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)
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.