The ocean heat graph (Part 2) – signal or noise?

In Part 1 I showed the ocean heat graph from the IPCC WG1 SPM, got rather distracted by the fact that I could not find the Supplementary Material that supposedly listed the datasets, showed the corresponding graphs from Chapter 3 and noted that they were described as ‘estimates’ there but not in the SPM.

The IPCC graph shows estimated total ocean heat content since 1950, in units of Joules, with the tick mark intervals on the axis showing an apparently enormous numbers such as 10^23 Joules.  What the IPCC does not tell the policymakers however, is what this corresponds to in degrees C.  In fact a very simple calculation shows that 10^23 Joules would correspond to a rise of only about 0.04C.

Lucia Liljegren produced a helpful version of the graph with additional axes showing the corresponding temperatures:

Her version is 0-2000m so is not directly comparable with the IPCC 0-700m one.

It’s no secret that the corresponding temperature increase is only a few hundredths of a degree. The website of the ARGO system for measuring ocean temperature mentions a temperature change of 0.06C since the 60s and the Levitus et al paper talks of  0.09C since 1955 in the abstract. But the IPCC SPM does not mention these small numbers, talking instead of a 0.11C/decade warming in the upper 75m of the ocean.

These very small temperature variations raise the question of whether there is any significant meaning in these graphs at all. How large are the error bars? How accurately can ocean temperature be measured? How accurately could it be measured in the 1950s? To within a hundredth of a degree? I think not.  As well as the temperature accuracy, there is the question of coverage. To be able to produce this graph accurately, you need to measure the temperature all over the ocean and at all depths, at all times back to the 1950s.

Here is an interesting graph from the Ishii and Kimoto 2009 paper, used as one of the IPCC sources

The lower curve, V6.7, surrounded by the grey band, is the version used in the AR5 report. The two upper curves, one solid and one dotted, V6.2 and V6.3, are from a 2006 paper by the same authors. Notice that the curves are quite different. The difference between them is of the same order as the variation in the graphs themselves! In the 2006 version, the heat content in the 1970s was about the same as it is now. So what is the difference? Well, the newer results incorporate what the authors refer to as ‘bias corrections’. An adjustment has been applied to the data, which just happens to work in such a way as to cool past temperatures, and produce a much more steadily increasing curve.

This all seems a bit familiar.

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4 thoughts on “The ocean heat graph (Part 2) – signal or noise?

  1. Hi Paul,

    “How large are the error bars? How accurately can ocean temperature be measured? How accurately could it be measured in the 1950s? To within a hundredth of a degree?”

    There’s a recent review of global ocean temperature observations that might be of help in answering your questions. It also provides background on the rationale for the bias adjustments and on the uncertainties of the estimated biases in the data.

    http://onlinelibrary.wiley.com/doi/10.1002/rog.20022/full

    Cheers,

    John

  2. Thanks John, that is indeed a useful resource, setting out the historical measurements very clearly. Clearly there are complications associated with the changes between different types of measurement systems, and some potential problems with the “XBT” data where the depth isn’t measured but has to be estimated from the speed at which the probe is thought to be sinking.

  3. I find the graph at: http://oceans.pmel.noaa.gov/ explaining it all. Ever since the ARGO-buoys have been collecting data, no evident warming of the oceans (uppermost 700 m) has occurred. The earlier data is very heterogeneous due to variations in sampling methods, sampling periods and stations and therefore does not fulfil the requirements of continuous systematic sampling covering all oceans.

    What bothers me more is related to the IPCC statement that global warming has stopped because the excess energy has “decided” to vanish into deep water. I wonder what kind of process is capable of deciding that 15-16 years ago a new process was introduced.

    Furthermore, the IPCC explains that global warming is due to increase in greenhouse gases (mainly CO2), but this warming is attributed to infra-red back-radiation. I also understand that infra red does not penetrate into water beyond the very thin uppermost surface “film”, and also that this energy will obviously be used to evaporate water and will not actually increase the water temperature.

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