The loss of a massive "tongue" of glacial ice on the Antarctic coast — a natural protective barrier nearly four times the size of New York City — could affect ocean circulation patterns and be a harbinger of changes to come from global warming, scientists on a mission to the frozen continent say.
Last February, the tongue of Antarctica's Mertz Glacier was rammed by a huge iceberg, causing the tongue section to break away and itself become an iceberg.
The tongue, sticking out into the Southern Ocean, had acted like a dam, preventing sea ice from moving into a permanently open section of water to the west.
But now, with the tongue gone due the collision, scientists fear it could trigger changes to the behavior of a major part of global ocean circulation patterns that shift heat around the globe via myriad currents at the surface and along the bottom.
The area around the glacier tongue, since halved in length by the collision, and to the west are one of the few places around Antarctica where dense, salty water is formed and sinks to the depths of the ocean, said mission leader Steve Rintoul.
This dense "bottom water" is a key driver of the global overturning circulation that includes the current that brings warm Atlantic waters to western Europe.
But he said there was a risk the area could now be less effective in producing the bottom water that feeds the deep ocean currents, which influence global climate patterns.
"This is one of the few places around Antarctica where the sea surface is made dense enough to sink to the deep ocean," Rintoul said from the icebreaker Aurora Australis.
"If the area is less effective in forming less dense water, then that salinity now should be lower than it was in the past."
Rintoul is leading an international team of nearly 40 scientists on a voyage studying the impacts of the loss of the glacier tongue as well as changes to ocean temperatures, salinity and acidity.
Oceans act as a brake on climate change by soaking up large amounts of heat and carbon dioxide, the main greenhouse gas, from the atmosphere. But the more CO2 the oceans take up, the more acidic they become, making it harder for animals such as sea snails to make their shells.
Rintoul, an oceanographer with the Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart, Australia, said the area around the glacier remains free of ice all winter. Such areas are called polynas.
"It remains free of ice because the winds blowing off the continent are so strong it blows the ice away as rapidly as it forms," he said.
"So in that sense, it is kind of a sea-ice factory and the more sea ice that is formed, the saltier the water beneath the sea ice becomes," leading to the creation of dense, salty water, he added.
"We think the presence of the glacier tongue was part of what made this a very active polyna," he said, adding he expected it would be less effective now there was no ice tongue.
A Japanese-led study published earlier this month estimated that the loss of the ice tongue led to nearly 25 percent less dense water formation.
Rintoul's team have been taking samples in the area measuring salinity, temperature, oxygen and carbon and the results will be studied over the coming weeks.
Studying the damage to the Mertz Glacier would help efforts to project future changes in climate, he said.
"It allows us to explore how sensitive the formation of that dense water is to things like the shape of the continent and change in the effectiveness in these polynas," he said.
As the oceans heated up, warmer water would likely increase the rate these floating tongues of ice melted, he said, exposing glaciers on the continent to warmer seas. This risked faster rates of melting and discharge of ice into the ocean, raising sea levels.
