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Ocean heat content revisions

Filed under: — gavin @ 19 June 2008

Hot on the heels of last months reporting of a discrepancy in the ocean surface temperatures, a new paper in Nature (by Domingues et al, 2008) reports on the revisions of the ocean heat content (OHC) data – a correction required because of other discrepancies in measuring systems found last year.

Before we get to the punchline though, it’s worth going over the saga of the OHC trends in the literature over the last 8 years. In 2001, Syd Levitus and colleagues first published their collation of ocean heat content trends since 1950 based on archives of millions of profiles taken by oceanographic researchers over the last 50 years. This showed a long term upward trend up, but with some very significant decadal variability – particularly in the 1970s and 1980s. This long term trend was in reasonable agreement with model predictions, but the decadal variability was much larger in the observations.

As in all cases where there is a data-model mismatch, people go back to both in order to see what might be wrong. One of the first suggestions was that since the spatial sampling became much coarser in the early part of the record, there might be more noise earlier on that didn’t actually reflect a real ocean-wide signal. Sub-sampling the ocean models at the same sampling density as the real observations did increase the decadal variability in the diagnostic but it didn’t provide a significantly better match (AchutaRao et al, 2006).

Other problems came up when trying to tally the reasons for sea level rise (SLR) over that 50 year period. Global SLR is a product of (in rough order of importance) ocean warming, land ice melting, groundwater extraction/dam building, and remnant glacial isostatic adjustment (the ocean basins are still slowly adjusting to the end of the last ice age). The numbers from tide gauges (and later, satellites) were higher than what you got by estimating each of those terms separately. (Note that the difference is mainly due to the early part of the record – more recent trends do fit pretty well). There were enough uncertainties in the various components so that it wasn’t obvious where the problems were though.

Since 2003, the Argo program has seeded the oceans with autonomous floats which move up and down the water column and periodically send their data back for analysis. This has at last dealt with the spatial sampling issue (at least for the upper 700 meters in the ocean – greater depths remain relatively obscure). Initial results from the Argo data seemed to indicate that the ocean cooled quite dramatically from 2003 to 2005 (in strong contradiction to the sea level rise which had continued) (Lyman et al, 2006). But comparisons with other sources of data suggested that this was only seen with the Argo floats themselves. Thus when an error in the instruments was reported in 2007, things seemed to fit again.

In the meantime however, calibrations of the other sources of data against each other were showing some serious discrepancies as well. Ocean temperatures at depth are traditionally made with CTDs (a probe that you lower on line that provides a continuous temperature and salinity profile), Nansen bottles (water samples that are collected from specified depths) or XBTs (eXpendable bathy-thermographs) which are basically just thrown overboard. CTDs are used over and again and can be calibrated continuously to make sure their pressure and temperature measurements are accurate, but XBTs are free falling and the depths from which they are reporting temperatures needs to be estimated from the manufacturers fall rate calculations. As the mix of CTDs, bottles, XBTs and floats has changed over time, minor differences in the bias of each methodology can end up influencing the trends.

(If this is all starting to sound very familiar to those who looked into the surface stations or sea surface temperature record issues, it is because it is the same problem. Almost all long historical climate records were not collected with the goal of climate in mind.)

In particular, analysis (or here) of the XBT data showed that it was biased warm compared to the CTDs, and that this bias changed over time, and was dependent on the kind of XBT used (deep versus shallow). Issues with the fall rate calculation were well known, but corrections were not necessarily being applied appropriately or uniformly and in some cases were not correct themselves. The importance of doing the corrections properly has been subject to some ongoing debate (for instance, contrast the presentations of Levitus and Gourteski at this meeting earlier this year).

So where are we now? The Domingues et al paper that came out yesterday, along with a companion paper from essentially the same group (in press at Journal of Climate) have updated the XBT corrections and dealt with the Argo issues, and….

… show a significant difference from earlier analyses (the new analysis is the black line). In particular, the difficult-to-explain ‘hump’ in the 1970s has gone (being due to the increase in warm-biased XBTs at that time). The long term trend is slightly higher, while the more recent trends are slightly lower. Interestingly, while there still decadal variability, it is much more obviously tied to volcanic eruptions than was previously the case. Note that this is a 3-year smooth, so the data actually goes to the end of 2004.

So what does this all mean? The first issue is tied to sea level rise. The larger long term trend in ocean warming reported here makes it much easier to reconcile the sea level estimates from thermal expansion with the actual rises. Those estimates do now match. But remember that the second big issue with ocean heat content trends is that they largely reflect the planetary radiative imbalance. This imbalance is also diagnosed in climate models and therefore the comparison serves as an independent check on their overall consistency. Domingues et al show some comparisons with the IPCC AR4 models in their paper. Firstly, they note that OHC trends in the models that didn’t use volcanic forcings are consistently higher than the observations. This makes sense of course because each big eruption cools the ocean significantly. For the models that did include volcanic forcings (including the model we used in Hansen et al, 2005, GISS-ER), the match is much better:

(Note that the 3-year smoothed observations are being compared to annual data from the models, the lines have been cut off at 1999, and everything is an anomaly relative to 1961). In particular, the long term (post 1970) observational trends are now a better match to the models, and the response to volcanoes is seen clearly in both. The recent trends are a little lower than reported previously, but are still within the envelope of the model ensemble. One interesting discrepancy is noted however – the models have a slight tendency to mix down the heat more evenly than in the observations.

This isn’t going to be the last word on OHC trends, and different groups are going to be publishing their own versions of this analyses relatively soon and updates to the most recent years are still forthcoming. But the big picture is that ocean heat content has indeed been increasing in recent decades, just like the models said it should.


165 Responses to “Ocean heat content revisions”

  1. 151
    Chuck Booth says:

    Re 147 and my response (148)

    Tom, I re-read your comments in 147 and now see the point you are making in citing the parachute article, and the point the authors of the parachute article were trying to make.

  2. 152
    Ray Ladbury says:

    Joeduck, I’d like to emphasize Gavin’s point re: Hansen’s position quote on the WAIS. Note that he does not attack the credibility of those who disagree. Where his position differs, he prefaces his statement with “In my opinion…” Moreover, his opinion is not beyond the pale of scientific opinion. We know that the models are significantly underpredicting melting, and Hansen’s ideas can be viewed as a reasonable bounding engineering estimate of how bad things could get. That, too, has value.

  3. 153
    Figen Mekik says:

    I would like to add my two cents by saying that though I am sure Joe doesn’t intend to insult anyone, posing questions like “what would happen to your career if you challenged Jim Hansen” is not only insulting to me (since my name was used in the original question) but it is also very insulting to Jim Hansen and the community of climate scientists as a whole.

    Scientists discuss and challenge ideas and scientific interpretations. There is nothing personal about that. And challenging ideas will not hurt anyone’s career when those challenges are based on observations and facts. Gavin said all this much more eloquently above.

    But I, for one, find it a little demeaning to assume that I or any of my colleagues would change their opinion about something or their inetrpretations of their observations because another, better established scientist disagreed with them. Challenging ideas are welcome in any intellectual pursuit, challenging another’s integrity or honesty without evidence is not.

    Here’s a quote from Eleanor Roosevelt: “Great minds discuss ideas; Average minds discuss events; Small minds discuss people.”

  4. 154
    Joe Hunkins says:

    Gavin – thanks, good enough for me. I have taken up enough of your time hassling the Hansen points. Huge bonus points to you for this quote which I really enjoyed:

    What gets tiresome is the continual parade of junk masquerading as neo-Galilaen revelations

    Figen you are right I didn’t mean to insult you, rather I am very interested in the social interactions in science as a potential source of bias (as well as a potential source of enlightenment). There is a new field trying to quantify this stuff by graphing relationships among decision makers.
    Eleanor quote is good and appropriate.

    Ray – thanks for keeping me level headed here at the RealClimate Club, where the drinks are cold and the science is hot.

  5. 155
    Joe Hunkins says:

    Tom re: mediaorology paper by Dr. Schneider. Very interesting, and a good intro to issues that have a lot more significance as climate change has risen to the top of the environmental agenda. I think everybody would agree that the press treatments of climate issues generally leave much to be desired.

  6. 156
    Brian Dodge says:

    There are a couple of things missing from the parachute paper, namely the role of informed consent and Institutional Review Board approval required for research on human subjects (although tossing chimps out of airplanes with & without parachutes may represent a suitable animal model).
    For more information, see http://www.hhs.gov/ohrp/humansubjects/guidance/45cfr46.htm, particularly section 46.116 -
    “(a) Basic elements of informed consent. Except as provided in paragraph (c) or (d) of this section, in seeking informed consent the following information shall be provided to each subject:”
    “(2) A description of any reasonably foreseeable risks or discomforts to the subject;”

    Unfortunately, these legal requirements aren’t being applied to what Roger Revelle noted in 1956 – “Human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future.”

    Arguably, there are well funded politically influential segments of our society and their allies in the current administration who have waged a campaign to obfuscate and minimize the “reasonably foreseeable risks” posed by anthropogenic CO2 and global warming.

  7. 157
    Martin Vermeer says:

    Re Gavin’s comment to #148, I wonder how much of this “palpable concern” at our lack of understanding of dynamic ice sheet processes is due to it being solidly within the domain of geophysics, whereas the threat of a collapse of the food production system under the combined pressures of climate, ecosystem degradation, population and peak oil, which may happen well before, is much more interdisciplinary. People easily miss the overview on things outside their field (goes for me too).

  8. 158
    franco reseghetti says:

    I have seen the paper published on Nature. I have a small comment.
    XBT temperature measurements are affected by several inaccuracies, not only depth errors because of wrong fall rate coefficients.
    At least, there is a contribution of instrumental biases (coupling among device, wire, thermistor,…), and accidental biases (namely launching conditions). For example, it seems that the fall rate equation is seawater temperature dependent (maybe, ship speed dependent too). In this case, the right evaluation of the XBT depth is more difficult than previously estimated.
    In addition, a significant part of historical XBT profiles stored in databases are without metadata, and details concerning the recording systems.
    The manufacturers of XBT probes quoted an accuracy of 0.1°C on the instrument and of 0.2°C on the recording system.
    Statistically speaking, it seems that biases and measurement errors produces XBT temperature values warmer than real.
    In conclusion, climatological analyses and extrapolation of ocean temperature trend by using XBT measurements, but without a correct use of XBT errors/inaccuracies, are critical.

  9. 159
    Chuck Booth says:

    Re # 156 Brian Dodge “although tossing chimps out of airplanes with & without parachutes may represent a suitable animal model”

    Not likely – federal animal care and use regulations are even more stringent than the federal policy on the use of human subjects in research (45 CFR 46, which you cite).
    http://www.aphis.usda.gov/lpa/pubs/awact.html
    http://books.nap.edu/html/labrats/

    Sorry for straying further off topic. :)

  10. 160
    Richard Sycamore says:

    How is “planetary radiative imbalance” calculated? Is it measured or inferred?

  11. 161
    Jan Dash says:

    Roger Pielke Sr. has an Opinion article in Physics Today, Nov08, page 54 entitled “A broader view of the role of humans in the climate system”, (cf. his website http://www.climatesci.org/publications/pdf/R-334.pdf). He presents 4 years of data on the global changes in upper ocean heat content, obtained from J. Willis at JPL, and the article draws some contrarian conclusions.

    I have three questions:

    1. Has this article been peer reviewed?

    2. Are these data reliable and consistent with other data on ocean heat content?

    3. What conclusions can be drawn reliably from only 4 years of such data?

    I would be interested in Real Climate’s judgment.

    ————–

  12. 162
    Ray Ladbury says:

    Jan, I am familiar with the piece. It is marked as “Opinion,” so it has not been peer reviewed. This is the same strategy they used to circumvent peer review with the Scafetta and West numerology crap a few months ago. Can’t say how consistent these data are, but 4 years of data is bupkis. It’s a pretty sad effort in my opinion.

  13. 163
    JCH says:

    Jan Dash, while others were puffing molehills and crew cutting mountains, I wandered off through Gavin’s links in that post (Mountains and molehills) on SLR and Argo, etc.

    I think you’ll find it worthwhile reading – eighth paragraph, starts with:

    In contrast to this molehill,

  14. 164
    Hank Roberts says:

    http://moregrumbinescience.blogspot.com/2008/10/pielkes-poor-summary-of-sea-ice.html

    That includes a pointer to Willis’s blog, where he comments on what Pielke says about his work, usefully.

  15. 165
    Jan Dash says:

    Just for completeness, here are excerpts from an exchange between Pielke Sr. and Willis on the JPL Blog “It’s a Sure Bet” – by Josh Willis:

    Roger A, Pielke Sr Says:
    August 14th, 2008 at 1:23 pm
    Josh-
    I am puzzled by your weblog, and have weblogged on it. You are ignoring the value of heat in Joules (not surface temperature) as the primary global warming metric, despite your pioneering research using heat content change in Joules in the upper ocean to diagnose the radiative imbalance of the climate system.
    Best Regards
    Roger

    Willis says:
    Roger, thank you for the comment and the cross-link to my blog…True, ocean heat content is the better metric for global warming, and the past few years of no warming are interesting. But tacked on to the 50-year-record of ocean warming before that, the last four years pretty much ARE just a wiggle…Between the long-term records of ocean heat content, land and ocean surface warming, global sea level rise (about 20 cm over the last 100 years) and the increase in atmospheric CO2, you get a pretty simple, consistent picture of man-made warming. No models required…

    Despite all the uncertainties, I think it is pretty clear that humans have already warmed the planet. And if we continue to add more CO2 to the atmosphere, we will warm it even further.

    Ref:
    http://blogs.jpl.nasa.gov/?p=8
    ———–


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