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How do you weigh an ocean?

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Marion O’ Sullivan writes: Oceanographers have come up with the unusual idea of weighing the global ocean at a single point to assess how big it is.

Professor Christopher Hughes from the National Oceanography Centre says that ‘making accurate measurements of changing pressure at a single point will indicate the mass of the world ocean.’

And he says he knows exactly where to place the pressure guage to take this measurement – the central tropical Pacific, where the deep ocean is at its quietist.

‘The instrument needs to be located away from land and oceanic variability, ‘ says Hughes. ‘The principle is rather like watching your bath fill; you don’t look near the taps where all you can see is splashing and swirling, you look at the other end where the rise is slow and steady.’

But why does he want to weigh the ocean – and why is it important to do so?’

‘We know that global sea level is rising by about 3 mm per year but we need to separate out the different causes of sea level change, ‘ he says.

The oceans can warm and expand, which means the same weight of water will take up more space. But if more water goes into the ocean from melting land ice, the weight will increase.

‘As we don’t know how much land ice will melt and go into the ocean in the future we need to develop long term monitoring systems to measure that, ‘ adds Hughes.

Using pressure measurements taken in the Pacific Ocean over the past ten years or so, Hughes and his team have been able to show that a massive six trillion tonnes of water enters the ocean between March and September each year. That’s enough to raise the sea level by 1.7cm.

Much of this will evaporate and leave the ocean over the following six months, but some will not.

Hughes explained that they can measure the annual cycle of water coming into and out of the ocean using drifting moorings, tide gauges and satellite. But none of these can give a really accurate measurement of the amount of water accumulating in the ocean each year.

He is hoping that engineers will rise to the challenge of developing a more sensitive instrument, capable of measuring fractions of a millimetre of water whilst coping with the immense pressure of four kilometres of water above it.

Hughes acknowledges that this would be a very challenging goal. ‘The pressure changes are smaller than the background pressure by a factor of about 10 million, and the deep ocean is a hostile environment for mechanical components, ‘he says.

So the gauntlet has been thrown down – it will be interesting to see if anyone takes up the challenge in the future.

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