Off the coast of Antarctica, trillions of tons of cold salt water sink to great depths. As the water sinks, it drives the deeper flows of the "tip" circulation, a network of strong currents that spans the world's oceans. The overturning circulation transports heat, carbon, oxygen and nutrients around the globe and fundamentally influences the climate, sea level and the productivity of marine ecosystems.
But there are worrying signs that these flows are slowing. They can even collapse. If this happens, it would starve the deep ocean of oxygen, limit the return of nutrients to the sea surface, and potentially cause further ice melt as the water near the ice shelves warms in response. There would be major global ramifications for ocean ecosystems, climate, and sea level rise.
Our new researchpublished in the magazine Nature, uses new projections from ocean models to look at changes in the deep ocean through the year 2050. Our projections show a slowing of the Antarctic overturning circulation and warming of the deep ocean in the coming decades. Physical measurements confirm that these changes are already underway.
Climate change is to blame. As Antarctica melts, more fresh water flows into the oceans. This disrupts the sinking of cold, salty, oxygen-rich water to the ocean floor. From there, this water typically spreads north to vent the far reaches of the deep Indian, Pacific, and Atlantic oceans. But all of that could come to an end soon. In our lives.
Why does this matter?
As part of this turnaround, about 250 billion tons of Antarctic surface water ice sinks into the ocean abyss each year. Subsidence near Antarctica is balanced by upwelling at other latitudes. The resulting upturnal circulation transports oxygen to the ocean depths and ultimately returns the nutrients to the sea surface, where they are available to support marine life.
If Antarctic overturning slows, nutrient-rich seawater accumulate at the bottom of the sea, five kilometers below the surface. These nutrients will be lost to marine ecosystems at or near the surface, damaging fisheries.
Changes in the overturning circulation could also mean more heat is getting to the ice, particularly around West Antarctica, the area with the highest rate of ice mass loss in recent decades. This would accelerate global sea level rise.
A capsize slowdown would also reduce the ocean's ability to take carbon dioxide, leaving more greenhouse gas emissions in the atmosphere. And more greenhouse gases mean more warming, which makes matters worse.
Meltwater-induced weakening of the Antarctic overturning circulation could also change tropical rain bands a thousand kilometers to the north.
Simply put, a slowdown or collapse of the circulation would change our climate and marine environment in profound and potentially irreversible ways.
Signs of worrisome change
The remote reaches of the oceans surrounding Antarctica are some of the most difficult regions in which to plan and undertake field campaigns. The journeys are long, the weather can be brutal, and sea ice limits access for much of the year.
This means that there are few measurements to track how the Antarctic margin is changing. But where there is sufficient data, we can see clear signs of increased transport of warm waters to Antarcticawhich in turn causes the ice to melt in key places.
In fact, the signs of melting around the edges of Antarctica are very clear, with increasing volumes of fresh water flowing into the ocean and making nearby waters less salty and therefore less dense. And that's all it takes to slow down the rollover movement. The denser water sinks, the lighter water does not.
How did we discover this?
Apart from few measurements, incomplete models have limited our understanding of ocean circulation around Antarctica.
For example, the latest set of global coupled model projections analyzed by the Intergovernmental Panel on Climate Change exhibit biases in the region. This limits the ability of these models to project the future fate of the Antarctic overturning circulation.
To explore future changes, we took a high-resolution image world ocean model that realistically represents the formation and sinking of dense water near Antarctica.
We ran three different experiments, one in which conditions remained unchanged from the 1990s; a second forced by projected changes in temperature and wind; and a third race that also includes projected changes in meltwater Antarctica and Greenland.
In this way, we could separate the effects of changes in winds and warming from changes due to melting ice.
The findings were surprising. The model projects that the tipping of the circulation around Antarctica will be reduced by more than 40% over the next three decades, driven almost entirely by pulses of meltwater.
Over the same period, our model also predicts a 20% weakening of the famous North Atlantic overturning circulation that keeps Europe's climate warm. Both of these changes would drastically reduce the turnover and overturning of the ocean interior.
We have known for some time that North Atlantic dump currents are vulnerable, and observations suggest a slowdown is already underway, and projections of a tipping point coming soon. Our results suggest that Antarctica looks poised to match its Northern Hemisphere counterpart, and then some.
Whats Next?
Much of the deep ocean has warmed in recent decades and the fastest trends have been detected near Antarcticain a pattern very similar to our model simulations.
Our projections extend only to 2050. Beyond 2050, in the absence of strong emission reductions, the climate will continue to warm and the ice sheets will continue to melt. If so, we anticipate that the capsizing of the Southern Ocean will continue to slow through the end of the century and beyond.
The projected slowdown of Antarctic overturning is a direct response to freshwater input from melting ice. Meltwater flows are directly related to how much the planet warms, which in turn depends on the greenhouse gases we emit.
Our study shows that continued ice melt will not only raise sea levels, but will also change mass overturning circulation currents that can lead to further ice melt and thus further sea level rise, and damage the climate and ecosystems around the world. It's yet another reason to tackle the climate crisis, and fast. MD
First Posted By The conversation.
Matthew England: Scientia Professor and Deputy Director of the ARC Australian Center for Excellence in Antarctic Science (Aceas), UNSW Sydney; Adele Morrison: Research Fellow, Australian National University; Andy Hogg: Professor, Australian National University7; Massachusetts Institute of Technology Qian Li (MIT); Steve Rintoul: Member of CSIRO.
