Melting ice in Antarctica will cause a rapid slowdown of a major global deep ocean current by 2050, new research shows.
Not only is Antarctic ice melting more quickly than new snow can replace it, but the rate of loss due to melting and calving is increasing. Each year, the ice sheet is shrinking more rapidly.
Scientists have calculated that the fastest-changing Antarctic region - the Amundsen Sea Embayment - has lost more than 3,000 billion tonnes of ice over a 25-year period.
And while the ice loss seems to have sped up, based on the latest research, there has been a slowdown of the deep ocean current, and this could have a disastrous impact on the global climate, the marine food chain, and even the stability of ice shelves.
New research published in the journal Nature on March 29, 2023, suggests if greenhouse gas emissions continue at today’s levels, the current in the deepest parts of the ocean could slow down by 40 percent in only three decades.
The peer-reviewed study modeled the impact of melting Antarctic ice on deep ocean currents that work to flush nutrients from the sea floor to fish near the surface.
The deep ocean current the study focused on is called the Antarctic overturning circulation – also known as abyssal ocean overturning. The slowdown of this current is expected to speed up ice melt and potentially end an ocean system that has helped sustain life for thousands of years.
“The projections we have make it look like the Antarctic overturning would collapse this century,” said Matthew England, deputy director of the Australian Research Council’s Centre for Excellence in Antarctic Science, who coordinated the study.
“In the past, these overturning circulations changed over the course of 1,000 years or so, and we’re talking about changes within a few decades. So it is pretty dramatic,” he said.
Understanding ocean currents
We can see the movement of ocean currents on the surface of the water, and these currents are continuous and directed movements. These currents are driven by global wind systems that are fueled by energy from the sun.
Differences in water density, resulting from the variability of water temperature (thermo) and salinity (haline), also cause ocean currents. This process is known as thermohaline circulation.
In the North Atlantic, ocean water loses heat to the atmosphere and becomes cold and dense. When ocean water freezes, forming sea ice, salt is left behind causing the surrounding seawater to become saltier and denser.
Because this water is heavier, it sinks to the ocean bottom. Surface water flows in to replace the sinking water – which in turn becomes cold and salty enough to sink.
We now have a the start of the global conveyer belt, a connected system of deep and surface currents that circulate around the globe on a 1000 year time span. This system is a critical part of Earth’s climate system as well as the ocean nutrient and carbon dioxide cycles.
The slowdown in the deep ocean current
It took the scientists three years of computer modeling to come up with what they are calling “headline news.” What they found was that deepwater circulation in the Antarctic could weaken at twice the rate of decline in the North Atlantic.
This brings us back to the Antarctic overturning circulation – also known as abyssal ocean overturning. The study relates to the amount of water that sinks to the bottom and then flows north.
Remember earlier when we talked about the ocean water in the North Atlantic? In a perfect world scenario, that cold current flowing to the North Pole would end up freezaing and forming ice.
When the ocean water freezes, forming sea ice, the salt left behind causes the seawater to become saltier and heavier – causing it to sink, starting the conveyer belt.
But what would happen if the South Pole became so warm that its ice cover began melting at a faster rate? Fresh meltwater is not salty, so this means that water does not sinkv to feed the Antarctic overturning circulation, just like what is now going on in the Arctic.
Ocean overturning allows nutrients to rise up from the bottom, with the Southern Ocean supporting about three-quarters of global phytoplankton production, the base of the food chain, said a second study co-author, Steve Rintoul.
“If we slow the sinking near Antarctica, we slow down the whole circulation and so we also reduce the amount of nutrients that get returned from the deep ocean back up to the surface,” said Rintoul, a fellow at Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO).
The study did not attempt to explain or quantify the knock-on effects, but the authors wrote the slowdown would “profoundly alter the ocean overturning of heat, fresh water, oxygen, carbon and nutrients, with impacts felt throughout the global ocean for centuries to come.”
