I thought I might write something about the recent paper by Drew Shindell, called Inhomogeneous forcing and transient climate sensitivity. The basic idea of the paper is to understand how an estimate of the transient climate sensitivity is influenced by inhomogeneous forcings.
The basic result of the paper is that the surface of the Earth is not the same in both hemispheres and the excess of land in the northern hemisphere (NH) extratropics (latitudes above 28 degrees North) means that this region has a greater temperature sensitivity to changes in forcing than other regions. Another result of the paper is that the aerosol, ozone, and land-use forcings are not uniformly distributed. In fact, they have a larger effect (negative) in the NH extratropics than elsewhere. This means that the global temperature response to a net change in global forcing is smaller than it would be were the aerosol, ozone and land-use forcings uniformly distributed.
This is probably best illustrated in this figure from the paper. It shows the global mean temperature change due to anthropogenic forcings when the aerosols, ozone and land-use forcings are uniformly distributed (dashed line) and when they’re inhomogeneous (solid line). The main point is that, globally, the change in forcing is the same, but the temperature response is different. However, as time progresses, the difference gets smaller because we expect the influence of aerosols and ozone to reduce with time. So, this inhomogeneity can change the temperature response during the course of the evolution of the anthropogenic forcing, but doesn’t particularly influence the transient climate response (TCR), because as we approach the point at which CO2 has doubled, the influence of this inhomogeneity gets smaller.
The influence of this homogeneity does, however, impact our ability to determine the TCR using the instrumental record – often referred to as energy budget constraints. A way of doing so is to use the following equation,
where ΔT is the observed change in temperature (0.68o averaged over 1990 – 2009 relative to 1860 – 1879), F2x is the change in forcing due to a doubling of CO2 (3.7 Wm-2), ΔF is the change in forcing due to anthropogenic GHG emissions (2.47 Wm-2), and ΔFa+ΔFoz+ΔFLU are the forcings due to aerosols, ozone and land-use (-0.64 Wm-2). In the estimate of the latter forcings, Shindell (2014) reduced ΔFa from -1.2 Wm-2 to -0.9Wm-2 to be more to be consistent with the AR5 best estimate.
In the above equation E is some factor that takes into consideration inhomogeneities in the aerosol, ozone and land-use forcings. If E = 1, then they’re assumed to be uniform. If we assume E=1 and solve the equation using the numbers above, then we get TCR = 1.37oC which is consistent with the estimates from Otto et al. (2013) which assumed that these forcings were uniform. The Shindell (2014) analysis, however, suggests that they aren’t uniform, and – to compensate – should use E=1.53, which gives TCR=1.7oC.
So, the main point of the paper is simply to point out that inhomogeneities in the distribution of aerosol, ozone and land-use forcings can result in energy budgets estimates (which use the instrumental record) under-estimating the TCR. I’ve ignored uncertainties in the forcings, but that doesn’t change the basic picture.
One of the reasons I thought I might write this is that Shindell (2014) has been strongly criticised by Nic Lewis on Climate Audit (Archived here). In this criticism, Nic Lewis says
The extensive adjustments made by Shindell to the data he uses are a source of concern. One of those adjustments is to add +0.3 W/m² to the figures used for model aerosol forcing to bring the estimated model aerosol forcing into line with the AR5 best estimate of -0.9 W/m².
What’s wrong with this? He was simply adjusting the aerosol forcing to be consistent with the AR5 best estimate. Furthermore, this adjustment was only applied to Nic Lewis’s favoured energy budget method. If Shindell (2014) hadn’t made this adjustment the energy budget estimate for the TCR using uniform forcings would have been even higher.
Nic Lewis continues with
In any event, since the AR5 uncertainty range for aerosol forcing is very wide (5–95% range: -1.9 to -0.1 W/m²), the sensitivity of Shindell’s TCR estimate to the aerosol forcing bias adjustment is such that the true uncertainty of Shindell’s TCR range must be huge – so large as to make his estimate worthless.
Well, yes, this is partly what the paper is trying to illustrate. The estimates in Shindell (2014) that Nic Lewis is commenting on here are estimates done using the energy budget method that Nic Lewis claims is the most robust and reliable of all methods. And yet, Nic Lewis has just claimed that the large uncertainty in aerosol forcing makes estimates using this method worthless. Does he realise that he’s just said this? If anything, most others think this is a bit extreme. The general view, as I see it, is that the energy budget method may be useful for estimating the TCR and for decadal predictions, but is likely worthless for estimating the ECS.
Nic Lewis also notes
I would note (without criticising it) that Drew Shindell is arguing against the findings of the Otto et al (2013) study,[ii] of which he and myself were two of the authors.
Indeed, it should be without criticism. Arguing against the findings of a paper on which you are an author is an entirely reasonable thing to do, especially if your more recent work highlights an issue with the earlier work. In fact, the main issue I have with Nic Lewis is that he seems completely unwilling to consider issues with his own papers.
Nic Lewis finishes with
In the light of the analyses of the characteristics of the models used in Shindell’s analysis, as outlined above, combined with the evidence that Shindell’s aerosol forcing bias-adjustment is very likely understated and that his results’ sensitivity to it makes his TCR estimate far more uncertain than claimed, it is difficult to see that any weight should be put on Shindell’s findings.
In a sense this is what Shindell (2014) was trying to illustrate. Energy budget estimates are unreliable because of the large unertainty in aerosol forcing and because it’s tricky to compensate for inhomogeneities in the forcings. I’m don’t think they’re worthless, but they certainly have issues that mean one should be careful in how such estimates are interpreted. Does Nic Lewis really know what he’s saying? Did he actually read the paper?
So, in my view the Shindell (2014) paper illustrates something interesting about how inhomogeneities in forcings can result in different temperature responses for the same change in global forcing and hence make energy budget estimates unreliable (unless this is compensated for). Nic Lewis, in his criticism, appears to agree with this, but it’s not clear that he actually realises that he’s done so. Interesting nonetheless.
Accordingly, LGM and earlier paleoclimate studies cannot reliably estimate ECS, one of the points made both in AR5 and in the Lewis & Crok report. That hasn’t prevented the folk at SKS attacking us for saying so, and claiming that paleo studies support an equilibrium climate sensitivity between about 2 and 4.5°C.
See my strong criticisms of the Shindell paper at:
So a criticism by SkS folk is an attack, while Nic Lewis’s critique of the Shindell paper is just strong criticism? Having read them both, it seems Nic Lewis has rather an odd way of distinguishing between an attack and a strong criticism.