Study Reveals Dangerous Antarctic Feedback Loop
The study, published April 18 in Science Advances, found that fresh water melting off of glaciers in some regions of Antarctica caused a layer of cold, fresh water to float above warmer, saltier water, both slowing ocean circulation and melting lower parts of the ice sheets.
Alessandro Silvano, the study's lead author and a Ph.D. student with the University of Tasmania's Institute for Marine and Antarctic Studies (IMAS), explained the mechanism in everyday terms.
"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," Silvano said in an IMAS press release.
But unlike mixing oil and water, this mechanism has global implications as the warmer, denser water leads to more 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," Silvano said.
The process also slows the circulation of ocean currents because it prevents the sinking of dense, cold water called "Dense Shelf Water." The study found that the formation of Dense Shelf Water had stopped off of East Antarctica's Sabrina Coast and in the Amundsen Sea in West Antarctica.
"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," Silvano said.
As The Washington Post pointed out, Silvano's research backs up a study co-authored by early climate change alarm-sounder Dr. James Hansen in 2016, which used computer models to predict that melting ice in Antarctica and Greenland would warm the oceans below the surface and increase the rate of melting, increasing storms and leading to sea level rise of "several meters over a timescale of 50 to 150 years," according to the abstract.
"Our study shows for the first time actual evidence of this mechanism. Our study shows that it is already happening," Silvano told The Washington Post.
Hansen himself also commented, saying "this study provides a nice small-scale example of processes that we talk about in our paper," the Post reported.
"On the large-scale issue, it is too early to say how these feedback processes will play out, based on empirical evidence," Hansen told the Post by email. "If we stay on business-as-usual [greenhouse gas] emissions rates, so that global warming continues to increase, I expect that the freshwater injection rate will increase (mainly via ice faster ice shelf breakup and underwater melt) and sea ice area will increase. This experiment will be playing out over the next years and decades."
This study comes days after reports that the Atlantic Meridional Overturning Circulation (AMOC), which brings warm water from the tropics and moves cool water south, is weaker than at any point in the last thousand years.
"Of those two key areas of deep water formation, the northern Atlantic one has been widely considered more vulnerable to global warming," Stefan Rahmstorf, a scientist at the Potsdam Institute for Climate Impact Research, told The Washington Post. "It is therefore of some concern that we now see increasing signs that the deep water formation around Antarctica is already being affected."
Because the study's authors do not have measurements for Dense Shelf Water formation in the regions studied going back far enough, they cannot be certain that the current changes are due to climate change, though the melting of fresh water from glaciers would still impact the process going forward, The Washington Post reported.