How do you weigh an ocean?

Oceanographers have come up with the unusual idea of weighing the global ocean at a single point to assess how big it is.
04 April 2013

Interview with 

Chris Hughes,National Oceanography Centre; Sue Nelson, Planet Earth Podcast


Gravity ocean map


Chris Hughes - We know that the sea level is rising. It's been rising at about 3 millimetres per year for the last 20 years or so and we're Gravity ocean mappretty certain it's going to continue rising but there are lots of different estimates of how much. So over the 21st Century it might rise by half a metre, a metre, maybe even more. Whether it rises by a lot really depends on how much ice melts. The main source of water is the ice that is locked up in Greenland or in Antarctica which can melt and fill the ocean. But the other way it can rise is you don't have water, it's just the water that's already there expands to take up more space because it is warming up. So we want to be able to separate out the two different sources of sea level rise and in order to do that is for me to measure the mass of the ocean separately from its volume. The mass tells us how much ice is melting, water pouring off the continents, whereas the volume tells us about the density change as well so we need to separate the two out to understand what is happening.

Sue Nelson - So is this why you want to actually build an instrument or device that would in a conventional sense of the word, quite literally, weigh the ocean by going on the seabed on the seafloor?

Chris Hughes - Yes, that's right. It gave us quite a surprise to find that this might even be possible. When you look at the sea level problem altimetry has been brilliant because it has measured the entire ocean, so the satellite measures almost the whole world and we know because we're measuring everywhere that we're measuring the global picture. If you don't measure the global picture you don't get the total. The ocean sloshes around in a very complicated way and if you just measured it at one place you wouldn't see global sea level rise you would actually see mostly the sloshing or the local effects. So you need to do that global average. But it turns out when we look at what ocean models can tell us that when you look at the bottom of the ocean things get much quieter and especially in the tropics there's so little of this sloshing going on on long timescales but it gets so quiet that if you just measure the pressure you're not measuring the sloshing anymore you are just measuring the slow rise of the ocean as the ice melts.

Sue Nelson - If you're measuring the pressure down, a force down on an instrument on the sea floor, how will that though make a global average? You're going to get a different measurement on the other side of the world.

Chris Hughes - That's the surprising aspect of it. I think of it like a little bit like when you're filling the bath. If you turn the taps on and look near where the taps are you can't actually tell whether the bath is filling because there's so much splashing around going on, but if you look a long way away all the ripples are damped out and there's just a gradual creep of the level up the side of the bath. That's what we're doing here, we finding a place where all that splashing around isn't really influencing what we see. So all that's left is the global average.

Sue Nelson - Right then, let's get to the nitty gritty. What will this instrument consist of and how are you going to build it?

Chris Hughes - Well that's the challenge. The same technology has been used for about the last 40 years which consists of a quartz crystal and the resonance of that crystal, the resonance frequency depends on the pressure but these have a problem, they have got much better over the years but even so when you put them down in five kilometres of water they are still slowly crushed by the pressure so we can't measure these changes in pressure on the very long timescales we would like to. It's not super difficult. We're talking about one in ten million accuracy, so you want to be in five kilometres of water measuring a pressure change equivalent to about half a millimetre. That would be the level we would aim for.

Sue Nelson - How deep are we talking about you going? Several thousand metres I would imagine.

Chris Hughes - Yes, typically it will be three to five kilometres. You want to be away from the edges of the ocean, so somewhere in the middle and that's the usual depth.

Sue Nelson - So if you can't use these quartz crystals what else are you going to use?

Chris Hughes - That's where we're asking for ideas. One possibility is using something to do with electromagnetism with light. We know there are great ways of measuring things to enormous accuracy if you're using light so maybe we can do something to do with the refracting index of a fluid which will change according to its pressure. Maybe there is something to do with the speed of sound that we could measure or ideas have been floated about using something which changes phase.

Sue Nelson - So this is a science and engineering challenge as well.

Chris Hughes - Yes, that's what we're trying to do. We want to get engineers around the world who have just not thought of this as a problem to bring their expertise to bear and come up with innovative solutions. There have to be lots of things that we haven't thought of and we've been stuck with the same technologies, as I say, for 40 years. It's a really fundamental piece of physics; pressure is not a complicated variable. I'm sure there is someone out there who has a brilliant idea that would solve our problem. If we can get these message out so that people understand that there is a challenge and this is what we need I am sure someone, somewhere, has the idea that will solve our problem.


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