Melting glaciers are one runaway symptom of global warming, but new research suggests that they might also be a contributing factor. Scientists have discovered evidence of a previously unknown vicious circle, whereby melted glacial water alters the chemistry on the surface of the ocean and drives further glacial melting, in turn accelerating the rise of sea levels.

The new research was led by the Institute for Marine and Antarctic Studies (IMAS) at Australia's University of Tasmania. The team used ocean modeling to explore how glacial meltwater affects the upper layers of the sea, finding that it made the surface less salty and more buoyant. This hinders the deep mixing of ocean water that normally takes place in the winter, enabling the lower, warmer layers of the ocean to retain their heat.

"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," explains IMAS PhD student Alessandro Silvano, who led the study. "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. We found that in this way increased glacial meltwater can cause a positive feedback, driving further melt of ice shelves and hence an increase in sea level rise."

Furthermore, the team found that the meltwater also hampers the formation of denser water in certain parts of the Antarctic, bodies of water that normally sink and help drive ocean circulation that contributes to ocean absorption of 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," says Silvano. "In combination, the two processes we identified feed off each other to further accelerate climate change."

The researchers say this process may have played out in nature before, having been floated as an explanation for the dramatic sea level rise of about five meters per century (16.4 ft) 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," says Silvano. "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 leads to rising sea levels as more ice flows to the ocean.'

The research was published in the journal Science Advances.