Ocean Temperature And Heat Content

  • Date: 26/02/13
  • Willis Eschenbach, Watts Up With That

The upper ocean temperature was roughly flat from 1993-1998. Then it increased by about one tenth of a degree in the next five years to 2003, and has been about flat since then.

Anthony has an interesting post up discussing the latest findings regarding the heat content of the upper ocean. Here’s one of the figures from that post.

pmel 0-700m heat content anomaly

Figure 1. Upper ocean heat content anomaly (OHCA), 0-700 metres, in zeta-joules (10^21 joules). Errors are not specified but are presumably one sigma. SOURCE 

He notes that there has been no significant change in the OHCA in the last decade. It’s a significant piece of information. I still have a problem with the graph, however, which is that the units are meaningless to me. What does a change of 10 zeta-joules mean? So following my usual practice, I converted the graph to a more familiar units, degrees C. Let me explain how I went about that.

To start with, I digitized the data from the graph. Often this is far, far quicker than tracking down the initial dataset, particularly if the graph contains the errors. I work on the Mac, so I use a program called GraphClick, I’m sure the same or better is available on the PC. I measured three series: the data, the plus error, and the minus error. I then put this data into an Excel spreadsheet, available here.

Then all that remained was to convert the change in zeta-joules to the corresponding change in degrees C. The first number I need is the volume of the top 700 metres of the ocean. I have a spreadsheet for this. Interpolated, it says 237,029,703 cubic kilometres. I multiply that by 62/60 to adjust for the density of salt vs. fresh water, and multiply by 10^9 to convert to tonnes. I multiply that by 4.186 mega-joules per tonne per degree C. That tells me that it takes about a thousand zeta-joules to raise the upper ocean temperature by 1°C.

Dividing all of the numbers in their chart by that conversion factor gives us their chart, in units of degrees C. Calculations are shown on the spreadsheet.

degrees pmel 0-700m heat content anomaly

Figure 2. Upper ocean heat content anomaly, 0-700 metres, in degrees C. 

I don’t plan to say a whole lot about that, I’ll leave it to the commenters, other than to point out the following facts:

• The temperature was roughly flat from 1993-1998. Then it increased by about one tenth of a degree in the next five years to 2003, and has been about flat since then.

• The claim is made that the average temperature of the entire upper ocean of the planet is currently known to an error (presumably one sigma) of about a hundredth of a degree C.

• I know of no obvious reason for the 0.1°C temperature rise 1998-2003, nor for the basically flat temperatures before and after.

• The huge increase in observations post 2002 from the addition of the Argo floats didn’t reduce the error by a whole lot.

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