Glen Paul: G'day and welcome to CSIROpod, I’m Glen Paul. The depths of the ocean have been in the news recently, with Hollywood Director James Cameron taking his submersible to the floor of the Mariana Trench's Challenger Deep, the deepest known point on earth. And while the landscape is described as basically featureless, the deep ocean remains a significant component of the Earth's climate system.

Changes in deep ocean conditions affect global climate, with deep warming contributing to sea level rise, and the deep ocean absorbing atmospheric CO₂. To assess change researchers create budgets to determine the amount of heat, water, and gases, including CO₂, entering and exiting the ocean.

To find out more I’m joined by CSIRO's Dr Bernadette Sloyan. Bernadette, how much of the world’s oceans are classed as deep oceans, and what’s the average depth?

Dr Sloyan: So the deep oceans we consider below 2000 metres, and if that’s your level, then about 52 per cent of the ocean is below 2000 metres.

Glen Paul: OK. And the deep water influences the earth’s climate system, but how much do we really know about what’s going on down there?

Dr Sloyan: So below 2000 metres we have to rely on observations from ship based programs, and in the world’s oceanography community at the moment we have a program where we try and get to specific locations in the deep ocean every five to seven years, so that is our basic time span of the data that we can get.

Glen Paul: Hmm, and obviously it’s a bit costly to go for trips under the sea in submersibles, so what sort of technology are you sending down from the ships to obtain measurements from the deep ocean?

Dr Sloyan: So we’re relying on a lowering of a package from the ocean surface, so from the ship right down to the deepest part of the ocean that we can observe. We mainly are observing the ocean in some parts, like the South West Pacific, from that ship from the surface right down to the bottom, which is about 5500 metres. Other parts of the deep ocean will only go to, you know, 3000 metres. We’re relying on an instrument which we call a CDT, which is a Conductivity Depth and Temperature instrument, and that measures the temperature, the salinity, and the pressure of the ocean as we move this package down through the ocean column.

Glen Paul: Righteo. And how much change have we seen in the ocean say over the past decade with the information that you’ve been able to retrieve?

Dr Sloyan: So in the last decade we’ve been able to measure changes in both the temperature of the deep ocean and its salinity, but also we’ve been recently able to measure that the water masses are actually decreasing in the deep ocean, so we’re actually getting less of the really dense Antarctic bottom water entering the deep ocean and flowing on to that circulation impact.

Glen Paul:  And what about what lies ahead; how are you projecting future warming?

Dr Sloyan: So in the international sphere Australia is occupying these hydrographic lines in the Pacific and southern oceans, and the Indian Ocean, and other colleagues from international researchers are doing other parts of the Pacific, and the North Atlantic, and the Indian, and all this data is freely available to the international community to use. So combining all of the international data that we have we can look at what types of changes we’re seeing on that five to seven year time scales in the deep ocean.

Glen Paul: Right. So how will, or how does the deep ocean actually impact on the earth’s climate system?

Dr Sloyan: So in Australia we have the southern ocean sitting to the south of us, and it’s where the deep oceans actually communicate to the atmosphere at the high latitudes, where inputting heat, and with inputting carbon into the deep ocean through the surface, which then because these waters are so dense and cold they actually sink down from that surface layer around Antarctica to the 3000, and then fill the basins northwards as they move into the Indian and Pacific ocean.

Glen Paul: OK. And how long can the deep ocean store the carbon?

Dr Sloyan: So the deep ocean can actually store the carbon for a long period of time, because the circulation is a little bit slower than in the upper ocean, and its ability to actually take up the heat and the carbon for long periods of time that we’re really interested in.

Glen Paul: Does that mean then the stuff we’re putting in now, the carbon we’re putting in now, someone else will have to pay for it tomorrow, like future generations will effectively cop our carbon?

Dr Sloyan: Well eventually everything that the ocean is continually circulating, and continually moving, so it will have an impact on the rest of the ocean’s circulation, and will eventually come back to the atmosphere.

Glen Paul: And what will that mean for the planet?

Dr Sloyan: Well depending on how we’ve changed the atmosphere; if we have now warmed the ocean a lot more, the ability of the ocean to actually absorb carbon will actually decrease, that impact will then go into how the ocean itself actually interacts with the ecosystems, and whether animals and corals can actually grow in those environments.

Glen Paul: Well, let’s hope they can. Thank you very much for discussing the research with me today, Bernadette.

Dr Sloyan: Thank you.

Glen Paul: Dr Bernadette Sloyan. For more information find us online at www.csiro.au. You can like us on Facebook, or follow us on Twitter at CSIROnews.