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When one thinks of Antarctica, one imagines a vast landscape in shades of blinding white, ice and snow stretching as far as the eye can see. But to really consider Antarctica is to consider its water.
The Southern Ocean, which encircles Antarctica, is where the ocean exhales. It is the primary place where the water of the deep oceans rises to the surface, mingles with the atmosphere in a kind of embrace, and then sinks back into the depths. In the course of this exchange, the Southern Ocean both consumes carbon from the atmosphere and releases some of the vast quantities of carbon stored in the deep ocean, the Earth’s largest natural carbon sink. The Southern Ocean therefore controls the global exchange of carbon between ocean and atmosphere, which is vital to regulating global carbon dioxide levels. But climate change may alter this dynamic.
We spoke to Elisabeth L. Sikes, a marine biogeochemist and paleooceanographer with Rutgers University who has received awards for her research on Antarctica and the Southern Ocean, about why the Southern Ocean is so vital to carbon regulation and how global climate change is influencing its role.
How does the Southern Ocean work as a carbon sink?
It’s both a source and a sink. Phytoplankton in the surface ocean take up CO2, through photosynthesis. And when they die, their bodies sink to the bottom of the ocean and somebody else eats them. Then that carbon—that they’ve pulled from the atmosphere, turned into organic matter, and sunk—is released back into the deep ocean as CO2. So that CO2 is sequestered in the deep ocean. And that process is called the biological pump.
That’s the first step in why the ocean can sequester CO2. The second step is sort of why it comes back out again.
And how does that happen?
The ocean has deep circulation that’s different from the surface circulation, which is wind-driven. This deep circulation is called thermohaline circulation. The main place this deep water forms is the North Atlantic.
The Gulf Stream delivers very warm, salty water to the North Atlantic. When you chill salty water—which is already dense from the salt—it gets really cold and really dense. And so that North Atlantic deep water travels around the bottom of the ocean. This deep water is cut off from the rest of the ocean, and it’s traveling along, and that biological pump is putting organic matter into it, which turns into CO2. So now you’re building up all of the CO2 in the deep ocean.
It’s about winds, and it’s about density.
So how does that carbon get back out into the atmosphere?
The Southern Ocean is the place where this deep water actually outcrops to the atmosphere. It’s about winds, and it’s about density.
This buildup of CO2 that occurs through the biological pump, and this deep conveyor belt circulation, gets released. Not all of it, but a large portion of it exchanges in the Southern Ocean, because the winds are strong, and the water outcrops with a CO2 load.
Those dynamics in the Southern Ocean actually control how much CO2 there is in the atmosphere. How much gets out depends on how much productivity there is in the Southern Ocean. If the biological pump is working in the Southern Ocean, it gets trapped back in. And if the bio pump’s not very efficient in the Southern Ocean—which it isn’t—some of it can get back out.
What determines how much is sunk and how much is released?
It depends on this wind and wave interaction. So when the winds and this upwelling are in the sweet spot, CO2 gets released. But when the winds aren’t aligned well with where this water is coming up—then it’s not going to release much carbon. The carbon is going to sink back down.
It sounds like a lot depends on the Southern Ocean. How stable is it?
So the next important thing about the Southern Ocean is its northern boundary is not a continent—its northern boundary is directed by winds. Basically, we have this band of winds around the southern hemisphere called the southern westerly winds. And the temperature of the atmosphere determines where they are. So when it’s cold, that southern westerly band moves north, and then the fronts move north, and the Southern Ocean expands. And when it’s hot, that heat presses those winds down south, closer to the Antarctic continent, and the Southern Ocean contracts. It’s very dynamic.
If you just think about the fact that you’ve got the same amount of energy going around the Southern Ocean, and you shrink it down—same energy, smaller area—the current is going to spin up. So some of the projections are that as the planet warms, and those southern westerly winds contract, that you’ll have more winds blowing over the Southern Ocean, and that will blow more CO2 out of the ocean, into the atmosphere. And that will create a positive feedback loop, making CO2 go up even more quickly.
It’s a little scary how interconnected it all is.
There is a running joke that being in the climate sciences and an oceanographer, there’s just no good news. But we don’t get into this business because we want good news. We want truth. 
Lead photo by Liz Greene
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