— Roger A. Pielke Sr (@RogerAPielkeSr) October 23, 2013
Tom Curtis then pointed out that Roger may not have blundered in the way that I thought he had. Given that this debate appears to regularly involve people making incorrect claims that they never correct, I will try to buck that trend and acknowledge that I did indeed misinterpret Roger’s post. I’ll even apologise for the tone of my previous post.
However, I do think that it was fairly easy to misinterpret what Roger was saying. Essentially, Roger wrote the following basic equation
Radiative Imbalance = Radiative Forcing + Radiative Feedbacks.
For the period that Roger was considering, the Radiative Imbalance was 0.71 Wm-2, the Radiative Forcing was 2.29 Wm-2 and hence the Radiative Feedbacks have to be -1.58 Wm-2. Therefore, the net Radiative Feedbacks are negative.
This is where the confusion comes in. The Radiative Forcing is due to external drivers (solar, geothermal, anthropogenic). The Radiative Feedbacks are normally the system responses to the resulting temperature change (water vapour for example). What Roger has done is include the flux due to the temperature change itself as part of the Radiative Feedbacks. There’s nothing fundamentally wrong with this, but – I believe – it is non-standard. Hence, I was lead to believe that Roger was implying that the water vapour feedback, for example, was negative. I realise now that he wasn’t, but it wasn’t as obvious as maybe it should have been.
Having acknowledged the above, I’m still confused as to what Roger is implying with all this. Firstly, the temperature change included in the Radiative Feedback term is about 0.8oC. This produces an increase in surface flux of around 4.4 Wm-2 (which, according to Roger’s definition, is a negative feedback). Hence, if the net Radiative Feedback is -1.58 Wm-2, there must be other factors (water vapour for example) that produce a positive feedback of 2.82 Wm-2. So, the positive feedbacks exceed the Radiative forcing as – I believe – expected.
Another issue is that if you include the temperature change in the Radiative Feedback term then the net Radiative Feedback is always negative for anything that would be relevant to our current climate. If the net Radiative Feedback were positive that would suggest that the response to the increased temperature would be producing a larger positive feedback than the negative feedback due to the increased temperature and we would, I think, be in an irreversible runaway state. As far as I can tell, given Roger’s definition, a negative net Radiative Feedback is precisely what we would expect. In fact, when we reach equilibrium the net Radiative Feedback will exactly match the Radiative Forcing since the Radiative Imbalance will be zero.
What Roger seems to be implying is that the IPCC has ignored this whole situation. I don’t actually think this is correct. If you consider the more standard form for the energy balance equation, it is
ΔQ = ΔF – λ ΔT,
where ΔQ is the Radiative Imbalance, ΔF is the Radiative Forcing, and – λ ΔT is equivalent to Roger’s Radiative Feedback term. However, λ is normally associated with the climate sensitivity. If you consider the period 2001-2010 (relative to the 1850s) then Otto et al. (2013) give ΔQ = 0.65 Wm-2, ΔF = 1.95 Wm-2, and ΔT = 0.75oC. Solving for λ gives λ = 1.733 Wm-2 per oC which one can invert to give 0.58oC per Wm-2. A doubling of CO2 produces a Radiative Forcing of 3.7 Wm-2 and hence the equilibrium temperature after a doubling of CO2 will be 0.58 x 3.7 = 2.1oC.
This is lower than other estimates (typically around 3oC) however if you include errors in the above calculation, the range is 1.2 – 3.9oC, largely consistent with other estimates. This lower estimate is also, I believe, one of the reasons why the IPCC has reduced the lower end of its ECS range. Furthermore, there is a lot of discussion about this. It is likely due to an element of internal variability and the influences of aerosols that are acting to reduce the system heat uptake (or reduce the overall Radiative Forcing). So, I don’t believe it is correct to argue that the IPCC have not considered all these various forcings and feedbacks. They just haven’t done it the way Roger seems to have wanted them to do it.
Anyway, the main point of this post was to acknowledge that I had mis-interpreted Roger’s post but to also point out that even though I now think I understand what he is suggesting, I’m not really sure why it’s relevant or that what he’s implying – with regards to the IPCC ignoring this – is correct. However, given that I did mis-interpret Roger’s post maybe I should consider that he is saying something important. So, if someone would like to clarify through the comments, I’m happy to be convinced.