Warm oceanic water in the southern seas gets trapped under a blanket of cold, fresh water from the melting glaciers above, and this allows the warm ocean to calve out ice from under massive shelves.
While it is common knowledge that warm water floats and cold water sinks, the opposite has been found happening in Antarctica. In spite of there being a significant temperature difference between the two waters, the density of freshwater over salty sea water keeps the warm water tightly trapped, a new study has found. This process was, till now undocumented, reports PhysOrg.
What this means, according to researchers, is that melting ice is changing the ocean in such a way that it has accelerated the rate at which glacial ice sheets melt, contributing to sea level rise. Glacial meltwater is essentially fresh and not salty, so it is buoyant, in spite of being really cold. In winter, this prevents the deep mixing of waters, which will stabilize salinity and temperature. By trapping warm water deep in the ocean it helps retain the heat, further calving the ice and increasing the melt, contributing to sea level rise.
"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," said Alessandro Silvano of the Institute for Marine and Antarctic Studies (IMAS).
"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," he explained.
Another spillover from this effect is that it is slowing ocean currents and circulation because it reduces the formation of dense water. This further increases heat storage 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. In combination, the two processes we identified feed off each other to further accelerate climate change."
"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," Mr Silvano said.
The research was first published in the journal Science Advances.