin and even break off, leading to rapid ice loss.

The researchers compared records of ocean temperatures and ice loss in the region between the years 1945 and 2009 - and they found a clear correlation. Nearly all the glaciers in the southern part of the peninsula, which flow out into regions with warm water - particularly, water that is warm at depths below about 100 yards or so - had experienced retreat during the study period. But glaciers farther north, in areas dominated by cooler waters, had experienced relative stability.

The warm water dominating the southern area comes from what’s known as circumpolar deep water, or CDW, a relatively warm water mass in the Southern Ocean. Research has suggested that the CDW has been growing warmer in recent decades, and also that its warm waters have been creeping higher onto the ice shelves in West Antarctica, potentially causing more melting.

“The ocean is warming, but much of the heat in the CDW has been in the ocean for a long time,” Alley, who was not an author of the new Science study, said by email. “Warming of the CDW may have contributed to the observed ice-sheet melting, but changes in ocean circulation bringing more CDW to the ice are clearly important, and probably most important.” He added that these changes in the CDW have probably been influenced in part by the effects of anthropogenic climate change and associated changes in winds and water currents.

“Our results are key for making predictions of ice loss in response to ocean warming in this region,” Cook said, noting that it’s important to the influence of the ocean in future ice models. “The Antarctic Peninsula is one of the largest current contributors to sea-level rise, and the glaciers here are highly sensitive, so they are key indicators of how the ice will respond to future changes.”  

Source: The Washington Post

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