Bishop Hill Thoughts on aerosols
Aug 7, 2015 Climate: Models I've been reading a bit about aerosols in recent days. As many BH readers will know, these are one of the great uncertainties in the Earth's climate and so they crop up all the time.
This interest was provoked in part by a conversation I was having with Ed Hawkins about his new paper. I had invoked Bjorn Stevens' study from which it is possible to infer a value of -0.5 Wm-2 for the overall effect of aerosols, around half of the IPCC's best estimate of -0.9 Wm-2. This of course would imply that climate sensitivity would have to be much less than the IPCC suggests it is.
Ed suggested that I was cherrypicking a single paper that gave me the answer I wanted, and I wondered what the range of estimates was. Here is Figure 7.19 from the Fifth Assessment Report (click for a slightly larger version), which shows the observational estimates from satellite measurements and the various GCM estimates ("with physics in") and how these are combined using "expert judgement" to give the best estimate of -0.9 Wm-2.
;)
We are interested in the two grey boxes on the right, which show the total of the direct and indirect effects of aerosols. In the top box, the markers with black borders are the subset selected for the expert judgement. The squares are the satellite estimates, and as you can see they are all packed into a corner, with none of them below -1 Wm-2 and some considerably closer to zero. The GCM estimates - the diamonds and stars - are all over the place, which is not surprising when you realise that the effect of aerosols is intimately bound up with clouds, which are little understood and cannot be properly modelled in a GCM.
So it is similarly unsurprising that by the time you get to the expert judgement subsets in the lower panel, there is a startling lack of overlap between observations and GCMs.
This kind of thing might have led more traditionally minded scientists to question the validity of the GCMs - the observations trumping the hypotheses as in other fields of scientific endeavour. But this being the IPCC, we get instead an "expert judgement" combining the observations with the hypotheses, in a process that seems to neatly mirror what happens on the climate sensitivity side of the equation.
This is particularly worrying in the light of Bjorn Stevens' thinking on these questions. Stevens' headline finding was that aerosol forcing values more negative than -1 Wm-2 were implausible. If he is correct, the majority of the GCM estimates of aerosol forcing would have to be junked. Stevens had several lines of reasoning for this conclusion, but one of his observations was that the temperature history of the past was quite hard to reproduce with a strong aerosol forcing. So in the first decades of the twentieth century, when there were lots of aerosols around but relatively little greenhouse gas forcing, we should not have seen any warming. But in fact that is just what we saw, with the planet warming from 1900 to 1940. In similar vein aerosols, being shortlived, should mainly effect the Northern Hemisphere where they are produced, particularly in the early part of the temperature record when the effect would be most pronounced. Again, this is not what is seen in practice.
Now undoubtedly there are issues with the satellite observations - with several adjustments required to reach a final figure. I haven't yet investigated the uncertainty bounds involved in these studies, which might, if very wide, provide some justification for what the IPCC has done. But with the satellite best estimates so tightly grouped one can't help but pick up something of a stench from Chapter 7 of the Fifth Assessment Report.
Reader Comments (59)
@Arno: Miskolczi used incorrect radiative physics, not his fault - he was taught it. The subtle difference from reality is that he assumes a radiant exitance is a real instead of a potential EM energy flux.
However, the water cycle does exactly offset intrinsic CO2 warming but by a mechanism different to what Miskolczi created. The proof of the variant physics is that it explains the hot-house earth and the Faint young Sun Paradox.
I consider that should the 'pause' continues to extend then initially, real scientists will question Climate Sensitivity and in particular whether the net feedbacks from water vapour are positive or negative. But it would not surprise me that should the 'pause' continue into 2030, some real scientists will take a second look at the radiative model, and will begin to question the very basis upon which the Greenhouse Theory is based.
Don't forget that should the 'pause' continue say to 2030 it is likely that the satellite data will by then have 50 years of data in which there was simply a one off and isolated warming event in and around the 1998 Super El Nino which event was a natural, not manmade event such that there will be 50 years of data with no signal to CO2 induced warming. During this period of time (ie through to 2030) it is probable that about 70 to 80% of all manmade emissions of CO2 will have taken place so if there is a signal to CO2 it would be surprising not to be able to see it in the satellite data.
Santer famously said that the theory should be questioned after 17 years with no warming. I do consider that should the 'pause' continue sooner or later real scientists will pick up on this. Interesting times may well lie ahead.
Arno
You need to read more closely. An atmosphere, not our atmosphere. Secondly, you confuse Greenhouse Effect with Global Warming. Then there is your more technical post...
Bush, not that your "cherry pie" does not taste good, but Ed needs to answer why does IPCC or any other coterie of cooks' cherry baking capabilities are superior. It seems to me that what Ed is "admitting" is that the state of climate science is not very much better than figuring out whether the puffy clouds above look like a dragon, serpent or a chicken.
Ssat: it is not that I do not wish there to be a greenhouse effect, it is just that I do not consider that what we are being told IS the greenhouse effect is actually a, er, greenhouse effect. Surely, if it were, then the atmosphere inside a CO2-enriched greenhouse should be noticeably warmer than in one with a normal atmosphere. The more I read and hear about it, the more convinced I become that it is the modern equivalent to phlogiston – i.e. a rather cack-handed explanation for an observed effect.
As to who I am, Arno Arrak: I am but an inquisitive mouse with the stubbornness of a rat, with the tongue of a shrew and the girth of a capybara, who happily squirrels away whatever information I can find. I no longer compare my beauty to that of a shaven beaver, as it seems to get people giggling.
@RR: there can't be two of us.......:0)
What odd logic, Mr 1701E. Of course there can be; you are you and I am me – there are two of us. In fact, should you look a bit further, you might find that there are considerably more than two of us… unless they (and possibly you, too) are mere constructs of someone’s imagination – or, to get even more philosophical, my own imagination. Mind you, I don’t really hold with that philosophy, as why would I create my own world where I can so easily be made to look stupid?
Hush, now. You will get us chastised by the Bishop for wandering too far off-topic.
RR
The GE is only the difference between measured temperatures and those that could (not should) be expected. The mechanism for the GE is where the problem lies with 'what you are told'. You are right to take nobody's word for that:)
On Aug 10, 2015 at 7:42 AM Commenter NCC 1701E says:
"@Arno: Miskolczi used incorrect radiative physics, not his fault - he was taught it. The subtle difference from reality is that he assumes a radiant exitance is a real instead of a potential EM energy flux. However, the water cycle does exactly offset intrinsic CO2 warming but by a mechanism different to what Miskolczi created. The proof of the variant physics is that it explains the hot-house earth and the Faint young Sun Paradox."
Let me understand this correctly. You say that Miskolczi used incorrect radiative physics but you don't hold it against him because he was taught that. Well, that is big of you. And then you go on and admit that despite his incorrect radiative physics he did get the water cycle right but your variant physics additionally explains the hot-house earth and the faint young sun paradox as well. Using incorrect physics to get the correct answer is quite a feat, don't you think? There must be a physical explanation for it, such as being only little bit wrong, like Newton and Mercury perhaps. Is any of this your work, may I ask? If so, have you published? The latter two feats should get front page treatment in Nature or in Science if true. To my knowledge there are no peer-reviewed articles criticizing any of Miskolczi's work and now comes your anonymous allegation. Why don't you stop hiding behind that silly pseudonym stuff and talk like a man? I have never hidden myself because I don't see the point of it.