A lower climate sensitivity from Ring et al.

I was wondering if anyone had any particular insights into the Ring et al. (2012) paper Causes of the Global Warming Observed since the 19th Century? The study use a simple climate model (SCM) to model the change in global surface temperature since 1850. The model seems to have 3 parameters, ΔT2x – the change in global-mean, equilibrium near-surface temperature for a radiative forcing equivalent to a doubling of the pre-industrial CO2 concentration – FA – The aerosol radiative forcing in reference year 2000 – and κ – the ocean thermal diffusivity.

The paper is interesting for two reasons. One is that it concludes that

Our findings have confirmed that human emissions are the main cause of the global warm- ing over the past 150 years. Since human emissions are the cause of the global warming, reducing emissions will reduce the amount of warming in the future.

The other is that it estimates the equilibrium climate sensitivity to be quite a bit lower than other estimates. This is shown in the table below

Table 1 from Ring et al. (2012)

Table 1 from Ring et al. (2012)


As far as I understand it, the paper uses estimates for the various forcings to model the change in global surface temperature and then varies the three parameters above so as to get the best fit to the various different surface temperature datasets. Although the paper claims that ΔT2x is the equilibrium climate sensitivity (ECS), it’s not obvious how this is implemented in this model. As far as I understand it, one would normally need to run a model until an equilibrium is reached, or use the current radiative imbalance to estimate the ECS. The paper seems to make no explicit mention of the radiative imbalance, although it does say

The ocean thermal diffusivity, κ, is estimated using the observed upper ocean heat uptake. For the temperature comparisons, we consider the four different instrumental temperature records mentioned in Section 1. The simulated upper ocean heat uptake is compared to [15].

where [15] is Levitus et al. (2010). I’m not sure what the upper ocean is defined as. If it’s only the upper 700m, then that might imply that this study is underestimating the radiative imbalance. Having said that, the results from this paper are not that dis-similar to the Otto et al’s (2013) observationally-based estimates. Otto et al., however, estimated that the ECS was closer to 2.4oC when the influence of anthropogenic aerosols was included.

The Ring et al. paper has already been criticised by Kevin Trenberth and John Fasullo, who say

“[Schlesinger’s] numbers have no sound or physical basis,” Trenberth said. “The problem is the paper uses a very simple model, one that has no hydrological cycle, and one where the ocean structure is fixed.”

Fasullo added: “Crude models such as the ones used in the [Schlesinger] study …. should not be used as a surrogate for GCMs as they are by their very nature simplistic and small changes in their basic assumptions can yield widely varying results.”

They also comment on the single-study syndrome, which is also a valid issue. Clearly, however, a lower ECS would be a good thing, but there are very few studies that support such a possibility. I, however, don’t understand – well enough – what Ring et al. have done to really understand why they’re getting a lower ECS than other studies would suggest. Hence, I was wondering if any of my regular commenters had any insights into this paper.

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13 Responses to A lower climate sensitivity from Ring et al.

  1. Given the journal it is published in, and who the publishing group is, it is most likely crap. Google Scientific Research Publishing, they are notorious. From Wiki:

    “The company generated controversy in 2010 when it was found that its journals duplicated papers which had already been published elsewhere, without notification of or permission from the original author.[6] Several of these publications have subsequently been retracted.[7] Some of the journals had listed academics on their editorial boards without their permission or even knowledge, sometimes in fields very different from their own.[8] In 2012, one of its journals, Advances in Pure Mathematics, accepted a paper written by a random text generator. However, the paper was not published, due to its author’s unwillingness to pay the publication fee.[9] The company has also been noted for the many unsolicited bulk emails it sends to academics about its journals.[1][8]”

    Or you can look at the Journal of Improbable Research.

    On the scientific side, it seems as though it would suffer from the same problems as the spate of recent papers using similar techniques, especially because this one seems to depend heavily on FA.

  2. I didn’t consider the journal, but thought the authors were quite reputable.

    I did notice the strong dependence of FA and that does seem to be one factor that is still quite uncertain. I also wondered if they were representing the radiative imbalance properly. If they’re only using the upper ocean, then presumably they’re over-estimating what fraction of the energy excess if available to heat the surface and hence would – I think – get a lower ECS than if they’d considered the full ocean heat content when constraining κ

  3. It may have been published elsewhere, I did recognize Schlesinger and wondered: what the heck is doing publishing in one of these journals? Perhaps purloined from elsewhere?

  4. BBD says:

    First, I’m sure Trenberth’s critique of the overly-simple ocean structure(1) in the SCM is valid, which would impact the ECS estimate. Second, R12 assumes zero volcanic forcing from 1999:

    We assume the volcanic forcing post-1999 is zero, as no eruptions of a scale that altered global climate occurred in the 2000s.

    According to others () that is wrong(2). Presumably this too will bias the estimate of S low. It’s probably a collection of simplifications and assumptions of this type that have pulled the estimate of S down.

    (1)

    The SCM calculates the changes in the temperatures of the surface air and ocean, the latter as a function of depth. The ocean is subdivided vertically into 40 layers, with the uppermost being the 67.7-meter-deep mixed layer and the deeper layers each being 100 m thick. The ocean is subdivided horizontally into a polar region where bot- tom water is formed, and a nonpolar region where there is upwelling. In the nonpolar region, heat is transported toward the surface by upwelling and downwards by physical processes whose effects are treated as an equivalent diffusion. Heat is also removed from the mixed layer in the nonpolar region by a transport to the polar region and downwelling toward the bottom, this heat being ultimately transported upward from the ocean floor in the nonpolar region.

    (2)
    Neely et al. (2013) Recent anthropogenic increases in SO2 from Asia have minimal impact on stratospheric aerosols

    http://onlinelibrary.wiley.com/doi/10.1002/grl.50263/abstract

    Vernier et al. (2011) Major influence of tropical volcanic eruptions on the stratospheric aerosol layer during the last decade

    http://www.agu.org/pubs/crossref/2011/2011GL047563.shtml

    Solomon et al. (2011) The Persistently Variable “Background” Stratospheric Aerosol Layer and Global Climate Change

    http://www.sciencemag.org/content/333/6044/866.abstract

  5. I didn’t read this in detail, but I think the SCM just amounts to a sophisticated curve-fitting. That’s about what Trenberth is saying.

  6. William, that may well be the issue. It seems that they just vary the 3 parameters until they match the observed surface warming. There is some mention of matching the ocean heat uptake in the upper ocean but, apart from that, no other mention (that I can see) of trying to match any of the other observations. However, they do seem to use the forcings so seems a little more sophisticated than simple curve fitting but I’m not quite sure in what way.

  7. BBD says:

    Karsten may feel that the treatment of aerosol forcings (all species) is not quite the thing…

  8. Well, I tend to think the other way around. Given that the best estimate for the current aerosol forcing is -0.8W/m2 (with respect to 1850), their method fails to reproduce that number in case of GISS and CRU4. That’s even more obvious if I were to look at the 1944-1976 cooling period in which case GISS and CRU4 produce almost no aerosol cooling. Extremely unlikely! Haven’t bothered to check their method (as it seems to be hidden in some DPhil Thesis), but stochastic noise over three decades in the order of +0.3K is a remarkably strange result. Seems this Singular Spectrum Analysis doesn’t care about phsyics too much ;). While they do make some interesting assumptions regarding the aerosol forcing, their estimate (which is then the starting point for their calculation) is too low. No idea why they made this assumption. After all, nothing particularly exiting. ECS from such studies is useless anyways and TCR is not provided.

  9. Thanks, Karsten. I was a little surprised that they didn’t discuss the TCR as it should have been straightforward to have determined using their method (I think). For a while I thought maybe they’d confused the ECS and TCR but that would seem unlikely given their credentials.

  10. BBD says:

    And there he is. Thanks, as ever, for the clarification.

  11. BBD says:

    And I cross with Wotts, as usual…

  12. dana1981 says:

    I could be wrong, but I think this is one of those papers where they actually calculate effective sensitivity and then assume it’s the same as equilibrium sensitivity, which is only true if certain conditions are true (i.e. feedbacks are constant over time). Moreover, there are really large uncertainties in the aerosol forcing and in ocean heat content. Levitus underestimates OHC by about 50% according to Balmaseda & Trenberth and AR5, for example. Hence Andrew Dessler has argued that such approaches simply can’t give us a precise estimate of equilibrium sensitivity.

  13. Thanks Dana. I did wonder if there was an issue with how they were defining/determining what they call the ECS. They don’t really explain how the model ensures that what they’ve called T2x is actually an equilibrium temperature. Andrew Dessler’s video is indeed very good.

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