Antarctic Deep Ocean Currents Slowing
Air Date: Week of July 7, 2023
Oceanographers used research vessels to measure the temperature and salinity of Antarctic waters and gather data about deep ocean currents. (Photo: courtesy of Steve Rintoul)
Climate disruption is showing up in the slowing of deep ocean currents that transport heat, carbon, and nutrients around the globe. Researchers found that deep ocean circulation around Antarctica has slowed by almost a third in the last 30 years, faster than predicted by climate models. Physical oceanographer Kathy Gunn joins Host Steve Curwood to explain what’s happening and why it may spell trouble for the entire climate system.
DOERING: It’s Living on Earth, I’m Jenni Doering.
CURWOOD: And I’m Steve Curwood.
The oceans are home to a system of deep currents that circulate heat, carbon, and nutrients around the globe. And the icy landmass at the southern end of our planet plays a vital role in driving that global cycle. But as the planet warms, deep ocean currents around Antarctica are changing, much sooner than climate models predicted. A recent study from Nature Climate Change found that deep ocean circulation there has slowed by almost a third in the last thirty years, decades ahead of projections. Here to tell us more is lead author Kathy Gunn, a physical oceanographer from CSIRO Environment, who joins us from Hobart, Tasmania, in Australia. Welcome to Living on Earth, Kathy!
GUNN: Thank you for having me.
CURWOOD: I was, well, frankly astonished to see that your research shows that Antarctic ocean currents are slowing much faster than predicted. What's going on?
GUNN: Yeah, so we had a study where we used observations to look at the Antarctic currents over the past 30 years or so. And we found this slowing of the circulation of about 30%. And that's something that wasn't predicted to happen by climate models until about 2100. And more recently by a model, which had a more realistic change in the ocean circulation, and they don't see it happening until around 2050. So, observational study suggests that the impacts from climate change are running a little bit ahead of schedule.
CURWOOD: So, talk to me about why the Antarctic ocean currents are slowing down.
GUNN: Of course. So around Antarctica, there's this very salty, very cold water formed. Because it's very salty and cold, it's very dense, and it falls off the Antarctic continental shelves and into the deep ocean. By that falling, it creates these currents. And those currents go on to fill the rest of the ocean. And they lie at the bottom, and they take about 40% of the ocean volume. So, they're very cold, and you can't really get them any colder. So, they're fixed there, but you can change their salinity at the source. And what we've seen is a freshening, so that makes these waters less dense and less heavy, so they're less able to sink. And that is effectively what's creating the slowing of those deep ocean currents.
CURWOOD: How concerning is this? I mean, what could long-term consequences for this change, this slowing of the deep ocean currents in Antarctica, what could it look like?
GUNN: So on the long-term, the consequences of the slowing is that we will start to see a decline in the health of our oceans. These deep ocean currents take oxygen down to the bottom of the ocean, where there will be creatures there that need the oxygen to survive. And with the slowing of the currents, there's less oxygen going to the deep ocean. So those creatures will have to change their behaviors or adapt where they're foraging for food to be able to survive. In a similar way to, you don't want a aquarium to stagnate, over time with this continued slowing, we'll see the ocean begin to stagnate. And that's as the oxygen doesn't reach the bottom. And also, these deep currents, they can stir up nutrients that have fallen from shallower places down to the bottom. But with a slowing of that deep ocean current, we'll see less of that stirring and refreshing, so the ocean will begin to stagnate. But that will play out over centuries or so.
CURWOOD: Or maybe a little bit faster, now, as we see that these changes are coming a little faster than we thought.
GUNN: Yeah, exactly. So, we might already be starting to see some of these changes, and we just don't really know exactly what's happening down there.
CURWOOD: What do you see as the relationship between these slowing Antarctic currents and climate disruption?
GUNN: The ocean as a whole plays a really important role, a critical role, in redistributing heat and carbon around the Earth. And these currents that we've been talking about are the deepest part of that ocean circulation. And if you start to get changes in one part of that circulation, because it's all connected, we would expect to see changes in the whole circulation system, which is what we call this overturning circulation, just really a north-south movement of water. So, we know that global surface warming is proportional to carbon change and carbon emissions. And that relationship is then partly determined by this ocean circulation. So, if we start to see changes in this ocean circulation, we expect that we'll see changes in how the carbon and heat is taken up in the ocean, and then redistributed around the globe.
CURWOOD: Talk to me about where we might start noticing impacts of these slowing ocean currents, not just there in Antarctica, but also out into the global ocean.
GUNN: So, one impact that's been overlooked so far is how the change in this deep circulation will change sea-level rise. So, as you get the deep circulation slowing down, there's just less of that cold water at the bottom. And that gets replaced by warmer water that's overlying it. And that warmer water takes up more space, and it will increase sea-level. So, some of the impacts from that sea-level rise will start to happen over the next 10 or so years. And that sort of change is going to slowly creep further northwards with time. So places, low-lying islands, such as, say the Pacific Islands, places like New Zealand and Tasmania, would start to see some of those signals first, because they're closest to Antarctica.
CURWOOD: What impact do these changes in the deeper currents have on the Antarctic ice, do you think?
GUNN: Around Antarctica, as you get this melting in ice, you get more freshwater input. And that changes the structure of the water around Antarctica. With more freshwater, it changes the stratifications of the water. And that could then cause some feedback loops for the melting of the ice. But it's hard to say exactly how that will play out.
CURWOOD: I'm wondering if what I'm hearing you saying is that, as things get warmer, it's going to make things warmer?
GUNN: Yes, there's definitely going to be some positive feedbacks with these processes going on. But the reason that we don't exactly know how things are going to play out is because we might start to see some positive feedbacks in some areas, and then some negative feedbacks or opposite feedbacks in other areas. So this comes back to what we mentioned at the start of this, which is that it's very hard to model these processes, because there's so many interacting different components. And they all can be acting in slightly different ways.
CURWOOD: Let's take a look into the future. What condition might these deep ocean currents be in another 10 or 20 years, do you think?
GUNN: So over longer terms, I expect to see a continue of this slowdown. Now, there's definitely going to be some ups and downs on that with natural variability. And other things like sea ice, for example, the amount of sea ice can affect those deep ocean currents as well. But over the next 10 to 20 years, I expect that we'll continue to see a slowdown.
CURWOOD: You have your research protocol already set out for yourself.
GUNN: I guess so, yeah.
CURWOOD: So, how did you get all this data? How do you know what you're telling me?
GUNN: So, we used three different types of data. So, the first was observations from these repeat sections, cross sections of the ocean, where you collect measurements of temperature and salinity, essentially using buckets. But they're more sophisticated than that. And they're repeated every 10 years or so. And then we combine that with these instruments called moorings, which are moored to the seafloor in different locations. And they give us information about the temperature and salinity, as well as the speed. And so, we combine those two to come up with the estimates of the strength of this deep circulation. And then we filled in some gaps using some model output, which gave us information about the structure of how that flow looked like.
CURWOOD: How do you feel about this data now that you've gathered it and written up this impressive paper for nature.
GUNN: I feel that this provides more motivation to reduce emissions as quickly as possible. So that's the first thing, and the second thing is that it really motivates us to try and understand more about this very remote, very interesting region of the Earth. Because we expect to see these long-term signals, but we still don't know how that's going to play out with the shorter-term and more natural variability.
CURWOOD: Now, I'm speaking to you from the northern hemisphere, you're in the southern hemisphere. Us folks in the north tend to forget that the biggest part of the oceans are towards the south of this planet. And there's an awful lot of ocean there. To what extent is the data you've gathered a warning to the rest of us that live in the northern part of the planet?
GUNN: It's a warning in the sense that the whole ocean is connected and changes that we see in one part, which we might think is very remote to us, will actually impact the whole ocean.
CURWOOD: Kathy Gunn is a physical oceanographer and climate scientist based with CSIRO Environment in Hobart, Australia. Thanks so much for taking the time with us today.
GUNN: Thank you very much for having me.
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