Scientists have discovered a worrying process that speeds up the melting of Antarctica's ice - and the rate of resulting sea level rise.

A just-published Australian study found how melting glacial ice sheets make the ocean's surface layer less salty and more buoyant.

That effectively stopped deep mixing in winter and allowed warm water at depth to retain its heat and further melt glaciers from below.

"This process is similar to what happens when you put oil and water in a container, with the oil floating on top because it's lighter and less dense," explained Alessandro Silvano, a PhD student at the University of Tasmania's Institute for Marine and Antarctic Studies.


"The same happens near Antarctica with fresh glacial meltwater, which stays above the warmer and saltier ocean water, insulating the warm water from the cold Antarctic atmosphere and allowing it to cause further glacial melting."

Silvano's study, just published in the journal Science Advances, found that, in this way, increased glacial meltwater could drive a positive feedback, causing further melt of ice shelves and hence an increase in sea level rise.

It further found fresh meltwater also reduced the formation and sinking of dense water in some regions around Antarctica, slowing ocean circulation which takes up and stores heat and carbon dioxide.

"The cold glacial meltwaters flowing from the Antarctic cause a slowing of the currents which enable the ocean to draw down carbon dioxide and heat from the atmosphere," Silvano said.

"In combination, the two processes we identified feed off each other to further accelerate climate change."

Silvano said a similar mechanism has been proposed to explain rapid sea level rise of up to five metres per century at the end of the last glacial period around 15,000 years ago.

"Our study shows that this feedback process is not only possible but is in fact already underway, and may drive further acceleration of the rate of sea level rise in the future," he said.

"Currently the ice shelves resist the flow of ice to the ocean, acting like a buttress to hold the ice sheet on the Antarctic continent.


"Where warm ocean waters flow under the ice shelves they can drive rapid melting from below, causing ice shelves to thin or break up and reducing the buttressing effect."

This process led to rising sea levels as more ice flowed to the ocean.

"Our results suggest that a further increase in the supply of glacial meltwater to the waters around the Antarctic shelf may trigger a transition from a cold regime to a warm regime, characterised by high rates of melting from the base of ice shelves and reduced formation of cold bottom waters that support ocean uptake of atmospheric heat and carbon dioxide."

Professor Tim Naish, the director of Victoria University's Antarctic Research Centre, said the paper addressed an important issue about feedbacks that occurred as the Antarctica ice sheet melted and discharged fresh cold meltwater into the ocean.

"Does this further enhance melting of the ice sheet or does the could fresh water slow it down? It seems that in the Amundsen Sea and Totten glacier region the freshwater discharge forms an insulating cap on the surface ocean because it is less dense," Naish said.

"This has the affect of reducing deep mixing and allowing warm circumpolar deep water to continue to upwell onto the continental shelf where it comes into contact with the marine margins of the ice sheet enhancing melting."

In the Ross Sea, however, scientists believed this fresh melt water may end up entrained by the Antarctic slope current, where it was drawn down to greater depths and formed a road block restricting warm ocean currents from reach the ice sheet.

This - an example of "negative" feedback, as opposed to the "positive" feedback highlighted by Silvano - then reduced melting.

"It's important to note that this is a high prioirty forefront of research today, and there are still many uncertainties," Naish said.

"What is clear is that different regions of Antarctica are behaving in different ways, and if we are to understand how fast the ice sheet will melt and contribute to future sea-level rise we need to understand the processes of ocean circulation on the ice sheet margin at the local scale."

Researchers within the New Zealand Antarctic Research Institute's Ross Ice Shelf programme had focused on that very issue.

"If the Ross Ice Shelf collapsed, the world's biggest ice shelf, it would have major implications for global sea level rise."

The wider West Antarctic Ice Sheet stores an equivalent 3.2m of sea level rise, while the much larger East Antarctic Ice Sheet is estimated to contain 58m of equivalent sea level rise.