Ocean Heat Content: How do you measure that?

Typical profiles of temperature and salinity measured as a function of depth by an ARGO probe.

Typical profiles of temperature and salinity measured as a function of depth by an ARGO probe. (From Wikipedia). Notice the interesting behaviour (called the thermocline) about 100 metres below the surface. Measurements such as this enable us to understand where heat is stored in the oceans.

Where does all the Sun’s energy go when it reaches Earth? We obviously need to be able to answer this question if we want to fully understand what is happening to Earth’s climate.

Because roughly two-thirds of the Earth’s surface is covered in water, we can make a first guess that the main effect of solar energy is to warm the oceans.

The mass of water in the oceans is 280 times larger than the mass of air in the atmosphere and so we can guess that the temperature of the atmosphere is then determined by exchange of heat with the oceans.

Of course it is all more complicated than I have implied. But even so, it is clear that one question that needs to be answered is this:

  • Is the amount of heat energy stored in the oceans changing?

To answer this we just need to measure the temperature of the oceans and see if the answer is changing.

We have a records of the temperature of the atmosphere above the land surfaces of the Earth that extend back over some parts of the globe for around 150 years. And we even have some records of the temperature of the atmosphere above the sea surfaces too. And through careful analysis we have been able to detect a temperature rise of about 1 °C over the last century or so.

However records of ocean temperature are much much poorer. In fact until the turn of the 21st Century – the measurements were really inadequate to detect changes in ocean temperature. The reason is that in order to detect changes one needs to measure the temperature of the entire column of water in the oceans. In lots of places. Over a long period. And one needs to make measurements very accurately.

The reason one needs measure the ocean temperature accurately is because of the contrasting properties of water and air.

  • Water is nearly a thousand times denser than air.
  • If you give equal amounts of energy to equal volumes of water and air, then if the air warms by one degree, then the water will warm by the less than one thousandth of a degree.

As a consequence, vast amounts of energy can be stored in the ocean without the temperature of the water rising  very much. I won’t go in to the fascinating details of this measurement covered in a recent paper, but simply tell you that  a solution to this problem now exists. and the answer is a network of more than 3000 ingenious ‘ARGO’ probes busy measuring the temperature of the ocean right now.

Schematic diagram of an ARGO probe

Schematic diagram of an ARGO probe

Each ARGO probe is fiendishly clever. Once deployed from a research vessel, the probe sinks to a depth of 100 metres, and waits for 10 days. It then adjusts its buoyancy to sink to 2000 metres depth and then ascends smoothly to the surface recording the pressure (from which the depth is deduced), the temperature and the salinity of the water. When it reaches the surfaces, it establishes a connection to one of a variety of satellites and sends its data back to base along with its location deduced by GPS. It then sinks to 100 metres depth again, drifts with the currents and repeats the whole exercise 10 days later.  It can keep going for 5 years or more.

Importantly the temperature probes are carefully calibrated so that their uncertainty of measurement is only 0.002 °C, and so even small changes in ocean temperature can be sensitively detected. Additionally the network of more than 3000 probes enables fair sampling over most of the world’s oceans. The data from the network is freely available and over the coming decades it is likely to reveal hitherto unmeasurable details about heat storage in the oceans.

Deployment Sites of Argo probes colour-coded by the country that deployed them. All the data is openly accessible.

Deployment Sites of Argo probes colour-coded by the country that deployed them. All the data is openly accessible.

And I take a personal lesson from the ARGO project too: just because something has always been done badly is no reason not to start doing it better. Or at least to try to.

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One Response to “Ocean Heat Content: How do you measure that?”

  1. Climate Change Diagram | worldwideclimatechange Says:

    […] Ocean Heat Content: How do you measure that? (protonsforbreakfast.wordpress.com) […]

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