Climate change and the acidifying, freshening, warming Southern Ocean

The Southern Ocean, as photographed during the Thomas G Thompson Southern Ocean Expedition. Flickr/Barbara L. Slavin.

The distinctive, planet-encircling flows of the Southern Ocean have played a role in moderating global warming, but change is at hand with the water heating up, getting less salty, storing more carbon, and growing more acidic. These changes could lead to rises in sea levels, changing weather patterns, and the inability of marine life to form shells, skeletons, and reefs.

These are some of the findings of Position Analysis: Climate Change and the Southern Ocean, a report released today by the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC).

Below is an interview with report co-author Dr Steve Rintoul, Oceans Program Leader at the ACE CRC, followed by analysis of the report from Professor Carlos Duarte, Director of the Oceans Institute at the University of Western Australia.

Report author Dr Steve Rintoul, Oceans Program Leader, Antarctic Climate and Ecosystems Cooperative Research Centre

The oceans as a whole are really important for climate because they can absorb huge amounts of heat and carbon dioxide. In a sense, when we talk about global warming that’s happening today, we’re really talking about ocean warming.

More than 90 per cent of the extra heat energy that’s absorbed by the Earth has gone into warming up the oceans, so if we want to track climate change we need to track the oceans. Similarly for carbon dioxide the oceans have absorbed about 30 per cent of the carbon dioxide we have emitted into the atmosphere, so that’s acted to slow the rate at which climate would have otherwise changed - it’s helped to mitigate the effects of climate change.

The Southern Ocean has a particulary important role in storage of both heat and carbon. About half of the extra heat energy that’s been stored by the Earth has entered the oceans through the Southern Ocean and about 40 per cent of the carbon dioxide that’s stored in the oceans enters though the Southern Ocean. Yet, the Southern Ocean represents only about 20 per cent of the surface area of the oceans. So it’s a factor of two for carbon and two-and-half for heat more effective than you might have expected for the area of the Southern Ocean in terms of storing heat and carbon.

The reason it can do that is because of a unique pattern of ocean currents. You can think of the Southern ocean as a window to the deep oceans [. It’s the only place where deep waters rise up and reach the sea surface, and it’s one of the few places where large amounts of water sink from the surface down into the interior of the ocean. It’s because of those rising and sinking motions that the Southern Ocean can store lots of heat and carbon.

There are two sets of ocean currents. The largest current in the world-ocean circles around Antarctica from west to east, and that extends from the sea surface right down to the sea floor and into the ocean basin. At the same time this water is whipping around Antarctica, there’s a slower but important set of flows that is moving towards Antarctica and rising towards the surface, or flowing away from Antarctica and sinking down away from the surface into the deeper ocean; we call that the overturning circulation. A lot of the critical climate aspects of the Southern Ocean have to do with the overturning circulation - how it works now and how it might change in the future. Given that the ocean plays an important role in today’s climate, if the Southern Ocean were to change that might have widespread consequences for climate and not just in the Southern ocean region.

Recent data shows that the Southern Ocean is changing now: it’s warming and it’s becoming less salty, and the amount of carbon dioxide that’s stored in the Southern Ocean is increasing - there’s more carbon accumulating in the Southern Ocean. Each of those three changes tell us something about how the system works and also acts as an indicator of what we might expect to happen in the future.

One of the effects of warming on the Southern Ocean is that it tends to increase the rate at which the ice that flows off Antarctica enters the sea and melts. The warmer the ocean the faster the ice around the edge of Antarctica melts. If the ice that’s already floating around Antartica melts it doesn’t make any difference to sea-level-rises directly, just like melting ice cubes in a drink don’t cause the drink to overflow the cup. But what happens when you thin or break up the ice that’s floating around the edge of Antarctica is that more ice flows off the continent and into the ocean, and that does increase sea-level rise. So the largest single uncertainty in estimating the future range of sea level rise is the question of what’s going to happen to the ice sheets of Antarctica and Greenland. And what happens to the Antarctic ice sheets is intimately tied up with the rate of warming of the Southern Ocean.

We’ve also observed the freshening of the Southern Ocean; it’s becoming less salty. This tells us two things. The fact that the surface waters are getting fresher tells us that there’s more rainfall than there used to be. At these latitudes we expect there to be more rainfall as the Earth warms up, and there to be less rainfall in the drier zones further north in the subtropics. That’s because warm air holds more moisture in, so as the Earth warms up we expect the whole water cycle on Earth to become more vigorous - more evaporation in the evaporative zones and more rainfall in wetter areas. So the Southern Ocean provides one of the clues but that’s already happening now.

We also see the very deepest layers around Antarctica becoming fresher. It’s an indication that some of the extra melting of ice around the edge of Antarctica is flowing into the sea and getting carried down to the deep ocean by the ocean currents.

The third change is the carbon concentration. What’s important about this is that the ocean’s picking up of carbon dioxide is in a sense a good thing for us - it soaks up CO2, and that makes the climate change less rapidly than it would otherwise. But it has another consequence and that is in changing the chemistry of the ocean. It makes the ocean more acidic and makes it harder for organisms to form shells or skeletons or reefs out of calcium carbonate.

As we put more carbon dioxide into the Southern Ocean eventually we’ll cross a threshold where the water will actually become corrosive to reefs and shells made out of calcium carbonate. The point at which you cross that threshold depends on the temperature of the water, and it will be crossed first in the cold waters of the polar regions, both north and south.

We’ve realised that that threshold will be crossed earlier than we thought, at least at some times of the year, and those corrosive conditions will exist in winter as soon as 2030 - only two decades from now. And this will happen at much lower levels of carbon dioxide: at levels of only about 450 parts per million. We used to think that threshold wouldn’t be crossed until we got to about 550 parts per million.