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Ocean heat content: latest numbers

Filed under: — gavin @ 16 August 2006

Net ocean heat content changes are very closely tied to the net radiative imbalance of the planet since the ocean component of the climate system has by far the biggest heat capacity. Thus we have often made the point that diagnosing this imbalance through measurements of temperature in the ocean is a key metric in evaluating the response of the system to changes in CO2 and the other radiative forcings (see here).
In a paper I co-authored last year (Hansen et al, 2005), we compared model results with the trends over the 1993 to 2003 period and showed that they matched quite well (here). Given their importance in evaluating climate models, new reports on the ocean heat content numbers are anticipated quite closely.

Recently, a new preprint with the latest observations (2003 to 2005) has appeared (Lyman et al, hat tip to Climate Science) which shows a decrease in the ocean heat content over those two years, decreasing the magnitude of the long-term trend that had been shown from 1993 to 2003 in previous work (Willis et al, 2004) – from 0.6 W/m2 to about 0.33 W/m2. This has generated a lot of commentary in some circles, but in many cases the full context has not been appreciated.

With any new data sets there are a number of questions that must always be asked: Are the measurements really representing what is claimed? (in particular, are there sampling or definitional problems?). Do related data provide some support for the results? If correct, what are the potential causes? and, most importantly, what part of the changes are related to predictable deterministic effects? This last question brings up the issue of model evaluation, because of course, the models can only be expected to reproduce the deterministic long-term component.

Given some of the ongoing discussion, it obviously still needs to be pointed out that year-to-year fluctuations in any of the key metrics of planet’s climate are mostly a function of the weather and cannot be expected to be captured in climate models, whose ‘weather’ is uncorrelated with that in the real world. So claims that two years worth of extra data of any quantity somehow prove or disprove climate models are simply erroneous. Clearly, life would be simpler without weather ‘noise’ cluttering up the system, but this is something that just needs to be dealt with. Dealing with it means paying more attention to long term changes than to short term fluctuations and making sure that enough ensemble simulations are made with the models to isolate the signal from the noise.

Going back to the data, are there any potential problems? Well, as addressed by the authors, this time frame is the period when the ARGO floating profilers really start to be important in improving the coverage of data (look at the difference in coverage in their figure 8 between 2002 and 2005). The profilers have clearly been the best thing to happen to ocean observations in decades. Not using the profilers gives a smaller recent change – but with increased error bars because of the deterioration of the sampling. Additionally, some parts of the ocean, particularly the Arctic are still not being sampled sufficiently. These effects may yet prove to be part of the story.

What about any supporting data? One problem is that if the ocean has lost heat at the suggested rate, then the thermal exapansion part of recent sea level rise should have decreased (i.e. sea level should have dropped). Overall, sea level however has continued to rise unabated according to the altimeter satellites. The only way to reconcile the results would be to have had a sharp compensating increase in freshwater from the ice sheets adding to sea level (from 0.7 mm/yr to 2.9 mm/yr). This is conceivable (though unlikely), but clearly would not be good news!

If however, we assume that the data are reasonably accurate, what could be going on? Some of the changes are clearly due to ocean circulation changes – an increased advection of warm water from the sub-tropical Atlantic to the North for instance, but the biggest contribution are the changes seen in the sub-tropical South Pacific. The heat can either have been subducted below the 700m level (the bottom depth for this analysis), advected sideways (no real evidence for that though), or lost through the surface (either to the atmosphere, or directly out to space). The third possibility is thought the most likely.

This in turn can have had a number of possible causes: ‘natural’ tropical variability – for instance, the winter (DJF) tropical Pacific cooled over these two years, possibly as part of larger-scale ENSO variability. Alternatively, it may be due to a change in the forcings. Possible candidates are an as-yet-unquantified increase in aerosol forcings from Asian sources. These haven’t been included in simulations since the data on emissions aren’t yet in.

On a larger point, the radiative imbalance in the AR4 models is a function of how effectively the oceans sequester heat (more mixing down implies a greater imbalance) as well as what the forcings are. Therefore, there is a variation in that modelled value across the models – some of which are smaller than our reported figure (all are significantly positive though).

A slightly more subtle (and slightly more valid) criticism is that the reported magnitude of decadal variability in the OHC numbers is larger than is seen in most coupled models. Some recent work has shown that sampling may play a role here, but it wouldn’t necessarily be surprising if this was so. Even in our paper last year we stated that earlier reported decadal variations were not well simulated. There is obviously much that remains to be understood about annual to decadal variability, however, it must be remembered that it is only on the longer time scales that we expect the forced signal to dominate over the internal ‘noise’. On this basis the ocean heat content changes remain a good validation of the climate model simulations.


107 Responses to “Ocean heat content: latest numbers”

  1. 101
    old jim hardy (aka analog) says:

    Thanks guys, for your kind remarks.

    I assumed, perhaps naively, that heat rejected by a hurricane to the troposphere would upset the equilibrium and quickly radiate away, the stratosphere being comparatively transparent to longwave.

    “The spectrally averaged effective emission temperature of the earth is about 252 K, which corresponds to the physical temperature near the 6 km level. This is also then the approximate location of the spectrally averaged TAU = 1 level from which most of outgoing flux can be said to originate from.” (http://www.realclimate.org/index.php?p=58, comment #17)

    old jim himself

  2. 102

    Re: Gavin’s response to #86

    Solar activity does have to have increased since 1940 to explain a significant part of the recent warming. All that is needed is unrealized climate commitment from continuation of the high levels of solar forcing reached by 1940. A constant forcing applied to a pot of water, can still result in an increase in the temperature of the water. The realization of the temperature increase from the high level of solar activity was delayed/interrupted by a period of aerosol cooling, and there was also a significant, but not necessarily large, contribution from the increase in GHG forcing, and perhaps a contribution from internal climate variation as well.

    Current models are not good enough to dismiss one of the highest levels of solar activity in 8000 years as a mere coincidence, or to apportion attribution.

    [Response: Think about it as a simple heating function with a large heat capacity. If you increase the heating and then keep it steady, you expect a delay in the response, but than an asympotoic trend to the new warmer state. Responses get smaller as time goes on. Now in the real world the temperature response is increasing through time - this is inconsistent with solar being the dominant driving. -gavin]

  3. 103

    Re: Gavin’s response to 102

    I agree it would be asymptotic to the new warming state if it were a simple heating function, however, if that was the case for the climate, we wouldn’t need the models. The climate commitment studies show that temperature increases are significant for the first century and equilibrium sea level rise can take a millenium. We know that an aerosol cooling period interrupted the warming trend. The same feedback mechanisms that are proposed to enhance the CO2 warming are also applicable to solar forcing, including warming induced increases in methane release. Non-linear positive feedbacks from reductions in ice and snow cover can also be operative.

    If we had better sea level rise data for the whole period, we might see that the heat storage curve into the ocean had a shape that better matched the simple function approximation than the land surface data does, or we might have better information on internal climate modes that confused or delayed the temperature response.

    If climate senstivity to CO2 is eventually shown (rather than just assumed) to be close to the sensitivity to solar, I think a case can then be made that the GHG attribution should be equal or higher than the solar attribution, despite the large uncertainty in our knowledge of the increase in solar forcing. I am hopeful that breakthroughs in the physical realism of the models, such as incorporating a parameterized version of the skin effect, will allow us to get confidence in this climate sensitivity to CO2, even if we still don’t have the data needed to validate the models to the accuracy needed for longer range projections.

  4. 104
    Hank Roberts says:

    I noticed this today, in the email newsletter from the inimitable Benny Peiser: CCNet 123/06 – 15 September 2006

    EXCERPT: (1) SCIENTISTS PREDICT SOLAR DOWNTURN, GLOBAL COOLING
    New Scientist magazine, 16 September 2006 http://www.newscientist.com/unpwlogin.ns

    “… Sami Solanki and his team at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, have looked at the concentrations of carbon-14 in wood and beryllium-10 in ice as far back as back 11,000 years ago. ….”It’s a boom-bust system, and I would expect a crash soon.”
    “…. the most recent calculations by Solanki’s team suggest that the sunspot crash could lead to a cooling of the Earth’s atmosphere by 0.2 °C …. as big as the most optimistic estimate of the results of restricting greenhouse-gas emissions until 2050 in line with the Kyoto protocol.”

    “… “What might happen is that the sun gives the planet a welcome respite from the ravages of man-made climate change – though for how long, nobody knows. During the Little Ice Age, the fall in average global temperature is estimated to have been less than 1 °C and lasted 70 years. The one before that persisted for 150 years, but a minor crash at the beginning of the 19th century lasted barely 30. For now, we will have to keep watching for falling sunspot numbers. “The deeper the crash, the longer it will last,” Weiss says.

    “There is a dangerous flip side to this coin. If global warming does slow down or partially reverse with a sunspot crash, industrial polluters and reluctant nations could use it as a justification for turning their backs on pollution controls altogether, makingmatters worse in the long run. There is no room for complacency, Svalgaard warns: “If the Earth does cool during the next sunspot crash and we do nothing, when the sun’s magnetic activity returns, global warming will return with a vengeance.”

    END EXCERPT, taken from CCNet 123/06 – 15 September 2006
    [My comment: the possible cooling from the Kyoto protocol alone has long been described as trivial by opponents, and as only a bit of what's needed by proponents. It appears all agree that the sun's effect over 11,000 years looks -- like Kyoto -- tiny compared to the anthropogenic warming expected -hr]

  5. 105

    Re: #104

    It would be interesting to know what climate model and scenarios Solanki’s 0.2 degree C temperature drop is based upon. This might be being achieved in the face of significant CO2 forcing in a model with significant anti-solar bias in its surface albedo. Even with these possible issues, it buys us 50 years of economic growth and technological development and a net reduction in the heat content of the ocean, that the future warming must overcome. A richer and more technologically capable civilization may not only be able to better afford to address warming, but may also be able to do it more cheaply and insightfully.

  6. 106
    Pat Neuman says:

    Re 103

    I disagree. We don’t “know” that an aerosol cooling period interrupted the warming trend.

  7. 107

    [...] At any rate, the article tells you that ocean heat content is not decreasing. Detailed in here and more recently in here. [...]


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