Brisbane, Australia
CNN
—
Melting ice in the Antarctic is not just raising sea levels
but slowing down the circulation of deep ocean water with vast
implications for the global climate and for marine life, a new study
warns.
Led by scientists from the University of New South Wales and published Wednesday in the journal Nature, 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.
Three years of computer modeling found the Antarctic overturning
circulation – also known as abyssal ocean overturning – is on track to
slow 42% by 2050 if the world continues to burn fossil fuels and produce
high levels of planet-heating pollution.
A slow down 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.
Most previous studies have focused on the Atlantic Meridional Overturning Circulation (AMOC),
the system of currents that carry warm water from the tropics into the
North Atlantic. The cold, saltier water then sinks and flows south.
Its Southern Ocean equivalent is less studied but does an
important job moving nutrient-dense water north from Antarctica, past
New Zealand and into the North Pacific Ocean, the North Atlantic and
Indian Ocean, the report’s authors said in a briefing.
The circulation of deep ocean water is considered vital for the
health of the sea – and plays an important role in sequestering carbon absorbed from the atmosphere.
According to the report, while a slowdown of the AMOC would mean
the deep Atlantic Ocean would get colder, the slower circulation of
dense water in the Antarctic means the deepest waters of the Southern
Ocean will warm up.
“One of the concerning things of this slowdown is that there can
be feedback to further ocean warming at the base of the ice shelves
around Antarctica. And that would lead to more ice melt, reinforcing or
amplifying the original change,” England said.
As global temperatures rise, Antarctic ice is expected to melt
faster, but that doesn’t mean the circulation of deep water will
increase – in fact the opposite, scientists said.
In a healthy system, the cold and salty – or dense – consistency
of melted Antarctic ice allows it to sink to the deepest layer of the
ocean. From there it sweeps north, carrying carbon and higher levels of
oxygen than might otherwise be present in water around 4,000 meters
deep.
As the current moves northward, it agitates deep layers of debris
on the ocean floor – remains of decomposing sea life thick with
nutrients – that feed the bottom of the food chain, scientists said.
In certain areas, mostly south of Australia in the Southern Ocean
and in the tropics, this nutrient-rich cold water moves toward the
surface in a process called upwelling, distributing the nutrients to
higher layers of the ocean, England said.
However, Wednesday’s study found that as global temperatures warm,
melting sea ice “freshens” the water around Antarctica, diluting its
saltiness and raising its temperature, meaning it’s less dense and
doesn’t sink to the bottom as efficiently as it once did.
The report’s co-author, Steve Rintoul from Australia’s
Commonwealth Scientific and Industrial Research Organisation and the
Australian Antarctic Program Partnership, said sea life in waters
worldwide rely on nutrients brought back up to the surface, and that the
Antarctic overturning is a key component of that upwelling of
nutrients.
“We know that nutrients exported from the Southern Ocean in other
current systems support about three quarters of global phytoplankton
production – the base of the food chain,” he said.
“We’ve shown that the sinking of dense water near Antarctica will
decline by 40% by 2050. And it’ll be sometime between 2050 and 2100 that
we start to see the impacts of that on surface productivity.”
England added: “People born today are going to be around then. So,
it’s certainly stuff that will challenge societies in the future.”
The report’s authors say the slowing of the Antarctic ocean
overturning has other knock-on effects for the planet – for example, it
could shift rain bands in the tropics by as much as 1,000 kilometers
(621 miles).
“Shut it down completely and you get this reduction of rainfall in
one band south of the equator and an increase in the band to the north.
So we could see impacts on rainfall in the tropics,” said England.
Earlier this month, the Intergovernmental Panel on Climate Change (IPCC)
warned in its latest report that the impacts of rising global
temperatures were more severe than expected. Without immediate and deep
changes, the world is hurtling toward increasingly dangerous and
irreversible consequences of climate change, it added.
The IPCC report found that the goal of limiting global warming to
1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial levels
was still possible, but it’s becoming harder to achieve the longer the
world fails to cut carbon pollution.
England points out that the IPCC predictions don’t include ice melt from Antarctic ice sheets and shelves.
“That’s a very significant component of change that’s already
underway around Antarctica with more to come in the next few decades,”
England said.
Rintoul said the study was another urgent warning on top of all the ones that have come before it.
“Even though the direct effect on fisheries through reduced
nutrient supply might take decades to play out, we will commit ourselves
to that future with the choices we make over the next decade.”
