Trillions of tons of cold salty water sinking to great depths off the coast of Antarctica drive the deepest flows of the “overturning” circulation – a network of strong currents spanning the world’s oceans. This circulation carries heat, carbon, oxygen, and nutrients around the globe and fundamentally influences climate, sea level, and the productivity of marine ecosystems.
However, there are concerns that these currents are slowing down and may even collapse, which could deprive the deep ocean of oxygen, limit the return of nutrients back to the sea surface, and potentially cause further melt back of ice as water near the ice shelves warms in response. Such a scenario would have major global ramifications for ocean ecosystems, climate, and sea-level rise. A new research published on March 29 in the journal Nature uses ocean model projections to show that the Antarctic overturning circulation will slow down, and the deep ocean will warm over the next few decades.
Physical measurements confirm that these changes are already well underway, and climate change is to blame. As Antarctica melts, more freshwater flows into the oceans, disrupting the sinking of cold, salty, oxygen-rich water to the bottom of the ocean. This disruption could end the normal spread of this water northwards, which ventilates the far reaches of the deep Indian, Pacific, and Atlantic Oceans, and all of this could happen in our lifetimes.
Antarctic Overturning Circulation: The Consequences of a Slowdown or Collapse
Approximately 250 trillion tons of frigid Antarctic surface water descends to the ocean abyss each year as part of the overturning process, which is offset by upwelling at different latitudes. The resulting overturning circulation brings oxygen to the deep ocean and eventually returns nutrients to the sea surface, providing support for marine life.
If the Antarctic overturning slows, nutrient-rich seawater will accumulate on the seafloor, potentially damaging fisheries. Furthermore, a change in the overturning circulation could increase the amount of heat reaching the ice, particularly in West Antarctica, accelerating global sea-level rise. A slowdown would also reduce the ocean’s capacity to absorb carbon dioxide, increasing greenhouse gas emissions in the atmosphere, which would worsen the situation.
A weakening of the Antarctic overturning circulation caused by melting could shift tropical rainfall bands northward by around 1,000 kilometers. In summary, a reduction or collapse of the overturning circulation would have significant and potentially irreversible impacts on our climate and marine environment.
Antarctic Ocean Melting
Planning and executing field campaigns in the remote oceans surrounding Antarctica is a daunting task due to the challenging conditions. The long voyages, harsh weather, and sea ice make it difficult to access the area, resulting in limited data on the changes in the Antarctic margin.
However, the available data indicates an increase in warm water transport towards Antarctica, leading to ice melt in critical locations. The signs of melting around Antarctica’s edges are evident, with a considerable amount of freshwater flowing into the ocean, resulting in reduced salinity and density of the nearby waters. Consequently, the overturning circulation slows down as denser water sinks, and lighter water does not.
How was it found out?
Insufficient data and incomplete models have restricted our comprehension of ocean circulation in the Antarctic region, as per a recent study. The latest analysis of global coupled model projections by the Intergovernmental Panel on Climate Change demonstrates biases in the region, limiting the models’ capacity to anticipate the future of the Antarctic overturning circulation. To explore potential changes, the researchers utilized a high-resolution global ocean model that realistically mimics the development and sinking of dense water near Antarctica.
They conducted three separate experiments to isolate the effects of changes in temperature, wind, and meltwater from Antarctica and Greenland. The study discovered that the overturning circulation around Antarctica is expected to slow by over 40 percent in the next 30 years, primarily due to pulses of meltwater. Moreover, the model also predicts a 20 percent reduction in the renowned North Atlantic overturning circulation, which moderates Europe’s climate, leading to a significant decrease in the renewal and overturning of the ocean interior. The study’s outcomes suggest that the Antarctic region will experience changes similar to those of its northern hemisphere counterpart.
According to research, much of the abyssal ocean has experienced warming in recent decades, particularly near Antarctica, which is consistent with model simulations. However, the projections for the future extend only until 2050, beyond which continued warming and melting of ice sheets are expected without significant reductions in greenhouse gas emissions.
As a result, the Southern Ocean’s overturning is predicted to continue slowing throughout the century and beyond, directly related to the influx of freshwater from melting ice. This meltwater flow is a direct consequence of global warming caused by greenhouse gas emissions.
The study also warns that continuing ice melt could alter the massive overturning circulation currents, leading to further ice melt, sea level rise, and climate and ecosystem damage worldwide. The study concludes that urgent action is necessary to address the climate crisis.