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Discussion > Climate Senstivity and All That

The Great Climate Sensitivity Knockabout continues here!

Roll up! Roll up!

Sep 23, 2011 at 7:00 PM | Unregistered CommenterBBD

Josh

I can't even begin to understand your logic here so I must be missing something. You seemed to be fixated on coming out of an ice age (and presumably going back in again) as though this gives you all you need to understand about climate sensitivity.

I just don't get it. Some kind of 'thermostat' is clearly at work (thus my earlier link to Willis' post) but as yet we don't know exactly how.

Well, I'm baffled too, because this is simple.

Low climate sensitivity = only slightly positive net feedbacks

High CS = strongly positive net feedbacks

Take one small forcing (Milankovitch) and apply to your climate system.

Amazingly, even if the climate system is in a glacial phase, after a couple of thousand years of gently increasing DSW, the glacial abruptly terminates.

This requires that feedbacks net strongly positive (high CS) or the modest increase in RF from Milankovitch would cause no significant change in T. So you stay in a glacial phase forever.

Once again: a low CS is incompatible with known past climate behaviour.

Willis' thermometer hypothesis (Lindzen Iris by another name, more or less) would effectively damp down the net effect of increasing RF. Were it a strong effect, then a low CS would result.

If Willis is correct, then we should be frozen solid.

Sep 23, 2011 at 7:07 PM | Unregistered CommenterBBD

One from earlier:

@BBD - the original Milankovitch theory is not without criticism: trying to make it work by invoking a high climate sensitivity seems to me to be begging the question.

At a qualitative level, the model for global climate oscillations over the last million years, in view of the good agreement of the OC diagram with oxygen isotope curves and the general mechanism adopted for orbitally forced climate variations, can be presented as follows. Orbital changes force insolation variations, which in turn induce initial temperature changes (mainly at high latitudes), resulting through positive feedback (due to variations in the volume and area of ice and snow at these latitudes) in still more considerable temperature oscillations involving larger areas.
These oscillations in turn influence more strongly the World Ocean temperature and the atmospheric concentrations of greenhouse gases, the combined action of the totality of factors then determining the resultant global oscillations in ice volume and temperature (climate oscillations). It thus follows that insolation variations and global ice volume changes, rather than variations in the atmospheric concentrations of greenhouse gases (in particular, CO 2 ), have been the underlying cause of global climate changes in the Quaternary.

http://rjes.wdcb.ru/v05/tje03116/tje03116.htm

So while greenhouse gases no doubt have a part to play, they are somewhat late to the party and demoted in importance, at least in this variation on the Milankovitch theory.

On that basis, we're back to questioning the value of paleo/glaciation evidence regarding climate sensitivity (to CO2), which is why Ron Cram's comment above seemed apposite.

Aug 31, 2011 at 12:40 AM | woodentop

Sep 24, 2011 at 1:03 AM | Unregistered Commenterwoodentop

And for context:

Ron Cram at Climate Etc on climate sensitivity (during the course of an interesting discussion):

Ron Cram | July 31, 2011 at 2:42 am | Reply
The point is you cannot calculate climate sensitivity unless you know a great deal about natural climate variability. Paleoclimate data cannot come anywhere close to providing us with enough information to determine if changes in the temperature were caused by atmospheric CO2, changes in clouds from cosmic rays, solar variation or any number of other things.
Gavin Schmidt’s remark would be true only if CO2 was the only driver of climate. It isn’t. Schmidt’s view the climate is driven only by greenhouse gases is a fantasy world created by Jim Hansen and the IPCC.

On many occasions I have seen Dr. Curry point out that uncertainties are higher than the IPCC likes to admit regarding attribution of climate change. If we cannot tell how much of climate change is natural and how much is anthropogenic now, we certainly cannot say anything specific about changes 1,000 to 2,000 years ago.

http://judithcurry.com/2011/07/30/spencer-braswells-new-paper/#comment-92188

Aug 30, 2011 at 9:11 PM | woodentop

All from this thread:

http://www.bishop-hill.net/blog/2011/8/30/nipcc-interim-report-2011.html

Sep 24, 2011 at 1:09 AM | Unregistered Commenterwoodentop

Woodentop

At a qualitative level, the model for global climate oscillations over the last million years, in view of the good agreement of the OC diagram with oxygen isotope curves and the general mechanism adopted for orbitally forced climate variations, can be presented as follows. Orbital changes force insolation variations, which in turn induce initial temperature changes (mainly at high latitudes), resulting through positive feedback (due to variations in the volume and area of ice and snow at these latitudes) in still more considerable temperature oscillations involving larger areas.

You are correct to highlight the requirement for strongly positive feedback (high climate sensitivity) to explain how small changes in insolation can terminate a glacial.

This is fine - it's not a variation on Milankovitch. Can I repeat that nobody is arguing that GHGs are the cause of the initial warming. Increased DSW does that. GHGs are one of many factors that amplify and prolong the initial warming from increased insolation:

- ice-albedo feedback is the dominant forcing in glacial climate states

- any change in the size of the major ice sheets will have an amplified effect on climate (decrease = increase in GAT; increase = decrease in GAT)

- change occurs at mid-latitudes, at the margins of the ice sheets

- a moderate change in insolation at higher latitudes is insufficient to explain ice-melt at mid-latitudes unless there are other, strongly positive feedbacks operating within the climate system

- these feebacks are amplified by ice-albedo feedback, which in turn raises GAT and further strengthens their positive effect

- this results in a faster rate of melt - until ice-sheet dynamics begin to take over and the ice sheet collapses

- these other feedbacks are water vapour and GHGs (CO2 and CH4)

Cram's obfuscation of the role of GHGs as factors in overall climate senstivity is comprehensively countered inHansen & Sato (2011), which shows exactly how wrong this argument is:

The point is you cannot calculate climate sensitivity unless you know a great deal about natural climate variability. Paleoclimate data cannot come anywhere close to providing us with enough information to determine if changes in the temperature were caused by atmospheric CO2, changes in clouds from cosmic rays, solar variation or any number of other things.

There's no point in my trying to summarize H&S here. It would only damage the analysis while failing to clarify it. The paper is clearly and accessibly written, so do please go over it when you have time and let me know what you think.

Sep 24, 2011 at 2:40 PM | Unregistered CommenterBBD

BBD:

The comparison of ice age periods with current climate is not generally useful, and the argument given is over simplistic. For example:

http://www.nap.edu/openbook.php?record_id=10136&page=1

http://pielkeclimatesci.files.wordpress.com/2009/10/r-260.pdf

In Hansen and Sato, there is the observation that estimates based on this comparison lead to climate sensitivity values similar to those obtained from models. However, it is not at all obvious that models can even simulate 20th C variations. We have already agreed on the weakness of the TSI argument, and the aerosol cooling argument is equally unconvincing. For example:

http://judithcurry.com/2011/02/16/mid-20th-century-global-warming-part-ii/

http://www.lanl.gov/source/orgs/ees/ees14/pdfs/09Chlylek.pdf

Curry has pointed out the circularity in the attribution argument, and this weakens the paleo argument as well. Cram is quite right if he claims that more understanding of natural variations is needed, before it is possible to convincingly argue for specific numbers.

To be honest, I doubt that arguments like this one will ever convince reasonable people of the urgent need for mitigation. That convincing must come from elsewhere. In general, approaching the energy issue through climate science is a very poor idea. It has far so failed, and I think it will continue to fail for a long time yet. Meanwhile, CO2 emissions continue to increase. According to IPCC figures there remain more than 200 years of potential fossil fuel reserves, even if 9 billion people enjoyed UK levels of energy consumption. A1FI seems more than plausible. Why politicize science like this, when all that people want is energy?

Sep 24, 2011 at 5:11 PM | Unregistered CommenterPhilip

Philip

Curry has pointed out the circularity in the attribution argument, and this weakens the paleo argument as well. Cram is quite right if he claims that more understanding of natural variations is needed, before it is possible to convincingly argue for specific numbers.

From H&S (2011):

Fortunately, it is not necessary to have a detailed quantitative theory of the ice ages in order to extract vitally important information. In the following section we show that Milankovitch climate oscillations provide our most accurate assessment of climate sensitivity.

Go on, you must be tempted... Read the rest.

Sep 24, 2011 at 6:33 PM | Unregistered CommenterBBD

[Continued from 'Try, try again' thread:]

Spence_UK

Do you even realise that Greenland was largely ice free during the Eemian?

Oh no! Something else the whole field has got wrong :-(

But wait... what's this?

To evaluate the area and volume of the Greenland Ice Sheet during the Eemian period, it is worth noting that we find Eemian ice in the Dye3 ice core in South Greenland, in the central Greenland Ice cores GRIP and GISP2, in NGRIP and in the little 350m thick ice cap, Renland, on the east coast of Greenland.

Additional information on the evolution of the Greenland Ice Sheet is found by observing the internal radio echo sounding layers in the ice along the main ice divide on which most of the deep ice core sites are located. A time dependent ice flow model along the divide is used to follow the paths of the ice containing the Eemian ice now found near the base at the boreholes. The model is constrained to match the dated ice from the boreholes and the internal RES layers can follow these layers between the boreholes further constraining the model.

It can be concluded that the there was an significant ice sheet covering Greenland during the warm Eemian period and that the reduction of the Greenland ice sheet at most contributed with a sea level rise of 1-2 m of the observed 5 m.

Finally studies of DNA from the basal ice from the DYE-3 ice core in south Greenland reveals that Boreal forest covered South Greenland before it was ice covered. Dating of the basal material indicates that the biological material found in the DYE-3 ice core is older than 400.000 years supporting the above conclusion that ice covered south Greenland during the Eemian period.

Sep 24, 2011 at 9:44 PM | Unregistered CommenterBBD

As noted in the "try, try again" thread, your comment is unresponsive to my point.

Because you seem to forget the point you were making just two comments back, I'll remind you. You were arguing the Greenland ice was useful for interpreting the glaciations. I pointed out it wasn't, because Greenland was LARGELY ice free in the Eemian. By "largely", I mean sufficient that the ice cores cannot provide continuous data beyond the Eemian. If we don't have continuous data, we have no data about the dynamics, which is what we were talking about.

So. Hopefully we have now some common understanding of what the word "largely" means. But please tell us. What can the Greenland ice cores tell us about the dynamics of climate prior to the Eemian.

I think you'll find, exactly as I pointed out, that the extensive melt in the Eemian prevents the type of analysis we were discussing earlier.

Also, your response includes one tangential point, and makes no attempt to address any other point I made in the previous comment. I assume you have no answer to those points? In that case, I guess we're done here.

Sep 24, 2011 at 10:40 PM | Unregistered CommenterSpence_UK

Spence_UK

You ask:

Tell me then. How much data have we got from ice cores prior to the Eemian about the climate of Greenland

How important is this? Data from the various Greenland drilling projects, especially GISP2 provides good evidence for abrupt climate shifts at the termination of the last glacial. Why would these be unique to that event and not others?

We still haven't dealt with the original basis for this conversation, which was that abrupt climate change happens and this requires a relatively high climate sensitivity.

Sep 24, 2011 at 10:41 PM | Unregistered CommenterBBD

Spence_UK

You were arguing the Greenland ice was useful for interpreting the glaciations. I pointed out it wasn't, because Greenland was LARGELY ice free in the Eemian. By "largely", I mean sufficient that the ice cores cannot provide continuous data beyond the Eemian.

Greenland was not 'largely ice free in the Eemian'.

Our posts crossed, so I won't repeat the rest of what I said above.

Sep 24, 2011 at 10:49 PM | Unregistered CommenterBBD

Spence_UK

I think you'll find, exactly as I pointed out, that the extensive melt in the Eemian prevents the type of analysis we were discussing earlier.

Why, exactly? I'm puzzled by this.

It can be concluded that the there was an significant ice sheet covering Greenland during the warm Eemian period and that the reduction of the Greenland ice sheet at most contributed with a sea level rise of 1-2 m of the observed 5 m.

The contribution to MSL from the GIS during the Eemian is interesting. Where did the other 3-4m sea level rise come from? Where else on Earth is there enough ice with an easy path to the (slightly warmed) ocean?

Sep 24, 2011 at 11:05 PM | Unregistered CommenterBBD

1. Yes, the evidence strongly suggests that Greenland was largely ice free during the Eemian. The website you link to fails to present a consistent alternative and I cannot find the publications that back it up. That there was some ice present is not doubted, hence the use of the word "largely". But the ice just a few metres from the base rock dates back to the Eemian and no later. If you want to believe the ice magically disappeared through some other mechanism, that's fine by me. But I can tell you without a shadow of doubt Antarctica had an ice sheet similar to today. We know because we can find ice from Eemian and before. The only ice older on Greenland is right at the base - within a few metres of the base. How can it be that 400kyr ice remained there, but 150kyr ice didn't? Simple answer - it was largely gone during the Eemian. There is nothing in the article (other than another deterministic model run tuned to give the answer first thought of, ho hum) that gives any other explanation. Your claims have to be consistent with the data, all of the data, and the claims you make (and made on that website) simply don't add up. There are many unexplained and inconsistent issues left hanging. But the claims on that website are far from clear. Where are the peer-reviewed publications backing up those claims, so we can actually scrutinise them? The lectures I provided you from Itia had a clear list of peer-reviewed publications supporting the claims.

2. Yes, the original question has been addressed. Dr Koutsoyiannis has offered a clear and viable alternative to the high climate sensitivity model. Many scientists support that alternative perspective. Until that credible, published alternative has been ruled out or falsified, we cannot rule out a low climate sensitivity. If you've got a flaw in Dr Koutsoyiannis' work, publish it. But please be warned you need to understand his claim first, and it is quite clear you haven't understood it yet.

3. What you arbitrarily term "abrupt shifts" are in fact part of a continuous variability quite consistent with the regional behaviour of a system exhibiting HK dynamics. I've already explained the inconsistent position of climate scientists with regard to abrupt unforced change in the holocene and abrupt change in the wisconsin glacial period. The model you are relying on needs kludges to make it work. Yet there is one model which is consistent with all of these changes, the model I have explained to you.

Sep 24, 2011 at 11:29 PM | Unregistered CommenterSpence_UK

Spence_UK


Yes, the evidence strongly suggests that Greenland was largely ice free during the Eemian.

No, it strongly suggests the opposite. Links to published studies would bolster your argument.

But the ice just a few metres from the base rock dates back to the Eemian and no later. If you want to believe the ice magically disappeared through some other mechanism, that's fine by me.

Ice sheets compress their lower layers and they spread and thin. Ice depth and timescale need to adjust accordingly. No magic required.

Sep 25, 2011 at 12:18 AM | Unregistered CommenterBBD

BBD,

Thank you, and I have read H&S. The context of my comment you quote was 20th C changes. My skepticism regarding the scientific claims in H&S derives from a number of sources: the comparison between ancient and modern climates may not be justified; the paleo data they use may not be reliable; the problems in attribution of 20th C changes.

You must be aware by now that I don't leap from here to rejection of the need for decarbonization. My point was to criticize the strategy of using science to fight arguments over energy, which is exactly what H&S do:


Rapid reduction of fossil fuel emissions is required for humanity to succeed in preserving a planet resembling the one on which civilization developed.
...

Climate change is likely to be the predominant scientific, economic, political and moral issue of the 21st century. The fate of humanity and nature may depend upon early recognition and understanding of human-made effects on Earth's climate.
...

However, the need for a CO2 target below the current CO2 amount, and the rapid emissions reduction that such a target implies, has not been recognized and acted on by the international political community. Thus there is an urgency to extract and clarify the implications of paleoclimate data for human-made climate change.
...

BAU scenarios result in global warming of the order of 3-6°C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain.

In other words, H&S also want to convince the reader regarding both how the problem should be viewed and what should be done about it. My concern is that they will succeed only in reassuring those who already agree with them, and will do nothing at all to persuade others to support their goals.

The problem for them is that the majority of people around the world want more energy, and not less. The solution, or at least the start of the solution, is to focus the world not on climate science, but on how best to increase energy supplies.

I feel the same way about the kind of discussion going on here. If the point is to develop scientific understanding, then why the heat? If the point is to work out how to deal with energy and CO2 emissions, then the discussion is ineffective, so why not tackle the issues directly?

Sep 25, 2011 at 10:37 AM | Unregistered CommenterPhilip

Spence_UK

I can see I needed to have read through your comments on the earlier posting as well. What you've said sounds very interesting, and I plan to go through your references to get a better understanding. I was aware of HK as a topic from when I was involved in research, but that was so long ago ...

It might help me orient myself a little, if I could position the Koutsoyiannis work relative to the references I suggested nearer the top:

http://www.nap.edu/openbook.php?record_id=10136&page=1

http://pielkeclimatesci.files.wordpress.com/2009/10/r-260.pdf

Don't go to any trouble please, but if you can offer anything to set the scene for me I would be very grateful.

Thanks.

Sep 25, 2011 at 11:47 AM | Unregistered CommenterPhilip

Philip

My skepticism regarding the scientific claims in H&S derives from a number of sources: the comparison between ancient and modern climates may not be justified; the paleo data they use may not be reliable; the problems in attribution of 20th C changes.

Why might not the comparision between modern and ancient climate be justified? The laws of physics do not change between glacials and interglacials. A basic point of HS11, surely?

Wrt the absolute reliability of the paleo data, HS11 explicitly states:

Fortunately, it is not necessary to have a detailed quantitative theory of the ice ages in order to extract vitally important information. In the following section we show that Milankovitch climate oscillations provide our most accurate assessment of climate sensitivity.

I'm a little surprised that you missed this.

I'm not really so sure there is a problem with attribution for the C20th warming from the mid-1970s onward. Agreed wrt the probably mistaken attribution of the 1910 - 1940 warming to solar variability and the shaky attribution of subsequent cooling to 'anthropogenic aerosols'.

In other words, H&S also want to convince the reader regarding both how the problem should be viewed and what should be done about it. My concern is that they will succeed only in reassuring those who already agree with them, and will do nothing at all to persuade others to support their goals.

The problem for them is that the majority of people around the world want more energy, and not less. The solution, or at least the start of the solution, is to focus the world not on climate science, but on how best to increase energy supplies.

Hansen's frightened. Hence the haranguing about emissions reductions not happening. But you're right: he's preaching to the choir. I assume you that Hansen isn't arguing for hair-shirts and windmills? He is strongly pro-nuclear and repeatedly cautions against energy fantasists over-hyping the potential of renewables.

I agree that the only energy policy that makes sense is one that leads to cheap abundance for the future. I also agree with HS11 on the likely value for CS. So let's get on with the necessary expansion of nuclear baseload, at least in Western democracies. Everything else is just pissing around and greenwash.

When exploring the idea of climate-as-unforced-natural-variation per Spence_UK, be aware of what is required. You may recall Deming's claim that the Team wanted to 'get rid of the MWP'. Well, Spence wants you to 'get rid' of Milankovitch and our entire understanding of the climate system and climate sensitivity. Presumably the physical properties of CO2 in the modern era will also have to get the bullet, as I suspect Spence is not too persuaded of the risk posed by AGW.

Oh, and you need to believe that the GIS largely disappeared during the Eemian, and that ice-sheed dynamics do not cause the lower layers to thin and spread. Nearly missed that.

Sep 25, 2011 at 1:30 PM | Unregistered CommenterBBD

BBD,

Why might not the comparison between modern and ancient climate be justified?

Because climate sensitivity is a linearization and because it likely depends on many factors that change over longer time periods. I might change my mind if you can offer some evidence that it is currently identical on Earth, Mars and Venus.

I'm not really so sure there is a problem with attribution for the C20th warming from the mid-1970s onward.

To clarify, I'm talking about GCM based attribution as per AR4. If that is what you understood, then what are your grounds for confidence in post-70s attribution?

I assume you [know] that Hansen isn't arguing for hair-shirts and windmills?

Yes, I do. But his behaviour still puzzles me, because it is so counter-productive with respect to his obvious goals. You said he is frightened, and I agree this may offer an explanation.

I agree that the only energy policy that makes sense is one that leads to cheap abundance for the future.

Good, I'm pleased you do. Of course, it has to be a carbon free future as well, and I think you also understand as I do that the two goals are linked.

When exploring the idea of climate-as-unforced-natural-variation per Spence_UK, be aware of what is required.

I'll try to be - I'm reasonably knowledgeable about physics in general and a number of physical topics within climate science. The HK ideas are quite close to areas I was involved in in the past, and I am interested to find out more. Whatever I find out, it is very unlikely to change my position on CO2 emissions. I think that argument is already resolved, and depends only on the fact of increasing emissions and basic physics. If there are any related articles you think I should read at the same time for the sake of balance, then please leave a link (must be public, I'm afraid, and ideally only one or two) and I'll be sure to read them as well.

Sep 25, 2011 at 3:01 PM | Unregistered CommenterPhilip

Philip

I might change my mind if you can offer some evidence that it is currently identical on Earth, Mars and Venus.

Difficult, given the extreme differences in atmospheric composition. A rather unfair request, I think ;-)

Because climate sensitivity is a linearization and because it likely depends on many factors that change over longer time periods.

HS11 addresses the factors that are influenced by climate state and the physical assumptions underpinning its estimation for CS are coherent. The fast-feedbacks estimate for +3C for CO2 is not derived from an invalid comparison of glacial and interglacial climate states. Do take another look when you get a chance.

Sep 25, 2011 at 3:22 PM | Unregistered CommenterBBD

A rather unfair request, I think

True, sorry.

Do take another look when you get a chance.

OK, I'll do that.

Sep 25, 2011 at 3:49 PM | Unregistered CommenterPhilip

Just for fun.

Milankovitch doubters will be interested in the acerbic defence of the theory by Douglas Keenan. It comes towards the end of the WSJ article by Keenan (which was featured at BH).

This is not the only instance of serious incompetence in climate science.

Over many millennia, the most important cycles in Earth's climate have been those of the ice ages, which are caused by natural variations in Earth's orbit around the sun. These variations alter the intensity of summertime sunlight. The relevant data are presented in Figure 6: One line represents the amount of ice globally and the other line represents the intensity of summertime sunlight in the Northern Hemisphere, where the effects are greatest. But notice that the similarity between the two lines is very weak.

Figure 6. Sunlight intensity (inverted) and global ice volume.

To understand what's going on, we have to consider the changes in the amount of ice globally. For example, if the amount of ice at different times were 17, 15, 14, 19, . . . , then we must subtract adjacent amounts to obtain the changes: 2, 1, -5, . . . . One line in Figure 7 shows these changes, while the other, as before, shows the intensity of summertime sunlight. Now we see that the similarity between the two lines is strong: one excellent piece of evidence that ice ages are indeed caused by orbital variations.

Figure 7. Sunlight intensity (inverted) and changes in global ice volume.

Serbian astrophysicist Milutin Milankovitch first proposed a connection between ice ages and orbital variations in 1920, though data on the amount of ice present in past millennia didn't become available until 1976. But not until 2006 did scientists first study the changes in the amount of ice. That is, it took 30 years for scientists to think to do the subtraction needed to draw the second line in Figure 7. During these three decades, scientists analyzing Milankovitch's proposed link based their studies on graphs like Figure 6, and they considered a variety of assumptions to try and explain the weak similarity of the two lines.

Sep 25, 2011 at 4:55 PM | Unregistered CommenterBBD

Keenan doesn't trouble to provide references, but the work he is referring to is: Roe, G. (2006), In defense of Milankovitch, Geophys. Res. Lett., 33, L24703, doi:10.1029/2006GL027817. Pdf here.

The paper begins:

The Milankovitch hypothesis is widely held to be one of the cornerstones of climate science. Surprisingly, the hypothesis remains not clearly defined despite an extensive body of research on the link between global ice volume and insolation changes arising from variations in the Earth’s orbit. In this paper, a specific hypothesis is formulated. Basic physical arguments are used to show that, rather than focusing on the absolute global ice volume, it is much more informative to consider the time rate of change of global ice volume. This simple and dynamically-logical change in perspective is used to show that the available records support a direct, zero-lag, antiphased relationship between the rate of change of global ice volume and summertime insolation in the northern high latitudes. Furthermore, variations in atmospheric CO2 appear to lag the rate of change of global ice volume. This implies only a secondary role for CO2 – variations in which produce a weaker radiative forcing than the orbitally-induced changes in summertime insolation – in driving changes in global ice volume.

Roe continues:

However, several challenges have recently emerged, with studies (1) arguing that variations of atmospheric CO2 and tropical sea surface temperatures played an important role in ice-age cycles [e.g., Shackleton, 2000; Lea, 2004], (2) questioning the timing of temperature changes relative to insolation and the global extent of the ice age climates [Winograd et al., 1992; Gillespie and Molnar, 1995], and (3) highlighting the relatively small fraction of total variance in global ice volume associated with insolation variations [Wunsch, 2004]. Moreover, progress has been impeded by the lack of a well-formulated, specific, and generally-accepted hypothesis. The term ‘Milankovitch hypothesis’ is used in a variety of ways, ranging from the simple expectation that one ought to see orbital frequencies in time series of paleoclimate proxies, to the implication that all climate variability with time scales between 103 and 106 yr is fundamentally driven by orbital variations. Somewhere in the middle of this are the more vague statements found in some form in many textbooks, that orbital variations are the cause, or pacemaker, of the Pleistocene ice ages. Phrases like Milankovitch curves, Milankovitch insolation, Milankovitch frequencies, Milankovitch forcing, and Milankovitch cycles pervade the literature, adding to the somewhat nebulous picture.

And concludes:

The prevailing view to date has been that ice sheet volume is the most important variable to consider. While this is obviously the case for global sea level, it is ice sheet extent that matters most for albedo, and ice sheet height that matters for atmospheric circulation [e.g., Broccoli and Manabe, 1987; Shinn and Barron, 1989]. Ice sheets are dynamic systems and these properties can vary quite differently from each other. However, the results presented here demonstrate the critical physical importance of focusing on the rate of change of ice volume, as opposed to the ice volume itself. The available evidence supports the essence of the original idea of Ko¨ppen, Wegner, and Milankovitch as expressed in their classic papers [Milankovitch, 1941; Ko¨ppen and Wegener, 1924], and its consequence: (1) the strong expectation on physical grounds that summertime insolation is the key player in the mass balance of great Northern Hemisphere continental ice sheets of the ice ages; and (2) the rate of change of global ice volume is in antiphase with variations in summertime insolation in the northern high latitudes that, in turn, are due to the changing orbit of the Earth.

Sep 25, 2011 at 4:57 PM | Unregistered CommenterBBD

Spence_UK

Wrt the evidence that there was an extensive Greenland Ice Sheet throughout the Eemian:

The website you link to fails to present a consistent alternative and I cannot find the publications that back it up.

Dorthe Dahl-Jensen is a professor of ice physics at the Niels Bohr Institute, University of Copenhagen, and project leader of the North Greenland Eemian Ice Drilling Project.

NEEM reached bedrock in July 2010. She and co-workers are preparing their findings for publication. Perhaps I should have said all this earlier, but it seemed a little OTT.

Dahl-Jensen summarises what has been learned at the link I gave you. Apologies for the misunderstanding.

Sep 25, 2011 at 9:55 PM | Unregistered CommenterBBD

I backed away from this thread because I was just getting angry at BBD switching argument every time he was unable to support his position. Such debate is like engaging with 9/11 troofers: interesting at first but ultimately maddening. Sorry BBD but your engagement here is absolutely like arguing with a troofer. Philip, I'm sorry I missed your questions because of this but (1) I had real life obligations to address and (2) the value of the debate here was near zero.

Just to go back to where we started from. BBD argued that the *only* way to explain the ice ages was with a high climate sensitivity. This is a strong claim and requires that all reasonable alternatives be rebutted. I provided clear and unambiguous evidence for an alternative. The best argument against this BBD put forward was "where does the energy come from": an argument EXACTLY analogous to idiot sceptic claims that the greenhouse effect breaks the second law of thermodynamics. When I see pseudoscientific drivel at this level, constructive debate is no longer possible and there is little point in continuing.

Having chilled about it a little bit, I'm going to answer some of BBDs questions about Greenland. Let's be clear: BBD is relying on an obscure and little held view of Greenland to support his contention that the ice ages show high climate sensitivity. First a couple of comments:

No, it strongly suggests the opposite. Links to published studies would bolster your argument.

Rubbish. Antarctica was covered in ice back beyond the Eemian, and what a surprise, the ice goes back much further than the Eemian.

Ice sheets compress their lower layers and they spread and thin. Ice depth and timescale need to adjust accordingly. No magic required.

Yes, of course the ice is compressed at the lower layers. That doesn't make it magically disappear, which would be required for your claims. It's still there, just compressed. Once again: look at Antarctica. It was covered with an extensive ice sheet during the Eemian, at (unsurprisingly) there is continuous ice found under the dome that dates back through the Eemian and earlier. The same would be true at Greenland.

Oct 22, 2011 at 2:07 PM | Unregistered CommenterSpence_UK

Right. Following on from my last post. Because BBD was too lazy to do the research into the viewpoint that Greenland retained its ice sheet during the Eemian, I went and did that research for him to understand the present position of scientists. Before going into this, remember that the reason for this is BBD's claim that low climate sensitivity is rejected from ice ages; so his view must be a strong one that no reasonable scientist would reject. In fact, his position is a minority one that just a few scientists hold. Astonishing. Of course, being open minded, we must consider it is plausible that this is correct: but being plausible and there being no other explanations are not the same thing!

The evidence that Dahl-Jansen provides in published papers all stems from Souchez 1998: A stacked mixing sequence at the base of the Dye 3 Core, Greenland. Following this, a couple of papers claimed that the sea level rise during the Eemian (assumed to be 4.5-5.5 metres) was probably largely from Antarctica, not Greenland. However, the figures 4.5-5.5 metres for Eemian sea level rise come largely from places such as Western Europe - further afield we see much larger rises during the Eemian (up to ten metres in some places). Why would we see lower sea level rise in Europe? Well, there are confounding factors (land rebound etc.) but the most obvious answer stares us in the face: change in the gravitational pull because the Greenland ice cap was largely gone.

Anyway, Cuffey wrote up a letter to Nature explaining why this view was misguided here. Huybrechts then published this study showing Greenland largely ice free, with an ice cap about 1/4 of the extent of the current one with very steep sides, contributing about 5.5m sea level rise in the Eemian.

The problem is trivial. The ice at the base of the current ice cap goes back no further than the Eemian, and if you want to retain a largely intact ice cap on Greenland during the Eemian, you need to explain this. A Greenland which is largely ice-free (except for a small, steep sided ice cap) explains this.

Dahl-Jansen needs to explain this as well. The link you provided BBD says she explains this by constraining her model to achieve this as an end result. That is, she dismisses the elephant in the room by loading her model with the assumption that the elephant isn't there. And I'm sorry, the claim that Dahl-Jansen knows because she drilled down to bedrock at Greenland is as nonsensical as suggesting that only people who core trees are capable of doing the statistical analysis required to create temperature reconstructions.

That might impress you, BBD. It doesn't impress me much, and doesn't give me any reason to doubt that there are entirely credible explanations for the ice ages other than high climate sensitivity.

Oct 22, 2011 at 2:34 PM | Unregistered CommenterSpence_UK