## Forcings and feedbacks

William Connolley had a recent post about making stuff up, that discussed the linear relationship between temperature change and change in forcing (ΔT = λΔF). I commented that I had thought that this linear relationship was simply an approximation based on the blackbody flux equation

where ε is the emissivity of the atmosphere (typically around 0.6) and σ is the Stefan-Boltzmann constant (5.67 x 10-8Wm-2K-4).

William Connolley correctly pointed out that this typically ignores feedbacks, so probably isn’t quite correct. However, I think that there are still some interesting things one can infer from this. For example, the coefficient 4εσT3 gives the climate sensitivity in the absence of feedbacks. If we use T=288 K, then 4εσT3 = 3.25Wm-2K-1. In the absence of feedbacks, a doubling of CO2 results in a change in adjusted forcing of 3.7Wm-2 which, using the value just calculated, would result in a rise in surface temperature of 3.7/3.25 = 1.1 K.

So, why am I writing this? Well, I noticed that Judith Curry had a recent guest post (by Steve McGee) called Seasonal radiative response. In this post, Steve McGee considers the seasonal variation in average global surface temperature and the associated variation in outgoing long-wavelength flux, to then produce the figure below.

Outgoing long-wavelength flux against average surface temperature (credit : Judith Curry, Steve McGee)

What Steve McGee then seems to do is to use this to estimate the climate sensitivity. What I think Steve McGee does is to use that a doubling of CO2 produces a change in forcing of 3.7Wm-2, to then determine the corresponding change in temperature. What his analysis suggests is that the error range indicated ( 1.6oC to 4.6oC ) is that of all the monthly data considered. The range of annual observed seasonal correlations is from 2.0oC to 2.7oC. The most frequent correlated response for the CFSR years was 2.5oC.

One might think that this all sounds quite reasonable as it is quite similar to other estimates of the climate sensitivity. I was, however, initially a little surprised (given what I did at the beginning of this post) that it wasn’t something closer to 1oC per 3.7Wm-2, rather than in excess of 2oC per 3.7Wm-2. If the emissivity isn’t changing much, then surely what I did at the beginning of this post would be a reasonable representation of how the outgoing flux should vary with variations in temperature. The reason for the difference is, I think, quite straightforward and is explained quite nicely in this Science of Doom post (H/T Pekka Pirila). It’s also essentially explained in Steve McGee’s post, but he doesn’t appear to realise it.

I don’t think that Steve McGee has really found a way to estimate climate sensitivity. What he’s actually illustrating – I believe – is the feedback due to water vapour. If you consider the figure below (from Science of Doom, taken from Ramanathan and Inamdar), during the course of a year not only does the average surface temperature vary, but so does the radiative forcing due to water vapour. The atmospheric water vapour concentration depends on temperature and increases with increasing temperature. The reason Steve McGee got more than 2oC per 3.7Wm-2 (rather than around 1oC per 3.7Wm-2) is because the emissivity (ε) is essentially increasing with temperature (i.e., the outgoing long-wavelength flux increases more slowly with temperature than it would if the emissivity remained constant).

credit : Science of Doom, Ramanathan and Inamdar

If one considers the top and middle panels in the figure above, then a 4.5oC increase in surface temperature produces about a 14Wm-2 increase in radiative forcing due to water vapour (i.e., around 3.1Wm-2 per oC). Therefore, if a doubling of CO2 produces – in the absence of feedbacks – a change in forcing of 3.7Wm-2 and an increase surface temperature of around 1oC, the associated increase in water vapour should then produce an additional increase in forcing of around 3Wm-2 and, consequently, a further increase in surface temperature. Or course, this will lead to additional water vapour and other feedbacks, but you probably get the point. It is essentially why many think that the Charney sensitivity can’t be below 2oC.

So, maybe I’m wrong and Steve McGee is correct that one can use seasonal variations to estimate the climate sensitivity, but I think all he’s really done is confused the seasonal variation in water vapour forcing (which is more representative of a feedback than a forcing) with the climate sensitivity. Given the Science of Doom post, it would seem that this is something that is already quite well understood and so I find it a little surprising that Judith would post this without comment. There may, however, be subtleties that I don’t quite get, so more than happy to have them clarified in the comments.

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### 112 Responses to Forcings and feedbacks

1. With posts like this no wonder people are starting to think you’re a sceptic.
All you need do now to become a true sceptic is to consider the possibility of negative feedbacks.

2. ScotScep, depends what you mean by the term “sceptic”. Sure, one can consider the possibility of negative feedbacks but there are some issues with assuming that they can be large enough to reduce the climate sensitivity significantly. Firstly, energy-budget estimates already show evidence for positive feedbacks that are comparable in magnitude to the anthropogenic forcings. Hence if there are significant negative feedbacks, where are they?

Also, as BBD was trying – I think – to get you to discuss, if there are strong negative feedbacks that largely negate positive external forcings, we’d likely still be in a snowball Earth.

So, yes I have considered the possibility for negative feedbacks and although some must exists I can’t see how they could act to reduce the climate sensitivity much below IPCC estimates.

3. Rachel says:

I don’t understand this post, sorry. Specifically, I don’t understand your objection to Steve McGee’s method. Shouldn’t calculations of climate sensitivity take into account the influence of water vapour? (or having read science of doom’s post, perhaps this should be rephrased to the influence of temperature on water vapour) I understand that water vapour is a feedback rather than a forcing but why is it wrong to dismiss this as a method to calculate climate sensitivity? Probably I have to give this some more thought….

4. Rachel, to be honest, I have worried that maybe I’m wrong or that there is a subtlety that I’m missing. Maybe in some sense his post does have a point.

Let me see if I can explain my thinking again. If there was no feedback associated with the seasonal variation in temperature then we should see a change in outgoing flux of around 3.7Wm-2 for every degree change in surface temperature. This would really just be the blackbody response to the change in temperature.

However, there is a change in water vapour concentration that reduces the outgoing flux (relative to there being no change in water vapour) so that a 1 degree change in temperature only produces about a 1.8Wm-2 change in outgoing flux. So, to me at least, that seems to be illustrating the feedback effect of water vapour, rather than the climate sensitivity.

In a sense it seems like he’s using it the wrong way around (i.e., he’s measuring the change in forcing due to a temperature change, rather than the change in temperature due to a change in forcing). Also, there’s no sense that the change in outgoing flux represents an equilibrium. It’s really just the change due to a combination of a change in temperature and a corresponding change in water vapour concentration. So, I still think he’s really illustrating the feedback response, rather than the climate sensitivity.

I should also have added what the Science of Doom post said. To actually determine the feedback due to water vapour you should really consider a climatically relevant time interval (decades) to see how the water vapour forcing has changed due to a long term change in surface temperature. The analysis here is probably relevant but is not definitively the feedback influence of water vapour.

Anders

It’s really just the change due to a combination of a change in temperature and a corresponding change in water vapour concentration. So, I still think he’s really illustrating the feedback response, rather than the climate sensitivity.

But isn’t temperature change + feedback response essentially what constitutes climate sensitivity? Or maybe I’m misunderstanding your argument somehow (not unlikely).

That’s not to say I’m convinced by Steve McGee’s method. It seems to me to be very unlikely you can make any meaningful estimate of climate sensitivity over such a small timescale and once you factor in slower feedback responses which would not be captured by his method you would get an unrealistically high number. Still, even if can merely be used to demonstrate the existence of water vapour feedback that would be useful, given that some “skeptics” refuse to accept that it exists at all.

6. Andrew,

But isn’t temperature change + feedback response essentially what constitutes climate sensitivity? Or maybe I’m misunderstanding your argument somehow (not unlikely).

In a sense, yes, and in thinking about this a little more, I may not have explained this as well as I could have. Fundamentally, I think the problem is that we don’t know that this temperature change here is associated with reaching some kind of equilibrium, hence we don’t know if the relationship between temperature and flux in the graph is representative of an equilibrium response. In fact, I suspect one would argue that it almost certainly isn’t – the timescales are too short. Hence, I think this analysis is more illustrating the feedback response of water vapour, than some kind of climate sensitivity (maybe it’s related to the TCR, but I think the timescales are even too short for that and there are lots of hemispheric variations that one would need to take into account, I think). As you say, though, this is interesting in itself and does indeed seem to indicate that one would expect water vapour feedback to provide a radiative forcing that is comparable in magnitude to the change in anthropogenic forcing.

Anders,

In that case I think we completely agree 😉

8. Rachel says:

You’re very good at explaining things, AndFizz. Very clear and easy to follow. This is one of the reasons I like your blog so much.

9. BBD says:

What ATTP says just above, plus a nice demo that WV is a *positive* feedback to increasing temperature – take note, Scottish.

10. > With posts like this no wonder people are starting to think you’re a sceptic.

For that Anders would need to be a true Scot, Scottish One:

http://en.wikipedia.org/wiki/No_true_Scotsman

11. Rachel, thanks.

BBD, I apologise for finally giving away what you were trying to illustrate with your questions to ScotScep many days/weeks ago. I thought it best to finally point out what you were trying to get ScotScep to recognise. Hope I got it right 🙂

12. BBD says:

No problem with that – I think I was wasting pixels anyway…

😉

13. RB says:

I think it shows that there is positive feedback due to water vapor, but I don’t see how you can estimate sensitivity when you don’t know how much heat is going into the ocean (i.e, feedbacks with lag time much greater than a month).

14. RB,

Yes, that would certainly seem to be the case.

15. andthentheresphysics: “Hence if there are significant negative feedbacks, where are they?”

Go outside look up into the sky … you see those fluffy things, now recall those glorious summer mornings which … cloud over as the day goes on and cool down. This is because increasing temperature causes thermals which in turn create clouds which in turn blocks out the sun.

And the proof that these negative feedbacks exist … if they didn’t we’d have fried 100s of times in the past and we simply wouldn’t be here to talk about this.

And the other thing which you’ve not been told is that the warmer it gets, the higher the negative feedback. As such it is relatively easy to go into an ice-age but it all but impossible to get much warmer than we are now. But then I have the advantage because I’ve actually designed temperature control systems and know what I’m talking about.

16. ScotScep,
I’m going to try not to respond in kind. Yes, clouds provide a negative forcing (cooling). However, feedback in this context means in response to a change in forcing, essentially the feedback response to an increase in anthropogenic forcing. According to the IPCC, clouds are providing a very weak positive feedback (measured relative to 1750, I believe).

And the other thing which you’ve not been told is that the warmer it gets, the higher the negative feedback.

Really? Care to provide some evidence? I don’t think you’re correct – at least not in terms of the total feedback. A basic energy balance calculation would show you that the net feedback is positive.

But then I have the advantage because I’ve actually designed temperature control systems and know what I’m talking about.

You may well know what you’re talking about when it comes to temperature control systems. If you can show how temperature control systems are analogous to our climate, maybe your experience would be relevant. Otherwise, not so much.

17. BBD says:

And the other thing which you’ve not been told is that the warmer it gets, the higher the negative feedback.

So the PETM never happened.

Scottish, this isn’t going to go well, so I advise some careful thought (and reading) on your part before trying this one one here again.

18. BG says:

And the additional negative feedbacks other than clouds are…? And are the sum of all the negative feedbacks greater than the sum of all the positive feedbacks?

19. BBD says:

Hansen et al. (2013) on water vapour feedback at higher global average temperatures:

We use a global model, simplified to essential processes, to investigate state dependence of climate sensitivity, finding an increased sensitivity towards warmer climates, as low cloud cover is diminished and increased water vapour elevates the tropopause. Burning all fossil fuels, we conclude, would make most of the planet uninhabitable by humans, thus calling into question strategies that emphasize adaptation to climate change.

20. pbjamm says:

Anders: “to be honest, I have worried that maybe I’m wrong or that there is a subtlety that I’m missing”
Which is why you are a true skeptic and not a true believer. You never hear such things from contrarians. They are typically far too sure of themselves.

21. Tom Curtis says:

Anders,
McGee does not indicate that he uses clear sky data only. If he has used all sky data, then the data included the change in outgoing IR radiation due to any changes in cloud response as well as those due to changes in humidity. As such he is not measuring either the climate sensitivity or the Water Vapour feedback. Rather he is measuring the combined effect of the Water Vapour feedback PLUS the Lapse Rate feedback PLUS that part of the cloud feedback which is due to changes in IR radiation.

To include all very fast feedbacks McGee would need to include the change in outgoing short wave radiation. That may be negative even with a net positive cloud feedback. It may also be positive with increased WV potentially resulting in faster precipitation of water resulting in larger cloud droplets (which reduces cloud albedo), or in thinner clouds. While the short wave only cloud feedback may be positive, I certainly would not assume that it is.

On the other hand, the method clearly leaves out the slower acting feedbacks such as ice and snow albedo; and the further response of the WV + LR + CIR feedbacks to those slower feedbacks. Given enough data, and it is not at all obvious that 31 years is enough data given noise levels, the acceleration of the LW plus SW response relative to time would give a method of estimating the relative strengths of the the slow to the rapid responses. Indeed, if the relative response for all intervals greater than a certain number of years was effectively constant, then restricting the data to intervals greater than x years would, if using IR plus SW data, give an estimate of the ECS.

For periods less than 30 years, the global mean temperature is dominated by essentially regional or hemispheric variations in temperature, such as those induced by ENSO (regional) or volcanoes (often hemispheric). Consequently the signal to noise ratio will be very low for most of the data comparisons used. As there are far more short comparisons (less than 5 year interval) than long comparisons (>20 years) in the data set, that is of particular concern. The ratio of short to long comparisons also greatly restricts any traces of slow feedbacks that may be found in the data.

22. Tom,
I’ve just had a very pleasant evening with the neighbours, which was very convivial and included the emptying of a few bottles of wine and a couple of bottles of proseco. I think I will need to read your comment again in the morning to fully appreciate what you are saying 🙂

23. Tom Curtis says:

Anders, my excuse for not commenting last night was that I was so tired I couldn’t make head nor tail of anything. That may be my excuse again for this morning once your well rested and sober 😉

Good to hear you had a pleasant evening.

24. Arthur Smith says:

In case this isn’t obvious (reflecting on Tom Curtis’ comment above), the basic condition for equilibrium in the climate is (averaged over suitably long periods to remove “weather” effects) that outgoing radiation be equal to incoming. Starting from a position of equilibrium and adding a forcing of magnitude x, say, means we are out of balance by that much. If the forcing is from a greenhouse gas, that means outgoing long-wave radiation has been reduced by an amount x (if it was from a change in the sun or aerosols etc it would be a change in short-wave rather than long-wave radiation). So – assuming short-wave radiation doesn’t change (most of Tom’s caveats were along those lines), the response of the system to bring things back into equilibrium has to be a temperature change that adds that same quantity x to outgoing long-wave radiation.

So under those assumptions, the graph really is a way of getting at equilibrium sensitivity.

25. Tom Curtis says:

Arthur Smith, I don’t think I would characterize what I said as caveats. The fact is that, except if by extraordinary coincidence there is a increase in cloud albedo equal and opposite to the reduction in snow and ice albedo from a warming climate, the short wave radiation will change. Further, because the snow/ice albedo feedback takes longer to reach equilibrium than the cloud albedo feedback (all else being equal), the balance of the change in shortwave radiation will change over time. I think given sufficient duration of observations and sufficient stability of forcings, that change over time could be used to set limits to the equilibrium response – but I don’t think McGee’s analysis occurs over sufficient time; and nor does he use the mathematical tools to tease out the information, even if it does. Consequently I think his analysis is an analysis of the strength of a compound feedback response. Treated as an estimate of climate sensitivity, it is an under estimate by an unknown amount.

26. Measuring climate sensitivity by looking at the OLR (outgoing longwave radiation) response to changes in temperature due to “natural” fluctuations or “forcings” like volcanoes (something of an arbitrary distinction) seems to have two basic shortcomings.

Before mentioning those, everyone who first hears about **measurements** of climate sensitivity has the reaction “of course, why didn’t anyone do that before.. crazy climate scientists using GCMs instead of real experiments..”

But no surprise, except to readers of particular blogs, climate scientists have been attempting to **measure** climate sensitivity ever since we had measurements of OLR. This is since the 80’s, satellite measurements having been in place since about 1979. Good measurements came with ERBE in the late 80’s for a limited period. Much better measurements came with CERES and AIRS in the early 2000’s.

What are the two problems?

First, as posed in Measuring Climate Sensitivity – Part One:

Can we measure the top of atmosphere (TOA) radiative changes and the surface temperature changes and derive the “climate sensitivity” from the relationship between the two parameters?

The answer from this and the subsequent two articles was basically “no”. The reason is that put forward by the very interesting paper: Potential Biases in Feedback Diagnosis from Observational Data: A Simple Model Demonstration, Spencer & Braswell, Journal of Climate (2008). That was my opinion anyway. I think many researchers disagree, but at least the great Isaac Held who was one of the reviewers of this – or was it the subsequent Spencer & Braswell paper? – thought it should be published so the ideas could be discussed.

Second, climate consists of feedbacks on many timescales. If you look at feedback within an El Nino (say 1-5 years) it looks like the climate has positive feedback. But then the El Nino ends and it looks like negative feedback. What period do we want to measure? We can only answer this when we know the different climate processes and their timescales. If we looked at the climate feedback during the start and subsequent acceleration of an ice age, like 115 kyrs BP, we might think there was positive feedback (see Ghosts of Climates Past – Part Six – “Hypotheses Abound”. But then we look at the climate feedback during the period when the ice age ends and we might think there was negative feedback. Along with the same points on varying timescales for all the “bumpy bits” in the temperature graph along the way from 115 ky BP to the present day.

I struggle to even understand what the real question is.

Is there even such a thing as climate feedback as a constant or is it totally dependent on the particular climate state at the time in question and the time period over which we are interested?

All good questions. Well, I think so. Generally, climate science seems to have adopted the view the feedback is a value that is useful and can probably be measured with a bit more care and a bit more time.

Some papers from real climate scientists disagree so my questions are not at all radical.

27. scienceofdoom, thanks for the comment. Your site is quite often referenced here as an excellent resource. I hope I correctly represented what you were suggesting in the post I linked to.

Before mentioning those, everyone who first hears about **measurements** of climate sensitivity has the reaction “of course, why didn’t anyone do that before.. crazy climate scientists using GCMs instead of real experiments..”

I agree. It often amazes me that people will present what they think is a radical new idea without having checked whether or not it’s been considered before and whether or not the reason that noone has tried before is that it actually isn’t nearly as simple as they think it is.

Your questions are interesting and I largely agree. If I understand the second one, then it is something I too have pondered. Many here have made the point that one issue with these ‘energy-budget’ estimates for the climate sensitivity is that they don’t capture the likely non-linearities in the feedbacks which, I think, would be a similar issue to what you’re suggesting in your second comment.

28. Tom Curtis says:

Science of Doom,
1) In the graphs in your blog post, is the Noise scaled in W/m^2 or as a ratio to the forcing of the model, ie, N/S?

2) I am not sure what you mean when you say:

“If you look at feedback within an El Nino (say 1-5 years) it looks like the climate has positive feedback. But then the El Nino ends and it looks like negative feedback. What period do we want to measure? We can only answer this when we know the different climate processes and their timescales. If we looked at the climate feedback during the start and subsequent acceleration of an ice age, like 115 kyrs BP, we might think there was positive feedback (see Ghosts of Climates Past – Part Six – “Hypotheses Abound”. But then we look at the climate feedback during the period when the ice age ends and we might think there was negative feedback.”

Taking the example of glacial cycles, are you suggesting that if we treat the deglaciation as a feedback on the initial cause of the glaciation, then we get a negative feedback? Or are you saying that treated as a response to an initial positive forcing, the end of a glaciation looks like a negative feedback?

If the former, I am puzzled why we would do such a bizarre thing. Clearly in that case the “signal” being examined preceded the “response” around a hundred thousand years, which 100 thousand years have been relatively stable in temperature. (Relative, of course, to glaciations and deglaciations, not to the Holocene) More importantly, there was a clear and opposite signal to the first signal immediately preceding the deglaciation, and ockham’s razor requires we treat the response as a response to that second signal rather than the first.

If the later, I am puzzled as to what feature of the deglaciation is the basis for the claim.

29. dana1981 says:

A note on the cloud feedback – climate contrarians rely on it because it’s the only potentially large negative feedback that still has significant uncertainty. We know the water vapor feedback is strongly positive, and that’s the largest single feedback. Likewise albedo is a significant positive feedback, and we know of a few others. We don’t know of many significant negative feedbacks, but for those who want to believe climate sensitivity is low, you need one to offset those big positive feedbacks. Hence they claim clouds must be a large negative feedback. However, research has shown that at least in the short-term, the cloud feedback is weak and probably slightly positive.
http://www.skepticalscience.com/reconciling-two-cloud-feedback-papers-dessler.html

That could change in the long-term, but at the moment there’s no evidence of a strong negative feedback, which is another reason (among many) why the argument for low climate sensitivity is so weak.

30. BBD says:

It seems difficult to reconcile an event like the PETM with the argument for negative cloud feedback. The Paleocene was much warmer than the present but a large increase in atmospheric GHGs still triggered a hyperthermal. This appears to demonstrate no significant negative cloud feedback for a very substantial range of global average temperature, but perhaps I’ve missed something here.

31. BBD says:

Science of Doom

I have to admit I’m a bit confused by some of what you say, specifically the points Tom Curtis raises in his (2).

32. Dana,
Thanks, that was my impression too as to why some are promoting the possibility of clouds providing a strong negative feedback.

Tom and BBD,
You may well be reading more into scienceofdoom’s comment than I am, but I had assumed the point was simply that the climate sensitivity may depend on the actual state of the climate system, rather than being independent of the climate conditions. Seems like a reasonable thing to suggest, and would just add to our uncertainty. Not sure, however, how significant this would be though.

33. Tom Curtis says:

anders, if that is what SOD is suggesting, I am not sure why the glacial/interglacial example is used in that conditions a the start of the last glacial were approximately equivalent to those now; and the rise in temperature at the end of the last glacial definitely appears to be a positive feedback of approximately the same strength of those at its start. Further, ENSO fluctuations occur within the same overall conditions. Never-the-less, I certainly may be overinterpreting SOD’s statement. That is why I asked for clarification.

34. Tom (from December 28, 2013 at 8:58 pm),

Taking the example of glacial cycles, are you suggesting that if we treat the deglaciation as a feedback on the initial cause of the glaciation, then we get a negative feedback? Or are you saying that treated as a response to an initial positive forcing, the end of a glaciation looks like a negative feedback?

I didn’t do a good job of explaining my thought processes.

What I’m trying to demonstrate is that depending on the start and finish point we can appear to find positive feedbacks or we can appear to find negative feedbacks.

At the start of an ice age some small effect (perhaps reduced solar insolation in the high latitude NH) gets amplified by snow/ice albedo, which feeds itself and we end up with massive ice sheets and a very cold climate. Positive feedback.

Now we look at what caused the termination – let’s suppose we know what that is. Some small effect gets amplified by the same process of snow/ice albedo, but now in the opposite direction, and ice sheets melt and the globe warms – positive feedback again.

But if we measure from say 30kyrs ago (10kyrs before the LGM) to 10kyrs ago we find that growing ice sheets and cooling climate are halted by something and when we come to determine our feedback we believe we have negative feedback. It all depends on the start and end point of our measurement.

Now, this is all presupposing that don’t have a forcing and a known response we can ascribe to each process.

If – and *only* if – we know of an external forcing or a perturbation, which, via a physics based model, produces the changed climate in accord with measurements – then we might determine that climate has a “consistent” feedback. So putting forward the “consensus” (see note 1) on ice age inception and termination, where distribution of solar insolation (the “Milankovitch” hypothesis) causes, via positive feedback, both the start *and* the end of an ice age – well, this might be claimed to simply be a positive feedback. So if you already know the answer, and you know exactly how all the climate processes work via physics based models, then you can be confident of your feedbacks.

But – and here’s the bit I am trying to get across – if you simply measure from time A to time B and then say, look, at A we had a natural increase in temperature and when we examine OLR vs temperature from A to B we get X feedback – well, it’s pretty much pot luck as to whether you will determine from measurements that you have a negative or a positive feedback.

That is the consequence of lots of different processes operating over lots of different time-scales, some of which ARE positive feedback (snow/ice albedo being a nice uncontroversial one, the THC is probably another one), and some of which ARE negative feedbacks (the climate naturally acts to reduce latitudinal temperature differences for example, which will oppose the snow/ice albedo).

Hypothesis A – Given that we don’t know *all* of these processes, or whether they are positive or negative (and if we do know direction we don’t really know strength), or what timescales they actually work over, or what modes they are currently in – pot luck when it comes to *measuring* climate sensitivity from time A to time B.

Hypothesis B – On the contrary, if you work on the basis that all of the different modes of climate are known, and the forcings are known – i.e., we have a fairly full understanding of climate and its variability, then we can measure over the *right* time period to determine by measurement what we already roughly know from theory.

Hypothesis A seems more likely than Hypothesis B (to me).

Note 1 – There is not quite a consensus, I am oversimplifying for the sake of reducing this complex question into something manageable in one comment.

35. Tom (from December 28, 2013 at 10:19 pm)

..I am not sure why the glacial/interglacial example is used in that conditions at the start of the last glacial were approximately equivalent to those now; and the rise in temperature at the end of the last glacial definitely appears to be a positive feedback of approximately the same strength of those at its start..

Definitely?

I’m having trouble finding evidence for such confidence in start and end of ice ages. And all the bumpy bits in the temperature graph on the way up and down.

Just to cite a couple of examples:

“The last glacial cycle: transient simulations with an AOGCM”, Robin Smith & Jonathan Gregory, Climate Dynamics (2012):

It is generally accepted that the timing of glacials is linked to variations in solar insolation that result from the Earth’s orbit around the sun (Hays et al. 1976; Huybers and Wunsch 2005). These solar radiative anomalies must have been amplified by feedback processes within the climate system, including changes in atmospheric greenhouse gas (GHG) concentrations (Archer et al. 2000) and ice-sheet growth (Clark et al. 1999), and whilst hypotheses abound as to the details of these feedbacks, none is without its detractors and we cannot yet claim to know how the Earth system produced the climate we see recorded in numerous proxy records.

“True to Milankovitch: Glacial Inception in the New Community Climate System Model”, Jochum et al, Journal of Climate (2012):

So far, however, fully coupled, nonflux-corrected primitive equation general circulation models (GCMs) have failed to reproduce glacial inception, the cooling and increase in snow and ice cover that leads from the warm interglacials to the cold glacial periods..

..The GCMs failure to recreate glacial inception [see Otieno and Bromwich (2009) for a summary], which indicates a failure of either the GCMs or of Milankovitch’s hypothesis. Of course, if the hypothesis would be the culprit, one would have to wonder if climate is sufficiently understood to assemble a GCM in the first place.

And on the abrupt climate change that we see in the proxy records, here are some food for thought extracts from “Mechanisms of abrupt climate change of the last glacial period”, Clement & Peterson, Reviews of Geophysics (2008):

The millennial time scale is perhaps the area in which ideas on the role of the tropics in abrupt climate change are least convincing. The main reason is simply that unlike high latitudes where glacial dynamics may introduce long time scales, there are no individual components of the tropical climate system that have such a long time scale..

..While Clement and Cane did not determine the particular mechanisms in the model that give rise to the low-frequency variability, it has been shown by Lorenz that in chaotic dynamical systems in general, very-long-period fluctuations, much longer than any obvious time constants appearing in the governing laws, are capable of developing without the help of any variable external influences..

..It would be useful to have multimillennia control simulations with coupled GCMs to study such an issue, but this may not yet be possible because of the computational cost of such experiments..

..without a specific forcing, however, it is difficult to design targeted experiments with CGCMs. One possibility is to simply run the state-of-the-art models for multiple millennia with constant external forcing and see whether the models produce millennial variability similar to the observed. For the present version of the fully coupled models, this is simply not feasible because of the computational cost. Another approach is to perturb the model in some way and study the transient response. The freshwater forcing experiments that have been extensively performed are only one way of perturbing the system, and given the uncertainty about the evidence for and causes of meltwater events, this may not even be the most relevant perturbation..

I don’t see a lot of “definites” about climate fluctuations.

36. andthentheresphysics says:

You may well know what you’re talking about when it comes to temperature control systems. If you can show how temperature control systems are analogous to our climate, maybe your experience would be relevant. Otherwise, not so much.

You clearly have very little experience in real world systems otherwise you would see the relevance because a temperature control system is a feedback loop. You would also know that anyone designing real world control systems can assess the type of feedback present just by looking at the signals.

And although I would like to explain, it really is pointless unless you have done a basic course on feedbacks. But as your questions shows, you haven’t any understanding or experience of feedback systems, so I would be wasting my time trying to explain to you.

And that just about sums up the situation. Your side haven’t a clue what they are talking about because you have no real world experience of feedback systems.

37. Tom Curtis says:

Scottish Sceptic, nothing like an argument from inscrutable authority…

38. Joshua says:

“Your side haven’t a clue what they are talking about because you have no real world experience of feedback systems.”

I never get tired of reading this kind of argument from someone who calls himself a skeptic.

39. Ian Forrester says:

But our blogger is a “citizen scientist” and they know much more than a mere academic scientist. Thus they are always right with their cherry picks and fraudulent claims. By the way, “Scottish” “Sceptic” do you consider Salby a “citizen scientist” or an “academic scientist” considering that he was essentially fired from his academic position?

40. Tom Curtis says:

SOD, “definitely appears” =/= “definitely” without further qualification, ever. Out of context quotation for rhetorical effect merely weakens your case.

I would argue that your quotation of Jochum et al is also out of context. It is taken from the introduction to the article, ie, the section that describes the state of knowledge prior to the article, and then goes on to indicate how that changes as a result of the article:

“The present study is motivated by the hope that the new class of GCMs that has been developed over the last 5 years [driven to some extent by the forthcoming Fifth Assessment Report (AR5)], and incorporates the best of the climate community’s ideas, will ﬁnally allow us to
reconcile theory and GCM results. It turns out that at least one of the new GCMs is true to the Milankovitch hypothesis. The next section will describe this GCM and illustrate the impact of changing present-day insolation to the insolation of 115 000 years ago (115 kya)”

Further, and contrary to Lorentz, we know that the glacial cycles are not unforced fluctuations. If they were, then either climate sensitivity is extraordinarily large; or very large energy imbalances can be sustained over tens of thousands of years without appreciable effect on surface or deep ocean temperatures. In the first case, the climate sensitivity must be sufficiently large that 5 C fluctuations in global temperature make almost no change to the energy balance, which given other evidence is absurd. The second alternative is absurd on its face.

This straight forward argument may not, indeed should not, be sufficient to end research on the subject in the field. Such straightforward arguments have been wrong in the past, so we expect scientists to dot the “i”s and cross the “t”s. But the number of occasions when such straightforward arguments have been false in the past is small. Consequently we can be very confident that as scientists dot the “i”s and cross the “t”s, they will find that glacial cycles are driven by milankovitch cycles.

41. Tom Curtis (from December 29, 2013 at 2:38 am),

Sorry to quote you apparently for rhetorical effect. That wasn’t my intention. You seemed certain from your comment.

Now you’ve clarified your position it strangely appears you are even more certain and there is no doubt? Scientists are just dotting the i’s and crossing the t’s on a certain theory?

Taking the hypothesis that you are certain, I’m not clear where your evidence is.

Further, and contrary to Lorentz, we know that the glacial cycles are not unforced fluctuations. If they were, then either climate sensitivity is extraordinarily large; or very large energy imbalances can be sustained over tens of thousands of years without appreciable effect on surface or deep ocean temperatures. In the first case, the climate sensitivity must be sufficiently large that 5 C fluctuations in global temperature make almost no change to the energy balance, which given other evidence is absurd. The second alternative is absurd on its face.

This is just a claim based on an “argument from absurdity”. Sometimes these types of claims are true and sometimes these mean that the writer of the argument doesn’t really understand *how* such a process can occur.

The very Milankovitch hypothesis that many/most accept actually works on the basis that with zero change in incoming energy we moved from the Eemian interglacial to the LGM and then back to the present day Eemian type climate. Simply a change in the distribution of solar insolation (by latitude and season) caused this, fueled by positive feedback from snow/ice albedo changing the absorbed solar radiation and CO2 changes affecting the surface energy balance.

So in fact, climate sensitivity was sufficiently large that 5’C fluctuations in global temperatures occurred with no change in the supplied energy. (So the Milankovitch hypothesis goes). Perhaps I am rhetorically weakening my case by attempting to repeat your argument in more detail?

Anyway, it’s one thing to put forward a hypothesis, it’s another to say that all alternative hypotheses are “absurd”. This requires more than a paragraph. I look forward to the evidence.

In the meantime I provide a couple more out of context quotes to indicate that the story is not quite one of dotting the i’s and crossing the t’s but more trying to understand the physics behind the process of glacial inception and termination:

Carl Wunsch had this to say in “Quantitative estimate of the Milankovitch-forced contribution to observed Quaternary climate change”, Carl Wunsch, Quaternary Science Reviews (2004):

The long-standing question of how the slight Milankovitch forcing could possibly force such an enormous glacial–interglacial change is then answered by concluding that it does not do so..

..The appeal of explaining the glacial/interglacial cycles by way of the Milankovitch forcing is clear: it is a deterministic story..

..Evidence that Milankovitch forcing ‘‘controls’’ the records, in particular the 100 ka glacial/ interglacial, is very thin and somewhat implausible, given that most of the high frequency variability lies elsewhere. These results are not a proof of stochastic control of the Pleistocene glaciations, nor that deterministic elements are not in part a factor. But the stochastic behavior hypothesis should not be set aside arbitrarily—as it has at least as strong a foundation as does that of orbital control. There is a common view in the paleoclimate community that describing a system as ‘‘stochastic’’ is equivalent to ‘‘unexplainable’’.

Nothing could be further from the truth (e.g., Gardiner, 1985): stochastic processes have a rich physics and kinematics which can be described and understood, and even predicted.

“Glacial terminations as southern warmings without northern control”, Wolff et al, Nature Geoscience (2009):

However, the reason for the spacing and timing of interglacials, and the sequence of events at major warmings, remains obscure. In most terminations, warming in Antarctica proceeds fairly steadily over several millennia, accompanied by increasing CO2 (refs 11,12), and decreasing deepwater isotopic values (representing deepwater temperature and ice volume). For the most recent termination (TI), Greenland temperature changed little during the Antarctic warming (Fig. 1b), jumping abruptly towards interglacial levels (towards the Bolling warm period) only rather late in the termination. In other terminations, if we treat rapid changes in methane as a proxy for rapid changes in Greenland temperature, a rapid northern-warming step coincided with the completion of Antarctic warming (Fig. 2).

“Antarctic temperature at orbital timescales controlled by local summer duration”, Huybers & Denton, Nature Geoscience (2008):

Southern Hemisphere climate proxies follow northern summer insolation intensity, leading to the common interpretation that northern insolation controls southern climate, at least at the obliquity and precession timescales. Proposed mechanisms include northern insolation influencing atmospheric CO2, North Atlantic Deep Water affecting the Southern Ocean and variations in the extent of northern glaciation influencing the south.

However, such explanations are problematic because conventional statistical techniques indicate that southern changes are in phase with or lead those in the north—a situation Mercer called ‘a fly in the ointment of the Milankovitch theory’.

Although northern forcing of Antarctic climate cannot be ruled out, particularly if small or otherwise difficult-to-detect northern perturbations precede those in the south, the lack of an obvious, operative mechanism to produce symmetric interhemispheric changes at the precession period prompts further examination of the connection between Earth’s orbit and southern climate.

42. Tom Curtis (from December 29, 2013 at 2:38 am),

I would argue that your quotation of Jochum et al is also out of context. It is taken from the introduction to the article, ie, the section that describes the state of knowledge prior to the article, and then goes on to indicate how that changes as a result of the article.

Out of context means when you read the whole paragraph, page, or article you get a different idea. My quotation of Jochum et al is a quotation which is obviously a review of the work to date. That’s not out of context.

In fact, in just about every published paper on ice age modeling by GCMs there is a review of past work and past problems, successes, and contradictory results – you can see more reviews in Ghosts of Climates Past – Part Eight – GCM II – followed by “here’s our success”.

I believe a review of the state of the work, by published climate scientists, is very useful.

Still it’s just my opinion. You can clearly see my bias and out of context quoting from actual published work in Ghosts of Climates Past – Part Eight – GCM II and Ghosts of Climates Past – Part Seven – GCM I.

Well, as you say, the situation has changed now as a result of the article. I should have pointed that out. It was a long time ago in.. 2012.

And prior to 2012, everyone thought what?

43. Tom Curtis says:

SOD:

From:

“This straight forward argument may not, indeed should not, be sufficient to end research on the subject in the field. Such straightforward arguments have been wrong in the past, so we expect scientists to dot the “i”s and cross the “t”s.”

(Emphasis)
you get:

“There is no doubt”

Clearly your intention is to misrepresent what I say for rhetorical effect. I do not like being lied too, or about.

44. Tom,

Wow.

You say: “Alternative hypotheses are absurd.”

I get “there is no doubt”.

Hypothesis A: Clearly I am lying and deliberately misrepresenting you.

Is there a Hypothesis B? I wonder..

45. Tom, scienceofdoom,
I have to say that I’m slightly confused as to what’s going on here. You’re both regarded (unless I’m mistaken) as people who are very knowledgeable about this subject, so I’m a little disappointed that the two of you seem to be having a bit of a spat here and also slightly confused as to precisely what the issue is. Is it possible to take a couple of steps back and re-evaluate the discussion?

46. ScotScep,

But as your questions shows, you haven’t any understanding or experience of feedback systems, so I would be wasting my time trying to explain to you.

Firstly, I think the term feedback used in the context of climate is not quite the same as the term used in typical feedback systems. Secondly, I don’t think I asked a question. Thirdly, I appreciate you no longer wasting your time here.

47. Andthentheresphysics,

“..I’m slightly confused as to what is going on here..”

Me too. You can read Tom’s claims – I am deliberately misrepresenting him. Not much that can be done when someone takes this kind of approach.

In return I haven’t claimed that Tom’s suggestion that I quoted a paper out of context is motivated by malice or lying. I simply point out it is not correct. Seems like a more practical approach.

But I’m happy to discuss climate sensitivity with other people, especially ice ages.

48. scienceofdoom,
I’m going to have to read the relevant comments in a bit more detail. I’ve been rather busy the last couple of days, so haven’t quite managed work out what the issue is. I shall do so and respond in due course.

49. Tom Curtis says:

Anders, it is possible for me to reevaluate to situation when SOD stop misrepresenting me as indicating greater certainty when I do, particularly after I have indicated I have a problem with his so misrepresenting me; and when he drops the facade that I indicated that “alternative hypotheses are absurd” (note the plural, and note also that he places the phrase in quotation marks even though it is his phrase, not mine) when in fact I indicated that one hypotheses, the idea that glaciations could simply be unforced large scale excursions in a chaotic system falls on the horns of a dilemma.

I take the view that the first time he misrepresented me, it was a misunderstanding – but then to immediately misrepresent me again on the same point, and to justify that misrepresentation with a fake quotation? It is difficult to regard that as other than intentional.

The fact of the matter is that even when scientists have a very good idea of what has happened in a particular instance, they push at the borders of doubt. As creationists and AGW deniers have shown time and again, it is trivially easy to find expressions of ignorance in the scientific papers as a result. It is also trivially easy to not track the scope of that doubt – to inflate it to make the doubt appear to cover that which you want to consider doubtful. In the case of the cause of the glacial cycle, the connection to the milankovitch cycle is well established. It is not clear, however, of the exact mechanism of that connection; and nor has it been the case that climate models are up to the task of exploring the connection in a way that resolves the disputes about different mechanisms.

SOD takes this limited doubt and inflates it so that the very limited doubt as to whether or not the glacial cycle is consists of unforced chaotic excursions (the possibility of which is allowed by Wally Broecker in 1990) is given the same weight as the doubts about the relative importance of (for example) variation in the strength of Agulhas Current leakage in glacial/interglacial transitions. Without that inflation of doubt, the fact that the precise mechanism of the transitions is not nailed down does not justify treating glacial/interglacial transitions as anything other than the results of positive feedbacks.

50. Tom,
From the reading I’ve managed to do, before having to rush out in a few minutes, this

SOD takes this limited doubt and inflates it so that the very limited doubt as to whether or not the glacial cycle is consists of unforced chaotic excursions

was also the impression I was getting. I too find it difficult to see how we could interpret the glacial/interglacial transitions as anything other than the results of positive feedbacks. I’ll aim to make a more substantive comment when I’m home again in a few hours.

51. BBD says:

So, if there is doubt that orbital dynamics triggers deglaciation, then how do we account for the distinct 41ka (obliquity) pacing of deglaciation from ~2.8Ma – 0.7Ma and the ~100ka pacing thereafter? Certainly we can argue whether the actual 82ka – 123ka interval range since the mid-Pleistocene transition is the effect of eccentricity modulating precession or obliquity or some mix of both, but not that this is the initial cause. So we appear to have positive feedbacks to orbital forcing.

If I’m being honest, I have to admit a moderately strong bias against the “self-propelling climate” school here.

😉

52. BBD says:

53. BBD,
That’s what’s confusing me. I’m not as well read on this subject as yourself and others, but I had thought that it was quite likely that the glaciation/deglaciation cycles were triggered by variations in orbital dynamics.

There seem to be a few other issues though. If you want some unforced chaotic excursion to be the trigger, then presumably this still needs to be large enough to result in an essential change in forcing. For example. ENSO cycles can produce changes in the surface temperature that then change the outgoing flux. However, these events seem small enough that they don’t trigger additional forcings and feedbacks and the excess energy (in the case of a heating event) simply radiates – quite quickly – back into space. So, I have always regarded these as simply internal variations, rather than forcing events.

That would seem to suggest that if a chaotic excursion is to trigger something like a de-glaciation it would need to be significant enough so as to actually change the Earth’s albedo (by melting polar ice for example) which could then lead to the additional forcings and feedbacks (GHGs, more melting) required. One could probably estimate how much energy would need to be associated with such a chaotic event so as to determine if it were possible or not. Also, as far as I’m aware, there isn’t much evidence for such events.

54. Rachel says:

Can I try to phrase this discussion in very simplistic terms so that I might understand what is happening here? Ok, thanks 🙂
Science of doom tells us the Milkanovitch hypothesis is just one hypothesis which attempts to explain the cycle of glaciation/deglaciation but that it appears to be inadequate or does not give the full story. Other possible explanations can be found here.

Tom, I think, seems quite certain that the Milkanovitch theory is a theory and is fairly certain and well established.

I think it’s great that we can have differences of opinion here and I hope that this might continue without people getting upset. For what it’s worth, I don’t think science of doom is deliberately trying to misrepresent or misquote you, Tom and nor do I think he is lying to you.

55. BBD says:

What’s like, rilly, rilly interesting too are the hints that orbital forcing was implicated in much more ancient paleoclimate events, eg:

DeConto et al. (2012) Past extreme warming events linked to massive carbon release from thawing permafrost.

Lourens et al. (2005) Astronomical pacing of late Palaeocene to early Eocene global warming events

Lunt et al. 2011 A model for orbital pacing of methane hydrate destabilization during the Palaeogene.

56. BBD says:

Here are some pretty pictures of sediments formed by the precessional cycle. It’s always nice having something you can actually see when it comes to rather abstract-sounding discussions of orbital dynamics and their effects on terrestrial climate.

57. BBD,
I guess my issues with it being some kind of chaotic excursion is what is driving it? Do we really think it possible that our climate could have some kind of really slowly varying cycle? It would seem that any such slow variations would be swamped by forced variations. Also, is it actually all that relevant anyway? Even if the Milankovitch cycles are initially unforced (which I would regard as unlikely, but shouldn’t completely discount anything) what we’re experiencing now seems clearly to be forced and to be largely/completely due to anthropogenic GHGs.

58. BBD says:

ATTP

Conservation of energy always wins, as you say:

If you want some unforced chaotic excursion to be the trigger, then presumably this still needs to be large enough to result in an essential change in forcing. For example. ENSO cycles can produce changes in the surface temperature that then change the outgoing flux. However, these events seem small enough that they don’t trigger additional forcings and feedbacks and the excess energy (in the case of a heating event) simply radiates – quite quickly – back into space. So, I have always regarded these as simply internal variations, rather than forcing events.

I’m entirely in agreement with you.

59. Rachel says:

I can’t help but come to the conclusion that the pretty pictures are for my benefit, BBD. 🙂

I don’t actually have a view on this. It’s not something I really understand very well – although the controversy has heightened my interest – and it doesn’t have any bearing on whether I think we need action to mitigate climate change which is a definitive YES.

60. BBD says:

Science of Doom

I noticed that you don’t reference Shakun et al. (2012), which does more than anything else I have read to explain the mechanisms operating during deglaciation, and directly addresses questions raised in the studies you mention earlier.

I also would like to add a quote from Wunsch (well, from Huybers & Wunsch 2005) to our collection:

The 100,000-year timescale in the glacial/interglacial cycles of the late Pleistocene epoch (the past ~700,000 years) is commonly attributed to control by variations in the Earth’s orbit1. This hypothesis has inspired models that depend on the Earth’s obliquity (~ 40,000 yr; ~40 kyr), orbital eccentricity (~ 100 kyr) and precessional (~ 20 kyr) fluctuations2, 3, 4, 5, with the emphasis usually on eccentricity and precessional forcing. According to a contrasting hypothesis, the glacial cycles arise primarily because of random internal climate variability6, 7, 8. Taking these two perspectives together, there are currently more than thirty different models of the seven late-Pleistocene glacial cycles9. Here we present a statistical test of the orbital forcing hypothesis, focusing on the rapid deglaciation events known as terminations10, 11. According to our analysis, the null hypothesis that glacial terminations are independent of obliquity can be rejected at the 5% significance level, whereas the corresponding null hypotheses for eccentricity and precession cannot be rejected. The simplest inference consistent with the test results is that the ice sheets terminated every second or third obliquity cycle at times of high obliquity, similar to the original proposal by Milankovitch. We also present simple stochastic and deterministic models that describe the timing of the late-Pleistocene glacial terminations purely in terms of obliquity forcing.

61. BBD says:

Rachel

It wasn’t meant to be a slur on your intellect. Even I’m not stupid enough to do that 😉

The expression is one I use regularly and the pretty pictures are there for all to see, but I was prompted to post them by your request that this doesn’t get too abstruse. Having been a silent witness to all-too-many threads that I struggled and failed to understand, I am very sympathetic to calls for simplicity and clarity. Plus Orera is a blinding bit of evidence that orbital forcing is the real deal 😉

62. Rachel says:

It is much appreciated BBD. As far as I’m concerned, pretty pictures are a good thing.

63. BBD from December 29, 2013 at 2:57 pm:

So, if there is doubt that orbital dynamics triggers deglaciation, then how do we account for the distinct 41ka (obliquity) pacing of deglaciation from ~2.8Ma – 0.7Ma and the ~100ka pacing thereafter? Certainly we can argue whether the actual 82ka – 123ka interval range since the mid-Pleistocene transition is the effect of eccentricity modulating precession or obliquity or some mix of both, but not that this is the initial cause. So we appear to have positive feedbacks to orbital forcing.

I am interested in what process links the orbital variations with the ice ages. So let’s look at the last 700kyrs which has 100kyr pacing of the ice ages.

This 100yrs period is the eccentricity variation. You can see more specifics on this in many places, also at Ghosts of Climates Past – Part Four – Understanding Orbits, Seasons and Stuff.

Citing from my previous article:

So eccentricity affects total TOA insolation, while obliquity and precession change its distribution in season and latitude. However, variations in solar insolation at TOA depend on e² and so the total variation in TOA radiation has, over a very long period, only been only 0.1%.
This variation is very small and yet the strongest “orbital signal” in the ice age record is that of eccentricity.

Annual solar insolation dropped by 0.1% over 50,000 years or 3 mW/m² per century. (This value is an over-estimate because it is the peak value with sun overhead, if instead we take the summer months at high latitude the change becomes 0.8 mW/m² per century).”

So how does let’s say 0.03 W/m² decrease over 1000 years cause an ice age?

Is it possible to find other changes of equal magnitude?

Separately, by the way in case there is confusion on one of my points, I am confident that there are positive feedbacks in the climate system. Obviously the start and end of ice ages demonstrates that.

64. BBD says:

Science of Doom

So how does let’s say 0.03 W/m² decrease over 1000 years cause an ice age?

A good point. How long does the diminution of summer insolation at 65N have to go on before ice sheets begin to grow? 10ka?

65. > What he’s actually illustrating – I believe – is the feedback due to water vapour

OK, I believe that. He’s graphed OLR against sfc T, and finds a relationship. You’d expect that, of course. And since sfc T affects/is affected by the atmos, and since this is monthly (which is a long time in the life of water vapour) then that relationship includes WV. It isn’t at all clear to me that we see “the other way round” from this data – he’s claiming we’re seeing a forcing-to-temperature relationship, but the *forcing* from increasing OLR is cooling, and of course we’re seeing the opposite – higher OLR is associated with higher temperature (could you get something useful by looking at transitions? If you looked at OLR vs temperature change?). So, therefore, we’re seeing higher OLR as an effect of higher temperatures in his graphs, which isn’t at all the relationship he is trying to see.

The lack of any attempt to see how incoming radiation might be varying due to variations in clouds, seasonailty, etc also looks like a pretty big problem to me; but I’m not familiar with this stuff.

[Insert std fling at Curry for hosting this kind of stuff without thinking.]

On the (somewhat bizarrely acrimonious) SoD/TC stuff: I think that no-one doubts that the ice ages are fundamentally paced by orbital variations, since the timing is so good. What’s far more mysterious, as SoD points out, is why 100 kyr dominates, since the forcing is so small at that period.

> clouds provide a negative forcing

They do in the SW, but in the LW they provide warming. My recollection was that the overall effect is near-neutral, but I checked and indeed is seems to be negative (e.g. http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch8s8-6-3-2.html).

66. BBD says:

What’s far more mysterious, as SoD points out, is why 100 kyr dominates, since the forcing is so small at that period.

I thought that the crucial (for ice sheets) summer TSI at 65N was at least 30W/m^2 higher at the start of the Holocene compared to present values?

67. BBD says:

Science of Doom

I’m not really sure what you mean here:

I am interested in what process links the orbital variations with the ice ages. So let’s look at the last 700kyrs which has 100kyr pacing of the ice ages.

So am I, which is why I mentioned the 41ka orbital pacing of interglacials from 2.8Ma – ~0.7Ma. The glaciations prior to the MPT were less profound than those that followed, but were still glaciations.

68. BBD on December 29, 2013 at 10:34 pm:

I’m not really sure what you mean here:

I am interested in what process links the orbital variations with the ice ages. So let’s look at the last 700kyrs which has 100kyr pacing of the ice ages.

So am I, which is why I mentioned the 41ka orbital pacing of interglacials from 2.8Ma – ~0.7Ma. The glaciations prior to the MPT were less profound than those that followed, but were still glaciations.

I don’t know anything really about the pre-700k timeframe which is why I can only comment on the later period. I have been working on the basis that the accuracy of dating, GHG concentrations, ice sheet size and temperature changes from proxy measurements is much better the closer we are to the present day. So that has been my focus.

In the last 700ka the 100kya time period is the strong one seen in the frequency domain of proxy records.

BBD on December 29, 2013 at 9:37 pm:

In response to my question:

So how does let’s say 0.03 W/m² decrease over 1000 years cause an ice age?

A good point. How long does the diminution of summer insolation at 65N have to go on before ice sheets begin to grow? 10ka?

There’s a lot in this small question. Which is why the ice ages is such a complex subject. (At least, I find them difficult to understand, so I have to break things into manageable parts and analyze each in turn).

Firstly, the hypothesis that the eccentricity variations cause the ice ages is impossible to support from any physics that I have seen postulated.

And just as a comparison, what other fluctuations can we find that result in an equally massive change in the radiative forcing? If over 1000 years CO2 decreased from 241 ppmv to 240 ppmv then we would have a similar magnitude change.

Secondly, once we ask the question instead about summer insolation at 65N then I think we are asking a different question. We are not then asking how eccentricity variations can cause ice age inception and termination. Instead we are looking at the Milankovitch hypothesis (“MH”). MH proposes reduced summer insolation at 65’N as the reason for the start of an ice age.

In Part Six – “Hypotheses Abound” I reviewed a few of the many hypotheses that go under the one banner. The reason there are so many hypotheses is that trying to match the start of more than one ice age to particular insolation minima/maxima starts to create counter examples.

This is why we need a GCM to try and understand what can cause an ice age to start. Let me explain a little further – the insolation minimum at 65’N in summer at 116 kyr BP corresponds to higher insolation at the same latitude in spring. So lower insolation might allow perennial snow cover through summer but how do we know the hotter spring doesn’t melt it? How do we know the increased temperature differential from the tropics to the high latitudes doesn’t provide enough negative feedback to counter this effect? (Remember that total annual TOA insolation is not affected at all by obliquity and precession changes). How do we know that the reduction in snow melt in summer isn’t more than compensated for by a reduction in snow fall due to the colder climate?

We can only claim to understand whether the MH is true by looking at the detailed physics.

And this is what climate scientists have been doing. There are computational issues involved and running a full GCM from say 130kyr BP to the present is not at all possible.

There are some positive results. But let’s take as an example one of the papers I cited earlier: “The last glacial cycle: transient simulations with an AOGCM”, Smith & Gregory (2012).

This paper uses the FAMOUS climate model which is a full GCM with half the spatial resolution and then the time speeded up by a factor of 10 so that they can run from 120kyr BP to the present.

They have reasonable success at reproducing the temperature records in Antarctica and Greenland but there are *three* forcings for the model:
– orbital changes
– GHG concentrations
– ice sheet sizes
So the model is “forced” by two factors that are actually feedbacks because we can’t simulate these essential feedbacks. So we have not simulated the major cause of reduced absorbed solar insolation, instead we have “assumed” it and shown that the GCM temperatures match the results (the paper points out that clearly their model must have additional missing feedbacks, let’s leave that for now).

That’s one example.

Other papers typically run in snapshot mode for a few hundred years to try and get perennial snow cover. So for example, “True to Milankovitch: Glacial Inception in the New Community
Climate System Model”, Jochum et al (2012), used CCSM4 (latest and greatest) at normal resolutions but can only run for a 700 year window, due to the high computational requirements.

This paper produced perennial snow cover at high latitudes. That’s a good start. What’s interesting to note when you delve into these papers is that actual specifics of why we get perennial snow cover are different from paper to paper. For example:
Thus, in contrast to the results of Vettoretti and Peltier (2003) the increase in snowfall is negligible compared to the reduction in snowmelt (not shown).
(I reviewed Vettoretti and Peltier 2003 in Ghosts of Climates Past – Part Eight – GCM II).

Jochum et al also show how important the negative feedbacks are and how much they cancel out the effect of the snow/ice albedo – which is one of the questions I was posing earlier. For example:

Thus, the negative feedback of the clouds and the meridional heat transport almost compensate for the positive albedo feedback, leading to a total feedback of only 0.5 W m2. One way to look at these feedbacks is that the climate system is quite stable, with clouds and meridional transports limiting the impact of albedo changes. This may explain why some numerical models have difficulties creating the observed cooling associated with the orbital forcing

And also when you have a combination of many opposing factors that result in a small value you do wonder about the impact of biases, like the cold bias at high latitudes in CCSM4:

Another bias relevant for the present discussion is the temperature bias of the northern high-latitude land. As discussed in the next section, much of the CCSM4 response to orbital forcing is due to reduced summer melt of snow. A cold bias in the control will make it more likely to keep the summer temperature below freezing, and will overestimate the model’s snow accumulation. In the annual mean, northern Siberia and northern Canada are too cold by about 1’C–2’C, and Baffin Island by about 5’C (Gent et al. 2011). The Siberian biases are not so dramatic, but it is quite unfortunate that Baffin Island, the nucleus of the Laurentide ice sheet, has one of the worst temperature biases in CCSM4. A closer look at the temperature biases in North America, though, reveals that the cold bias is dominated by the fall and winter biases, whereas during spring and summer Baffin Island is too cold by approximately 3’C, and the Canadian Archipelago even shows a weak warm bias (Peacock 2012).

So I said earlier “There’s a lot in this small question” and mostly that means I don’t know the answer. Some GCMs can produce perennial snow cover at high latitudes simply by using a pre-industrial GHG climate with 115 kyr BP orbital forcing when run for a few hundred years in snapshot mode. They don’t usually have ice sheet models and can’t anyway be run for long enough to see an ice sheet grow.

Can the same model without alteration produce the ice age termination that led to the Eemian interglacial?

I have this and many other questions myself.

Hopefully some of these points are helpful.

I will take a look at the Shakun et al paper and comment in due course. Thanks for the reference, I haven’t seen it before.

69. William,
Unless I’m mistaken, Figure 9.43 in AR5 straddles zero with the CMIP5 mean being very slightly positive.

SOD and BBD, Thanks, I’ve leanrned quite a lot from this. I was aware that the change in overall insolation was small and hence had assumed that the main influence was variations in high-latitude insolation changing the albedo. Also, as William mentions, the timing is so good, it’s hard to imagine that it is something else.

I had, however, been considering this the other way around though last night. If the temperature variations are not driven by orbital variations then it would seem that it has to be some kind of large internal variation that can lead to sufficient changes (in albedo for example) that can then lead to the necessary feedbacks. On the other hand, it seems that there are large variations in insolation at high-latitudes (for examples). If some large internal variation could drive albedo changes, then surely these large variations in insolations would do the same.

I guess there’s still the problem of why the dominant signal is 100kyr which coincides with the eccentricity cycle which seems to small (alone) to explain the temperature variations, but it would still seem that it’s just the difficultly trying to understand a complex cycle rather than any suggestion that the driver could be something completely different. Of course, that could be wrong, but it would seem remarkable if it turned out that Milkanovitch temperature variations were not driven by orbital variations.

70. BBD says:

I guess there’s still the problem of why the dominant signal is 100kyr which coincides with the eccentricity cycle which seems to small (alone) to explain the temperature variations,

This is not what is argued. Rather that eccentricity *modulates* obliquity and precession and the combined effect on high latitude NH summer insolation triggers the cascade of positive feedbacks described in Shakun et al. which is still being ignored.

71. BBD says:

Nope – misspoke – SoD says he’s going to look at it. Sorry.

One point here that cannot be over-stated is that the models clearly lack the fidelity to reproduce the full range/power of positive forcings hence their inability to simulate glacial inception/termination under orbital forcing. Now that should give us pause. IIRC Steve Bloom made exactly the same point about model inability to reproduce mid-Piacenzian warming.

And we *can not* just dump 2.1Ma of obliquity-paced deglaciations because we don’t much know about it.

72. Arthur Smith says:

I’ve really enjoyed the fascinating discussion here… A few thoughts:

First – whether or not “Scottish Sceptic” returns, the point made about “temperature control system is a feedback loop” should be a familiar one to most of us who have been involved in climate discussions, it certainly comes up pretty frequently. As our host noted, the term “feedback” in climate discussions has a slightly different meaning from the term in control theory. In control-theory terms, Earth’s climate system is dominated by the strong negative Planck response, under which outgoing long-wave radiation naturally increases as temperature increases, naturally counteracting any energy imbalance. So the climate is naturally stable due to a strong in-built negative feedback with this Planck response. In climate discussions, this essentially constant Planck response is assumed as a baseline, and then “feedback” refers only to further adjustments in response to the temperature response, such as water vapor, cloud, and ice changes we’ve been discussing. Positive climate feedbacks such as water vapor act to reduce the negative Planck response and increase climate sensitivity; negative feedbacks act to increase the Planck response and decrease sensitivity.

But there is a fundamental issue at play here – in principle, positive climate feedbacks could be strong enough to overwhelm the Planck response, and switch the Earth climate system from a negative-feedback control-theory state (i.e. stability) into a positive-feedback state (instability). The increased sensitivity under the action of positive climate feedbacks is a sign of approaching instability; decreased sensitivity under negative feedbacks is a move to greater stability of the system.

Two other issues complicate the picture, both of which have been discussed above. First, time period matters. There are fast feedbacks and slow feedbacks. If Earth’s climate is stable under fast feedbacks (and it surely seems pretty stable on the century timescale), it could still be unstable – or at least closer to instability – under slow feedbacks like ice sheet changes. Carbon cycle feedbacks could also lead to greater sensitivity or instability in the long-term response. Second, the various parameters involved in response and feedbacks are not constant. ATTP’s black-body flux equation up above has a sensitivity (the Planck response essentially) that varies as T^3. Water vapor varies exponentially with T. Ice albedo depends on the area covered by the ice sheets, with allowed ice sheet areas varying significantly according to Earth’s geography and the climate state. Ocean current and atmospheric circulation responses likely also have nonlinear effects that have strong effects on clouds, etc.

From a big-picture control-theory viewpoint, which might be where SoD is coming from here, Earth’s history for the past few million years looks awfully like the behavior of a bi-stable system, glacial and interglacial, with the transitions between the two states looking like (over sufficiently long periods of time) a climate response that is in the positive-feedback regime of control theory, that is actually unstable. Can Earth remain stably in an only half-glacial state, with northern Eurasia and North America only partially covered in ice? It sure doesn’t look like it. But once in the two end states, climate seems to remain stable for thousands to tens of thousands of years, so negative-feedback regime again.

But the feedback relevant there is the full Earth response over thousands of years. If you specify CO2 and ice albedo as forcings, then the response to those gives the short-term feedback, and that should stay within the “stable” regime of limited positive climate feedbacks that aren’t overwhelming the Planck response.

So I think the issue is we’re talking about several different definitions of feedback, and different timescales for understanding sensitivity and response.

The control-theory bistability argument is worrying though, it would be good to understand it a lot better, and it sounds like GCM’s aren’t the way to go on that…

73. BBD,

“Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation”, Shakun et al (2012) is a fascination paper.

Thanks again for highlighting it.

To be honest, I’m still reviewing but everyone here seems much faster than me, and not commenting on it might be misinterpreted as lack of interest. Here’s my very preliminary summary of the paper.

As a number of other climate scientists have noted in the last decade, the previous models of ice age termination don’t match the data when we look at the excellent record now compiled for the last 20ka.

Global CO2 concentrations lead the global warming from 17kyrs into the more recent past. That is, almost all the warming from the LGM to the present was driven by the radiative forcing associated with increases in CO2 from about 190ppm to 270ppm.

The global CO2 concentrations themselves were led by Antarctic increases in temperature, indicating a strong positive feedback from an initial temperature rise in Antarctica that continues to drive global temperatures. For what caused this initial rise in Antarctic temperatures:

..An important exception is the onset of deglaciation, which features about 0.3’C of global warming before the initial increase in CO2 ,17.5 kyr ago. This finding suggests that CO2 was not the cause of initial warming.

..Substantial temperature change at all latitudes (Fig. 5b), as well as a net global warming of about 0.3’C (Fig. 2a), precedes the initial increase in CO2 concentration at 17.5 kyr ago, suggesting that CO2 did not initiate deglacial warming. This early global warming occurs in two phases: a gradual increase between 21.5 and 19 kyr ago followed by a somewhat steeper increase between 19 and 17.5 kyr ago (Fig. 2a). The first increase is associated with mean warming of the northern mid to high latitudes, most prominently in Greenland, as there is little change occurring elsewhere at this time (Fig. 5 and Supplementary Fig. 20). The second increase occurs during a pronounced interhemispheric seesaw event (Fig. 5), presumably related to a reduction in AMOC strength, as seen in the Pa/Th record and our modelling (Fig. 4f, g).

..In any event, we suggest that these spatiotemporal patterns of temperature change are consistent with warming at northern mid to high latitudes, leading to a reduction in the AMOC at ~19 kyr ago, being the trigger for the global deglacial warming that followed, although more records will be required to confirm the extent and magnitude of early warming at such latitudes.

So it’s a very interesting paper, with lots of support and especially the SH temperatures leading CO2 leading global temperatures seems very clear.

Fig 5 seems one of the key graphs, which breaks out high latitude NH and we can see a temperature rise there preceding the Antarctic rise followed immediately by the CO2 rise (the CO2 rise shown in other figures).

Then a “plausible scenario” is presented for the initial NH warming:

A possible forcing model to explain this sequence of events starts with rising boreal summer insolation driving northern warming. This leads to the observed retreat of Northern Hemisphere ice sheets and the increase in sea level commencing,19 kyr ago (Fig. 3a, b), with the attendant freshwater forcing causing a reduction in the AMOC that warms the Southern Hemisphere through the bipolar seesaw.

Presumably this is the “Milankovitch hypothesis” being supported ?

First, note that this is a “possible forcing model” and not the paper’s subject or apparently supported by the paper.

Second, note that their model runs, fig 2c, don’t support this hypothesis – they show NH temperatures trending down over this critical period. Compare 2b and 2c.

Third, I don’t think I can put up a graph here (can the moderator put up the graph)? But I have the calculated July insolation at 65’N from 100kyrs to the present for everyone to take a look at. I posted it at http://scienceofdoom.files.wordpress.com/2013/12/toa-65n-july-100k-present.png

Take a look at ask in advance if you feel able to identify the period of NH ice sheet reduction due to July insolation. Would you pick 21 kyrs ago? I’m pretty confident no one would pick that as the start of the deglaciation.

So, in very preliminary summary, a great paper with much to learn and it has definitely advanced the understanding of the end of the last ice age.

Was the end of the last ice age caused by the high latitude (NH) insolation change (the “Milankovitch hypothesis”)? The paper provides no support for it.

74. Arthur Smith,
Interesting comment, thanks. I’m going to have to give it some more thought before responding in any more detail.

SOD,
Similarly interesting, I think I’ll have to try and read Shakun (2012) in more detail to try and see what I get from it.

75. BBD on December 30, 2013 at 2:11 pm

“..And we *can not* just dump 2.1Ma of obliquity-paced deglaciations because we don’t much know about it.

Perhaps I’m visiting the wrong blog. To me, not knowing about one part of a large complex subject doesn’t mean you have “dumped it”.

The further back in time we go the worse the dating of all proxy records, the worse the coverage and the worse the accuracy. Why not focus on the best data to understand how climate works? If we can do that, then we can go back further and then learn yet more.

As an aside, many people get confused with the older chronologies like SPECMAP because they assumed the “orbital hypothesis” as a starting point because they didn’t have accurate dating methods, simply locked in the time periods for ice ages as being those indicated by the hypothesis for the start and end of ice ages. I have not even researched what the accuracy is of dating pre-700 kyrs BP. I’m just a beginner.

And I realize that to most people the causes of ice ages are pretty much a slam dunk and we’re just looking at details. But, I believe in questioning, being skeptical and asking for evidence. Just because most people, or even everyone, says something is true does not make it true.

There’s accepting authority.. and then there’s physics.

76. SOD

and then there’s physics.

Thought that was my line 🙂

Maybe I can ask you a more general question about this. Let’s say we don’t quite understand the trigger for the glaciations/de-glaciations (which I guess we don’t). What’s the significance of that? My one thought is that maybe it doesn’t really make much difference to our understanding of climate sensitivity as it still appears as though the changes are still then driven by a combination of CO2 forcing and albedo changes. Alternatively, it could imply that our climate is more sensitive that we might realise (i.e., something relatively small can trigger glaciations/deglaciations). Are there other implications that I’ve missed (quite likely)? Also, what are the implications with respect to today?

77. BBD says:

Science of Doom

Thanks for this detailed and interesting comment. My apologies for what will have to be piecemeal responses.

Second, note that their model runs, fig 2c, don’t support this hypothesis – they show NH temperatures trending down over this critical period. Compare 2b and 2c.

Did you mean Fig. 4? If you did, the NH model runs show more cooling than the proxy reconstructions but the cooling is still there in the proxies. My understanding of this is that it is the cooling caused by interruption of the AMOC by freshwater flux and is consistent with what S12 is saying about that and a bipolar seesaw warming of the SH in response to the shutdown of the NH “heat sink”.

Can we clarify this before we go on to discuss whether S12 really provides no support for the MH?

78. andthentheresphysics on December 30, 2013 at 9:52 pm,

There are lots of competing and complementary hypotheses I have bubbling away, based on what we find from how well we understand and can model past climate.

I haven’t yet tried to write them down and think about them properly, but for the sake of discussion I’ll suggest a few here, with the caveat that I haven’t given them due thought:

1. Climate is super-sensitive to small changes therefore we are heading for catastrophe

2. We can’t model past climate therefore we have no idea what the future holds

3. Climate models can produce most results when we know in advance what the answer is – i.e., there are sufficient degrees of freedom to model most things

4. Climate models can’t model abrupt (past) climate change, and given that climate is super-sensitive to small changes we therefore have no idea what the future holds [note: these abrupt changes haven’t been much discussed in this thread but as well as the challenge of modeling the major ice age curves there is even less success in modeling very significant higher frequency change (over decades & centuries)]

5. Climate is very non-linear and while apparently small changes have both caused and ended major ice ages, at other times, even bigger changes have had no effect, therefore we are either doomed or not in a very unpredictable way

6. Major positive feedbacks – which indicate a super-sensitive climate – are then over-turned by other small changes which indicates that the strength of the feedback varies significantly with time or with the state of the climate.

7. Even though modeling past climate is a challenge where quite small changes have started and ended ice ages, the 150 year increase of atmospheric GHG concentrations is so much greater that it overwhelms all these much smaller effects

8. Climate is chaotic but transitive, meaning that even though climate is not deterministic we can identify statistics for future climate states.

9. Climate is chaotic and intransitive, meaning that climate is not deterministic and the statistics of future climate states cannot be known.

Well, it’s a bit of a mess, and needs some work.

But I believe hypothesis 10 is false:

10. We understand why ice ages start and finish.

79. BBD says:

Also, I would like to proceed cautiously about the choice of month when considering TSI at 65N.

I know you mentioned this upthread, but I’d like to add this from Hansen & Sato (2012):

A satisfactory quantitative interpretation of how each orbital parameter alters climate has not yet been achieved. Milankovitch argued that the magnitude of summer insolation at high latitudes in the Northern Hemisphere was the key factor determining when glaciation and deglaciation occurred. Huybers (2006) points out that insolation integrated over the summer is affected only by axial tilt. Hansen et al. (2007a) argue that late spring (mid-May) insolation is the key, because early ‘flip’ of ice sheet albedo to a dark wet condition produces a long summer melt season; they buttress this argument with data for the timing of the last two deglaciations (Termination I 13-14,000 years ago and Termination II about 130,000 years ago).

80. And on my comment on hypothesis 10 – that doesn’t mean no currently pubilshed ice age inception/termination hypothesis is true. It means that “hypotheses abound” and most of these hypotheses contradict each other, even though they all (vaguely) claim to be the same hypothesis.

81. BBD on December 30, 2013 at 10:30 pm:

Exactly. I thought that was *my* point – what hypothesis are we putting forward?

If summer insolation at 65’N is the key but then someone else points out that termination X or inception Y doesnn’t match the data and instead it’s spring or autumn or winter or 45’N or the tropics or the gradient between latitudes, or the minimum where that value starts rising that causes it, or the maximum because that is the peak.. is this *support* for the hypothesis of orbital variations causing and ending ice ages or a cookbook where you can invent any new recipe you like so long as you still call it a cake?

Check out the curves in figs 5, 6, 7 in Part Four.

By the way I can produce the high resolution insolation curves thanks to the Matlab model of Jonathan Levine.

Here is May at 65’N (pls can moderator insert the graph, thanks):

82. BBD on December 30, 2013 at 10:24 pm

Did you mean Fig. 4? If you did, the NH model runs show more cooling than the proxy reconstructions but the cooling is still there in the proxies. My understanding of this is that it is the cooling caused by interruption of the AMOC by freshwater flux and is consistent with what S12 is saying about that and a bipolar seesaw warming of the SH in response to the shutdown of the NH “heat sink”.

Sorry I did mean fig. 4.

The point I was hoping to make was that their model – at least what they show in the graph – didn’t produce the initial NH warming they discuss that is there in the proxies.

The question they posed – what is the cause of the initial Antarctic warming? They answer – it is a temperature increase in the NH high latitudes. The model result 4c doesn’t show any temperature increase in NH.

But I also notice that their breakdown by latitude band, fig. 5, shows a warming that doesn’t appear in the overall NH proxies. It’s *possible* that their model *did* show an NH warming in high latitudes but they don’t produce it in a graph.

**I don’t think this is the main point of their paper**. I think they have done a great job with correlating a lot of data, including doing a Monte Carlo sensitivity analysis, and shown what leads what in time. I had a small set of papers on this specific topic (termination of last ice age using latest EPICA data from Antarctica and correlating with ice core data from Greenland) that I had scanned but not really dug into and I’m glad I didn’t spend lots of time because this is the first one I’ve seen (there may be others) that presents it so clearly.

To demonstrate the NH warming was caused by [insert appropriate MH variant here] probably requires more work.

Also, I haven’t dug into their “inter-hemisphere” seesaw section yet. This is a well-established theory, but a recent “latest & best model” paper couldn’t make this seesaw produce much linkage in temperature between the two polar regions. I have to find that paper as it may have been modeling ice age inception.

83. Following my earlier comment from December 30, 2013 at 10:50 pm here is the graph for March insolation at 65’N. It’s definitely March – insolation in March at 65’N peaks at 22kyrs. We have proven the theory!

Here is the graph:

84. BBD says:

Science of Doom

To demonstrate the NH warming was caused by [insert appropriate MH variant here] probably requires more work.

Agreed, of course. But to suggest that orbital dynamics are not the fundamental trigger for terminations is to ignore just about everything else we know about both topics. Specifically, the ’41ka world’ prior to the MPT and the [82ka – 123ka] world thereafter. Obliquity is always there.

I wholeheartedly agree that there is probably a stochastic component to deglaciation and will armwave at the increasing instability of large ice sheets as they evolve. The right forcing at the right time (debate ongoing)…

What is difficult to accept is that we do not understand, at least in outline, how glaciations start and finish.

85. BBD says:

**I don’t think this is the main point of their paper**

Also agreed. The authors appear to accept the Milankovitch hypothesis as theory.

86. Tom Curtis says:

Briefly on Shakun et al:
1) The see saw is initiated by an initial rise in temperature at 60-90 degrees north, and to a lesser extent at 30-60 degrees north (see figure 5), ie, the “northern mid to high latitudes” as specified in the paper – not the “NH” as SOD repeatedly misstates. The model results and observations SOD refers to are for the entire NH, including 0-30 north which does not show the rise. Consequently, and due to relative area, the NH does not show the rise even in observations. Unfortunately the paper does not show the temperature response by 30 degree latitude bands for the model run.

2) The mechanism for the Northern Warming assumed in Shakun et al is a rise in high latitude NH summer insolation (they show June 65 degree North insolation values). That rise is not by itself supposed to be able to end the glacial. Rather, it merely slows the AMOC, thereby setting up the seesaw. Thus presumption is that high NH temperatures track the summer insolation, instituting a melt when the insolation reaches a minimum and starts rising again. That rise causes ice melt, and hence a fresh water pulse in the North Atlantic slowing the AMOC. Equally low temperatures just a few thousand years before would not cause that pulse because temperatures were falling.

It is plausible that the seesaw would not have lead to an end to the glaciation without the ongoing rise of mid to high NH summer insolation to restart NH warming after the slowing of AMOC (in Shakun et al’s scenario). The paper does not specifically say.

3) Skahun et al show the 65 degree north June 21st insolation in figure 3. They are, therefore, clearly aware that 21 kya coincides with the minimum of insolation. It is implausible, therefore, that their theory was based on the strength of the increase in insolation (as SOD would have it), rather than the direction of change as per (2) above.

87. Earlier (December 30, 2013 at 11:12 pm) I said:

“Also, I haven’t dug into their “inter-hemisphere” seesaw section yet. This is a well-established theory, but a recent “latest & best model” paper couldn’t make this seesaw produce much linkage in temperature between the two polar regions. I have to find that paper as it may have been modeling ice age inception.”

The result is in one of the papers I’ve already cited, Smith & Gregory (2012). They are commenting on the ice age termination:

An important aspect of the glacial climate system is the idea of the bipolar see-saw mechanism (Broecker et al. 1985), inferred from proxy data as affecting surface temperature during millennial scale climate events. Such a see- saw is not seen in our runs, either in terms of abrupt events, or anti-correlated surface temperature variations on any timescale. There is some evidence in our experiments, though, for the physical oceanic mechanism by which such a see-saw might work, in the anti-correlated interplay between the AMOC and the ACC.

Support for an AMOC-mediated Greenland/Antarctic temperature see-saw is often claimed to have been provided by modelling studies, although this is perhaps overstated, with only models of intermediate complexity with simplified atmospheres showing a significant surface temperature change on Antarctica itself (e.g. Ganopolski and Rahmstorf 2001). Other studies using coupled AOGCMs have found differing results, which they attribute to internal variability, or different mechanisms (e.g. Rind et al. 2001; Vellinga and Wood 2002).

Wolff et al.’s (2009) recent hypothesis linking southern and northern hemisphere climate events requires a physical ocean teleconnection, and one in which the coupling breaks down at the LGM, initiating deglaciation. In our runs, the degree of the coupling between the two hemispheres does change through the glacial, with the coupling reducing in strength as the northern hemisphere ice-sheets build and provide a strong local influence on the AMOC, and the ocean fills with cold glacial deepwater.

Wolff et al. (2009) require the see-saw to be led by the southern hemisphere, which is not seen clearly in our experiments.

[And by the way, Wolff et al (2009) is the paper, “Glacial terminations as southern warmings without northern control” I quoted from earlier in this thread – December 29, 2013 at 4:05 am. Wolff is the lead author on many EPICA [Antarctica ice core] papers and is interpreting the same data as Shakun et al 2012 that we have been discussing.]

88. .. and by the way, bold in the last quotation from Smith & Gregory was emphasis added.

89. Tom Curtis says:

The abstract of Liu et al, 2009, “Transient Simulation of Last Deglaciation With a New Mechanism for Bolling-Axelrod Warming”, ie, Reference 25 of Shakun et al, and the source of their model experiments.

“We conducted the first synchronously coupled atmosphere-ocean general circulation model simulation from the Last Glacial Maximum to the Bølling-Allerød (BA) warming. Our model reproduces several major features of the deglacial climate evolution, suggesting a good agreement in climate sensitivity between the model and observations. In particular, our model simulates the abrupt BA warming as a transient response of the Atlantic meridional overturning circulation (AMOC) to a sudden termination of freshwater discharge to the North Atlantic before the BA. In contrast to previous mechanisms that invoke AMOC multiple equilibrium and Southern Hemisphere climate forcing, we propose that the BA transition is caused by the superposition of climatic responses to the transient CO2 forcing, the AMOC recovery from Heinrich Event 1, and an AMOC overshoot.”

The paper describes a polar seesaw with a reversal of early warming in the Arctic leading to a 4 C temperature fall over Greenland matched by a 2 C temperature rise over Antarctica. The polar seesaw is induced by a slow down of the AMF due to Melt Water Flux. The model used is the NCAR CCSM3. Consequently Smith and Gregory’s claim that “…with only models of intermediate complexity with simplified atmospheres showing a significant surface temperature change on Antarctica itself…” is simply false, whether emphasized or not.

90. Tom Curtis says:

Anders or Rachel, apologies for failing to close the link. I would appreciate it if you could correct my error.

91. Tom Curtis says:

Speaking of Smith and Gregory, they write:

“Variability at millennial frequencies identiﬁed in the EPICA record, such as the warm events of so-called Antarctic Isotopic Maxima events (EPICA Community Members 2006) or the warming/cooling signal of the Bolling-Allerod/Younger Dryas period do not appear to be present in our simulations.”

That and the fact that they use FAMOUS, a low resolution version of an AOGCM suggests Liu et al are probably more reliable on this feature, particularly given that Liu et al accurately reproduce AMOC current strengths as indicated by a proxy at Bermuda (Fig 1 D, DGL-A ie, the sudden shut off of melt water experiment).

92. Rachel says:

Anders or Rachel, apologies for failing to close the link. I would appreciate it if you could correct my error.

No worries. All fixed. Let me know if I’ve closed it in the wrong place.

93. Tom Curtis says:

Thanks, Rachel, your placement is perfect.

94. SOD,

Thanks, a lot of what you say is what I was thinking myself.

Climate is super-sensitive to small changes therefore we are heading for catastrophe

This is one possibility, but I would have thought that we have enough past climate evidence to suggest that it’s unlikely that our climate is super-sensitivie.

We can’t model past climate therefore we have no idea what the future holds

Certainly if we can’t model past climate it would reduce our confidence in what models suggest our future holds. We do, however, understand basic radiative forcings and have some idea of feedbacks (from energy budget calculations) so “no idea” would seem a little strong, in my view at least.

Climate models can produce most results when we know in advance what the answer is – i.e., there are sufficient degrees of freedom to model most things

This is always a problem with modelling. My view is that whether this is an issue or not depends partly on how constrained the parameters are. Lots of free parameters, and this may be a real issue. If the parameters are constrained by physics or observations, it may be less of an issue.

Climate models can’t model abrupt (past) climate change, and given that climate is super-sensitive to small changes we therefore have no idea what the future holds [note: these abrupt changes haven’t been much discussed in this thread but as well as the challenge of modeling the major ice age curves there is even less success in modeling very significant higher frequency change (over decades & centuries)]

I am aware that this is an issue, so may well be a valid criticism. I’m not really sufficiently aware of this issue to really comment further.

Climate is very non-linear and while apparently small changes have both caused and ended major ice ages, at other times, even bigger changes have had no effect, therefore we are either doomed or not in a very unpredictable way

This goes back to point 1. There I suggested that maybe there is evidence that our climate isn’t super-sensitive but, I guess, it is possible that non-linearities mean that some small changes can have a large effect while other larger changes have no effect. One comment I would make is that it was my impression that the general view was that the system is chaotic on short-timescales, but less so on longer-timescales. What you’re suggesting here presumably implies it’s chaotic on all timescales.

Major positive feedbacks – which indicate a super-sensitive climate – are then over-turned by other small changes which indicates that the strength of the feedback varies significantly with time or with the state of the climate.

There is presumably some truth in this. However, given the existence of Venus presumably we can’t bank on some major negative feedback kicking in sometime in the future if we warm sufficiently. It could, but we have evidence (from Venus) that a runaway process is possible (although I realise that we’re not in danger of that ourselves, yet). Furthermore, a major negative feedback kicking in and stablisiing our climate in the future presumably also implies a major change to our climate anyway, so doesn’t really imply that we should necessarily consider this a positive outcome.

Even though modeling past climate is a challenge where quite small changes have started and ended ice ages, the 150 year increase of atmospheric GHG concentrations is so much greater that it overwhelms all these much smaller effects

Basic physics would seem to indicate that this is true at the moment. The dominant forcing is anthropogenic GHGs.

Climate is chaotic but transitive, meaning that even though climate is not deterministic we can identify statistics for future climate states.

Again, this goes to an earlier response of mine. It was my understanding that the general view was that it was chaotic on short timescales but not really on longer timescales. Maybe this is not certain though.

Climate is chaotic and intransitive, meaning that climate is not deterministic and the statistics of future climate states cannot be known.

Again, would be interested to know more about how chaotic the climate it. Also I presume you mean “deterministic chaos” so even a chaotic system is determined, we just can’t know what the future state is because of the sensitivity to the initial conditions.

Well, it’s a bit of a mess, and needs some work.

But I believe hypothesis 10 is false:

10. We understand why ice ages start and finish.

A bit of a mess maybe and more work needed, but I would argue (as I think others have too) that even though we can’t state that we understand why ice ages start and finish that doesn’t mean we don’t have some idea. I guess we can’t rule out that it might not be orbital variations, but that does appear to be the preferred explanation (in a general, rather than a specific sense).

I guess, overall, I agree with much of what you’re saying (in a general sense at least) but there seems to be very little to suggest that continuing to add GHGs to our atmosphere won’t make quite significant changes to our climate.

95. andthentheresphysics on December 31, 2013 at 8:59 am:

Just to be clear to readers that these are all hypotheses that might be put forward, it’s not a claim that any, all or none are true. Some contradict others, for example. The idea is to put forward hypotheses that can be falsified. With sufficient evidence, counter arguments, etc the number of hypotheses can be reduced. Perhaps only one or two survive.

In The Confirmation Bias – Or Why None of Us are Really Skeptics I commented on The Black Swan by Nassim Nicholas Taleb. He talks about the confirmation bias – how (in general) we try to find evidence to support our theories rather than trying to find evidence to shoot down our theories.

Are we skeptics or just supporters of our favorite football team/theory? It’s a challenge.

My hypothesis 9 – Climate is chaotic and intransitive, meaning that climate is not deterministic and the statistics of future climate states cannot be known.

You said (I’m going to try a html tag here hope it works, not sure what tags work or not on this blog):

Again, would be interested to know more about how chaotic the climate it. Also I presume you mean “deterministic chaos” so even a chaotic system is determined, we just can’t know what the future state is because of the sensitivity to the initial conditions.

If a system is chaotic then small changes in initial conditions mean that the future state cannot be known. However, many chaotic systems have deterministic statistics. That is, we can confidently state the probability of the system being in state X, or between values x & x+Delta.x.

Then there are other systems where the statistics are not deterministic. That is, a system can switch between different modes such that even the statistics are not reliable over time.

You said “Again, would be interested to know more about how chaotic the climate is.”

There might be a rich literature on this but I haven’t yet found it. From the small amount I have studied on chaotic systems the problem is usually that unless you can define the exact equations you can’t determine where the chaotic regions lie. Small changes to parameters can turn a deterministic system into a chaotic system and a transitive system into an intransitive system.

Lorenz has many excellent papers on this from the 1960s through to the present – including his 1968 paper I highlighted in Ghosts of Climates Past – Part Two – Lorenz. Most of his sample equations come from primitive equations of climate.

96. SOD,

However, many chaotic systems have deterministic statistics.

Sorry, I meant to add to my comment that I knew what you meant. I realise that it is very complex. I don’t know enough about chaos theory or about climate theory to really comment much further. My impression was, however, that many thought that chaos applied more to weather than to climate. However, I suspect that this may apply to global changes rather than to regional (where chaos may still play some role).

97. dana1981 says:

Crap. New study by Sherwood et al. suggests the cloud feedback is positive, implying equilibrium climate sensitivity >3°C.
http://www.theguardian.com/environment/2013/dec/31/planet-will-warm-4c-2100-climate?CMP=twt_gu
http://www.nature.com/nature/journal/v505/n7481/full/nature12829.html

98. Dana,
Shall I guess who the “crap” is aimed at 🙂

Yes, I saw the Guardian article. Haven’t had a chance to read the paper yet though.

99. Arthur Smith says:

Chaotic or non-chaotic may not actually be a terribly relevant issue for climate if the changes in boundary conditions (solar/orbital parameters, Earth’s continents, volcanic aerosols, CO2 or other GHG pulses from various sources) dominate. That is, internal dynamics of Earth’s climate (chaotic or not) are fixed only under fixed boundary conditions. If the boundary conditions change, the dynamics can change significantly – from chaotic to non, or to chaotic with very different time constants, etc. Earth (and the whole solar system)’s orbital parameters themselves are chaotic when you look over 100 million-year time-scales, but the orbital parameters follow a simpler pattern for the time periods we’re likely to care about.

Significant rapid climate change in the past seems to be pretty rare, and when you look at the examples we seem to be able to find causes – volcanoes or asteroids or an event associated with ice sheet melting. In the latter case (Younger Dryas for example) is that part of the deterministic dynamics of the Earth system, or just random stochastic behavior? There is much about the world that will always be unpredictable…

Either way, it seems like you need something like the intermediate Earth system models SoD discusses in his latest post (nice collection of reviews on this, by the way!) to get at least a rough picture of the millennium-time-scale dynamics and see to what degree there may be real instability or chaos at play, or whether it’s all just responses to external forcings of one sort or another. This does seem pretty important to understand better…

100. Arthur,

millennium-time-scale dynamics and see to what degree there may be real instability or chaos at play, or whether it’s all just responses to external forcings of one sort or another. This does seem pretty important to understand better…

Yes, I agree it would be important to understand better, but would you expect it to imply anything with respect to the next century or so? It’s always scientifically interesting to understand something better and it’s clear (after discussions with various people about millenial reconstructions) that there is much we don’t understand about millenial reconstructions and that there is much that such work could tell us about forced versus unforced variability. However, it stills seems unlikely (even if not impossible) that some negative feedback will suddenly kick-in so as to prevent significant warming (2 – 4 degrees) by 2100 if we follow a BAU emission pathway.

101. dana1981 says:

Hah sorry, I just meant crap as in “oh crap, that’s not good”. And the Sherwood paper is consistent with several other studies, which I’ll put together in my next post.

102. I wrote an article on the paper, Shakun et al (2012), that we discussed here.

Ghosts of Climates Past – Eleven – End of the Last Ice age

It turns out that Eric Wolff, lead author on a number of EPICA papers, has similar criticisms of the paper to those I expressed.

103. BBD says:

SoD

104. SOD,
Thanks, I’ve just seen that.

105. ATTP,

The original post contains a reference to me. That makes me add a few comments. I haven’t read nearly all of the discussion. What I’ll write is probably already been discussed in some way, but not summarized as follows.

The text of Steve McGee tells about climate based the following assumptions:

1) changes in albedo can be dismissed
2) OLR at TOA depends only on GMST (for fixed GHG concentration)
3) slow feedback need not be considered

All points are questionable, but the problems in (2) have perhaps not got enough emphasis. The two states being compared are NH-summer/SH-winter and NH-Winter/SH-summer. The assumption (2) means that OLR is assumed to be the same, if GMST happens to be the same. The state of atmosphere is, however, very different in those two cases and the properties of those surface areas which are warmer in the first case are quite different from areas which are colder. Thus the assumption that OLR would be the same is really questionable. Those neglected differences might very well be large in comparison to the change in OLR from the observed seasonal variations in GMST. This point alone means that we cannot a priori expect that the method leads to anything meaningful.

That the resulting value for climate sensitivity is not very far from other estimates of TCR tells that the combined effect of the three assumptions is not huge, but this is really the outcome of the exercise, not a new significant estimate of climate sensitivity, and that tells only about the combined effect, not about any of the assumptions separately.

106. Pekka,
Thanks. Hope you don’t mind me referencing you. It was your comment that lead me to the SoD post, so it only seemed right to give due credit.

I think I agree with what you’re saying in your final paragraph. This may well be a reasonable estimate for the TCR – although I still think the timescales are still too short for it to be definitive in any way. Do you agree, though, that it’s also illustrating the existence of water vapour feedback. My view was that that was a more interesting aspect than anything that Steve McGee was trying to say.

107. ATTP,
Of course you can refer to my comments on net, when you feel that appropriate.

In my above post I tried to say that we cannot draw essentially any conclusions about the climate sensitivity or feedbacks from the McGee analysis alone, because the differences between the atmospheric patterns between the two seasons may be large enough to overwhelm other effects.

Interpreting the result as being close to other estimates of TCR leaves still open the question, whether all the requirements are satisfied, the alternative being that the agreement is accidental.

Looking at the spatial patterns might lead to further conclusions, but that’s a different and much more complex study.

108. Pekka,

In my above post I tried to say that we cannot draw essentially any conclusions about the climate sensitivity or feedbacks from the McGee analysis alone, because the differences between the atmospheric patterns between the two seasons may be large enough to overwhelm other effects.

Okay, yes I probably agree. I certainly wasn’t suggesting that it was some kind of definitive proof of feedbacks, simply an illustration of how water vapour concentrations can vary with temperature. You’re probably correct, though, that given the short timescales it’s probably not really possible to eliminate all the other influences that may well overwhelm everything else.

109. Pekka,
I should probably also apologise for not using the correct character in your surname. I shall endeavour to do that properly in future if I do reference you again 🙂