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Climate Hustle

CO2 lags temperature - what does it mean?

What the science says...

Select a level... Basic Intermediate

CO2 didn't initiate warming from past ice ages but it did amplify the warming.  In fact, about 90% of the global warming followed the CO2 increase.

Climate Myth...

CO2 lags temperature
"An article in Science magazine illustrated that a rise in carbon dioxide did not precede a rise in temperatures, but actually lagged behind temperature rises by 200 to 1000 years.  A rise in carbon dioxide levels could not have caused a rise in temperature if it followed the temperature." (Joe Barton)

Earth’s climate has varied widely over its history, from ice ages characterised by large ice sheets covering many land areas, to warm periods with no ice at the poles. Several factors have affected past climate change, including solar variability, volcanic activity and changes in the composition of the atmosphere. Data from Antarctic ice cores reveals an interesting story for the past 400,000 years. During this period, CO2 and temperatures are closely correlated, which means they rise and fall together. However, based on Antarctic ice core data, changes in CO2 follow changes in temperatures by about 600 to 1000 years, as illustrated in Figure 1 below. This has led some to conclude that CO2 simply cannot be responsible for current global warming.

Figure 1: Vostok ice core records for carbon dioxide concentration and temperature change.

This statement does not tell the whole story. The initial changes in temperature during this period are explained by changes in the Earth’s orbit around the sun, which affects the amount of seasonal sunlight reaching the Earth’s surface. In the case of warming, the lag between temperature and CO2 is explained as follows: as ocean temperatures rise, oceans release CO2 into the atmosphere. In turn, this release amplifies the warming trend, leading to yet more CO2 being released. In other words, increasing CO2 levels become both the cause and effect of further warming. This positive feedback is necessary to trigger the shifts between glacials and interglacials as the effect of orbital changes is too weak to cause such variation. Additional positive feedbacks which play an important role in this process include other greenhouse gases, and changes in ice sheet cover and vegetation patterns.

A 2012 study by Shakun et al. looked at temperature changes 20,000 years ago (the last glacial-interglacial transition) from around the world and added more detail to our understanding of the CO2-temperature change relationship.  They found that:

  • The Earth's orbital cycles triggered warming in the Arctic approximately 19,000 years ago, causing large amounts of ice to melt, flooding the oceans with fresh water. 
  • This influx of fresh water then disrupted ocean current circulation, in turn causing a seesawing of heat between the hemispheres.
  • The Southern Hemisphere and its oceans warmed first, starting about 18,000 years ago.  As the Southern Ocean warms, the solubility of CO2 in water falls.  This causes the oceans to give up more CO2, releasing it into the atmosphere.

While the orbital cycles triggered the initial warming, overall, more than 90% of the glacial-interglacial warming occured after that atmospheric CO2 increase (Figure 2).

Shakun Fig 2a 

Figure 2: Average global temperature (blue), Antarctic temperature (red), and atmospheric CO2 concentration (yellow dots).  Source.

Basic rebuttal written by dana1981


Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

 

Last updated on 8 January 2016 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Related Arguments

Further reading

That CO2 lags and amplifies temperature was actually predicted in 1990 in a paper The ice-core record: climate sensitivity and future greenhouse warming by Claude Lorius (co-authored by James Hansen):

"Changes in the CO2 and CH4 content have played a significant part in the glacial-interglacial climate changes by amplifying, together with the growth and decay of the Northern Hemisphere ice sheets, the relatively weak orbital forcing"

The paper also notes that orbital changes are one initial cause for ice ages. This was published over a decade before ice core records were accurate enough to confirm a CO2 lag (thanks to John Mashey for the tip).

Also, gotta love this quote from Deltoid in answer to the CO2 lag argument: See also my forthcoming paper: "Chickens do not lay eggs, because they have been observed to hatch from them".

Further viewing

Comments

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Comments 451 to 500 out of 568:

  1. davytw @447, you have three excellent responses so far.  Let me add that it is often worthwhile taking denier cherry picks or other selective data at face value.  If you do in this case, for example, you see that the lag shown is about 1 year.  So, assume CO2 lags temperature by one year, and assume also, as per denier dogma, that there has been a pause in global temperatures.  It follows that from one year after the pause started (1995 and 1998 seem to be the preffered values) there has been a pause in the rise of global CO2 concentrations.  If your interlocuter cannot show you that pause the raw CO2 data, then it follows that either:

    There has been no pause in global temperatures; or

    CO2 does not simply lag global temperatures and the graph has been deceptively constructed to show an appearance that is not real; or both.

    In other words, even at face value combined with the raw data, the claim refutes itself.

    On a more subtle point, the partition of CO2 between the three main surface reservoirs (atmosphere, ocean and terrestial biosphere) is governed by temperature.  In particular, increased temperatures will shift CO2 from the ocean to the atmosphere.  If there are no other relevant factors, this is the primary short and medium term controller of CO2 concentration, and given the size of the effect relative to the increase in global temperature, dominates CO2 concentrations at sub decadal time scales.  However, anthropogenic emissions over a decade or more far exceed the variation in CO2 concentration caused by small temperature effects so that anthropogenic emissions control almost completely the long term rise.

    Long term temperture trends (such as occur at the start and end of glacials) can result in long term changes in CO2 concentration.  However, taking the graph your denier interlocuter provided, the ratio appears to be 1 ppmv of CO2 increase for every 2.5 C increase in temperature.  (Again the advantage of face value.  Your interlocuter must either admit that figure or something very close, or admit the graph has been deceptively constructed.)  Based on that figure, the approximate 1 C temperture increase since the preindustrial can only be responsible for about 0.5 ppmv increase on CO2 concentration ;)

    Of course that figure is way to low, because the graph is deceptively constructed.  Using the ratio between temperature and CO2 concentrations in the glacial, the actual increase in CO2 due to the 1 C increase in temperature may be as much as 20 ppmv.  Of course, that increase is itself no more natural than the warming.

  2. davytw @447.

    I have to say that your denialist has a pretty poor graph. There is a far better one, also from Wood-for-trees, up-thread @391.

    As folk here describe, there is a tiny CO2 effect due to rising global temperature but also due to ENSO which wobbles temperature & CO2 both, small (or perhaps more correctly very small) effects when compared with the direct anthropogenic inputs of CO2.

    These tiny wobbles are used by some, with added smoke and mirrors, to 'demonstrate' that rising CO2 is natural. I think Murray Salby takes the saddo prize in these works of lunacy as he has actually managed to line all his mirrors up to link this modern-day lagging of CO2 with the measured lags in CO2 in the ice core data. (If you can cope with such madness, there is a 68 minute video of his presentation here. I think it helps if you wear tin-foil hat, just to get into the spirit of the thing.)

    Encountering such mind-blowing nonsense once too often prompted me to produce this graphic (usuallt 2 clicks t 'download you attachment'). I forget whose equasion it was, possibly Humlum's. And I'm pretty certain there is a SkS version as well, somewhere.

  3. This one is isn't specifically related to "lags temperature", but it's another "CO2 / temperature - no correlation" graph, ostensibly from Doctor Richard Alley, showing the past 11,000 years from Greenhouse ice core data.  

    LINK

    My knowledge-free guess" effects from the sun are in the driver's seat here... am I somewhat right on that?  The "discussion" as it were is happening here:

    How to Talk to a Climate Change Denier

    Response:

    [RH] shortened link.

  4. Well anything from climate4you is likely.misinformation. This is discussed here and here.

    You might like to put "humlum" into the search box to see other stuff.

  5. :

    And here is the CO2 reconstruction we should be using:

    The immediate thing to notice is that the CO2 rises by 20 ppmv from 260 ppmv, leading to an expected temperature increase of 0.3 C.  Instead we get a temperature decrease of about the same amount.  Why?

    Well, here is the temperatures over the last 800,000 years:

    You will notice that after each large peak, the temperatures plummet rapidly, as we would expect with the rapidly declining NH summer insolation (which drives the timing of glacials).  There is one exception, that doesn't fall rapidly, that that is the last 10 thousand years.

    It is a natural supposition that the rise in CO2 levels (itself something we would not expect naturally) has counteracted the natural fall in temperatures we would expect from the declining NH summer insolation and greatly reduced the natural decline in temperatures.

    There is a recent post discussing just this topic. 

  6. Has this study been debunked on any other thread?

    http://file.scirp.org/Html/4-4700320_51443.htm

  7. It looks to me like these authors have been trying to promote this strange idea of adiabatic causes for warming for a long time, and have been repeatedly debunked. 

    http://wah-realitycheck.blogspot.com/2008/09/khilyuk-and-chilingar.html

    They even make the completely bizarre claim that increased methane concentrations would result in greater cooling. It's rather loopy stuff they're promoting.

  8. Quick

    From the introduction to the paper, discussing what they claim the standard model of the GH effect says -  "In particular, it assumes that the heat transfer in atmosphere occurs exclusively by radiation."

    Megafail right there.

    All the climate models include the basic physics that most heat transfer within the lower atmosphere is through conduction and evapotranspiration.

    What radiative exchange controls is the external energy balance with space. Ultimately setting the balance temperature for the mid troposphere at around 5 km up. The lapse rate then propogates  a temperature profile from that down to the surface.

  9. I haven't read the whole thing and only perused for a preliminary inspection. The paper has all the hallmarks of pseudo-science. The journal is definitely low impact. It does, however, details its review process, accessible through a few links. The list of citations of the paper includes ridiculous stuff like the Landscheidt piece in E&E, as well as the Robinson and Soon thing and an Soon/Baliunas/Robinson piece. The authors cite themselves on multiple occasions, several of these cites are books instead of peer-reviewed papers. In light of all this, I'm sure many will decide there are better uses for their time than reading and analyzing the piece to then explain where it goes wrong.

  10. Quick @456, the first thing to note is that the article is published by Scientific Research Publishing, number 405 on Jeffrey Beal's list of Predatory Publishers.  That means that despite the appearance the publisher attempts to give, the article is not in fact peer reviewed.  Rather, the article was published on the basis of a very careful peer review of money recieved from the authors to pay for posting it on an internet site (there is no associated print version).

    Turning to the paper itself, the core of the paper is found in their equation 2.  For the simplest rebutal, it is only necessary to note that the first term on the right hand side of the equation is bα, where "b is the scaling factor determined by the planet’s given surface temperature Ts in degrees Kelvin (for Earth Ts = 288.2 K)".

    In simple terms, that means that their equation does not determine the Global Mean Surface Temperature (GMST).  Rather, it requires an arbitrary value to inflate the temperature to whatever value that happens to be.  Indeed, as b is arbitrary, raising it to the power of alpha is a needless complication, and simply represent camouflage for the arbitrary nature of the term.  Further, as b > 1 for Earth, and >> 1 for Venus, that makes it plain that without this fudge factor, solar radiation plus pressure underpredicts the GMST.  Consequently, despite this being purportedly a refutation of the greenhouse effect, they need a greenhouse effect to make their theory work.

    The actual value of their fudge factor is 1.093 raised to the power of (γ-1)/γ, where γ is cp/cv, ie, the heat capacity under constant pressure divided by the heat capacity under constant volume.  In air, on Earth, cp/cv = 1.4.  Therefore, b is raised to the power of (approx) 0.286, so that the combined fudge factor is approximately 1.026.  

    Clearly that is not enough to account for the greenhouse effect, so they introduce the Earth's angle of precession,ψ, into the denominator of their middle term in a rather complex equation, reducing the denominator by a factor of 0.88, according to their claim (I have not done the maths).  Introducing this factor represents, in effect, a claim that tilting an effectively spherical body will increase the amount of sunlight it will intercept by 13.6%.  It might be supposed that their is some justification for this in that the Earth is an oblate spheroid - but it is simply not that oblate.  More importantly, they include no equation for the oblateness of the sphere in the equation, thus treating all planetary objects as having the same, very large oblateness.  As the introduction of the angle of precession cannot be justified in terms of the actual oblateness of the Earth, it is consequently just another fudge factor to account surreptitously for the greenhouse effect.  This fudge factor raises the zero greenhouse effect value for GMST from 255.8 K to 264.1 K, or an inflation by a factor of 1.03.  Including the first fudge factor raises the non-greenhouse GMST according to their model to 271 K.  That is, they predict a GMST 17 K below the actual value.  Despite this, in their figure 2 they show the surface temperature at 288 K, which strongly suggest their line in that graph was not calculated using equation 2, contrary to their claim.

    You may think I am ignoring the bulk of the paper in this critique.  The rest of the paper, however, simply plugs values into equation 2.  That is, it assumes there is not greenhouse effect to prove that CO2 has no significant effect on planetary atmospheres.

    I do agree with one decision by the authors of this paper.  If I had such a dogs breakfast as this paper, I would certainly not submit it to a proper journal for peer review either.  But neither would I submit it to the vanity press, for this paper can only deflate the reputations of its authors - however useful it may be as propaganda.

  11. @Rob, Glen, Philippe, and Tom.

    Thank you, gentlemen! My physics is a bit rusty, so at cursory glance this paper seemed to have some meat, but alas, my instincts prove correct. I knew some of you would have the proper resources readily available. Time to go educate 2 persistent skeptics. Thanks again!

  12. What do you guys make of this? The guy who posted it is trying to promote himself as a rogue genius or something.

    "First off they claim that CO2 is causing warming by absorbing infrared. They say it's preventing infrared that's in the infrared window range from being radiated away. But CO2 only absorbs 2 wavelengths of IR in that range. One peak at 9.4 microns and the other at 10.4 microns. Both peaks have absorption coefficients just less than 1%. Wien's displacement law shows temperatures radiating those wavelengths to be 95.25 deg F and 41.9 deg F. So it doesn't exactly involve much energy. Not only that, but one of those is BELOW mean global temperatures. Basically, they're trying to claim that a heater that only gets up to 95 deg F and radiates at 1% efficiency is warming the planet. LOL Hardly.

    The next thing they claim is that CO2 "absorbs" and then "re-emits" photons of IR coming from the surface, and THAT action "traps" heat by sending some of those IR photons heading back towards the surface. What they're desperately trying to describe is something called Stokes-Raman scattering, which is a type of inelastic scattering. Unfortunately for the AGW cult, there are only 2 wavelengths that get scattered by CO2, one at 7.20 microns and another at 7.78 microns. NEITHER of which are in the infrared window range. So that process doesn't stop ANY infrared from being radiated away.

    It gets even more interesting when you know how the process works. It works like this: A photon of 7.49 microns interacts with a photon that's ALREADY vibrating at the correct temperature (kinetic energy) as the photon. The photon gives up a bit of energy to the molecule which causes the molecule to now have a rovibrational component rather that just a vibrational component and the photon is scattered away at a lower energy level; now at 7.78 microns. This is called the Stokes shift.

    The next photon that's at 7.49 microns that interacts with that molecule will now absorb just a bit of energy (from the new rotational component) and will be scattered away at a slightly higher energy level; now at 7.20 microns. This is called the anti-Stokes shift. The molecule will then lose some energy, will stop rotating, and will drop back down to its original vibrational mode.

    The really emotional...err...I mean bizarre thing is, 7.49 microns isn't coming from the ground. The ground radiates between 8 and 14 microns. So 7.49 microns is INCOMING infrared. So the process is actually scattering some INCOMING infrared back out. The claim that it's scattering IR coming from the ground is a BIG LIE.

    I also said that the molecule must ALREADY be at the temperature (kinetic energy) of the photon. The temperature needed to scatter a photon that's at 7.49 microns is 236.76 deg F. On a hot day, only about 1 in 10 million CO2 molecules would be at that precise temp. On a cold day, even less. Perhaps 1 in every 30 million CO2 molecules."

    Response:

    [JH] Exactly where was this posted? 

    We want to check this out because recently we've had a spate of commenters who are attempting to disguise why they are posting denier memes on our comment threads.

  13. Did the guy cite any sources for his claims about absorption/emission spectra?

  14. I have responded in a more appropriate thread here. Please do not have any further responses to Quick on this thread.  Quick's issue is not about CO2 lagging temperature.

  15. JH, He posted as "Jim in CA" here: http://news.yahoo.com/emotions-not-science-rule-u-climate-change-debate-184843028.html?bcmt=1423418737492-0c1201c8-469f-4767-b168-0d3f364b5729_00023b000000000000000000000000-e9d055cf-f4cc-497d-9308-4e539fc56968&bcmt_s=u#mediacommentsugc_container

    Response:

    [JH] Thanks.

  16. Sorry..;link didn't work. It was in the comments of a yahoo repost of this Reuters peice: http://www.reuters.com/article/2015/02/05/us-climatechange-science-politics-idUSKBN0L92IN20150205

    Response:

    [PS] Fixed link

  17. Above you state: "The Southern Hemisphere and its oceans warmed first, starting about 18,000 years ago. As the Southern Ocean warms, the solubility of CO2 in water falls. This causes the oceans to give up more CO2, releasing it into the atmosphere."

    How does this jibe with the ocean acidification narrative which postulates that in a warmer world with a higher CO2 atmosphere the ocean will absorb more CO2 and become acidic?

    Response:

    [DB] Your question on ocean acidification is dealt with in the 18-part 'OA is NOT OK' series, written by subject matter experts in that field, as summarized in Parts 1:

    http://www.skepticalscience.com/Mackie_OA_not_OK_part_19.html

    And Part 2:

    http://www.skepticalscience.com/Mackie_OA_not_OK_part_20.html

    If you have questions, place them on the appropriate thread and the subject matter experts will respond to them.

    [PS] Its about equilibrium - outgassing from warm ocean is something for centuries in future. The above links give the details.

  18. Patrick,

    You ask about how during the ice ages the ocean could outgas CO2 while warming but now it is absorbing CO2 while the temperature increases.  You can calculate the solubility of CO2 using the formula here if you want more accuracy.   

    The issue here is that on the "skeptical" blogs they do not consider the magnitude of processes.  They presume that any effect that seems to minimize the AGW problem is the dominate one.

    According to figure 1 in the OP, the CO2 changes from about 190 ppm to about 280 ppm during an ice age.  This is a change of about 90 ppm.  The global temperature change is about 6C during the same time.  Thus the CO2 changes about 15 ppm for each 1C change in temperature.

    For AGW we have changed the CO2 from about 270 to 400 or about 130 ppm of CO2.  The temperature has risen about .8C so far.   We might expect ocean outgasing to decrease CO2 concentration in the ocean by the equivalent of about 10 ppm CO2 in the atmosphere. (the deep ocean has not come to equilibrium.  This explaination is a rough estimate)  

    According to Henry's law, the solubility of CO2 is directly proportional to pressure of CO2.   The increase in pressure has increased the solubility of CO2 about 130 ppm which is over 10 times as much as the solubility of CO2 has decreased from the increase in temperature.

    The ocean acidification narrative is correct because the increase in solubility from pressure increase is so much greater than the decrease in solubility from temperature.  The situation was different during the ice ages because the change was slow and the difference in CO2 concentration was smaller.

    As the moderator points out, the increase of temperature may cause major problems in the future as the deep ocean heats up.  After enough temperature rise the ocean will not accept any more CO2 (the temperature affect increases as the temperature increases).  Then the CO2 in the atmosphere will rise more rapidly.  The surface few hundred meters will stay acidic in any case.  The rapid change in acidity is difficult for ecosystems to adapt to.

    This calculation is old news to people who have researched the facts about AGW.  Your posts would come across better if you asked questions about what you want to learn instead of suggesting that scientists have made major errors.

  19. Maybe this isn't the appropriate thread - this article might apply to several arguments, but CO2 lag seemed to fit. Brief on LiveScience on observed greenhouse effect(vs. modeled):http://www.livescience.com/49950-greenhouse-effect-measured-us.html

  20. (Moderator, forgive the funky link. Policies/firewalls here break some features of websites - I see a basic comments box, no tabs.)

    Response:

    [RH] If I'm not mistaken, you can also just type in the html code to embed links. If you know a little html that might be a workaround.

  21. Looking at the Temp and CO2 chart, the striking thing to me is the regularity of the events and the peaks.  Seems hard to refute that we AREN'T just in another peak cycle.  How can this be refuted when all we have is relatively miniscule time slices of human impact to compare?  Empircally incompatible.

  22. TGU, we are "in another peak cycle"... otherwise known as an interglacial. The current interglacial began about eleven thousand years ago when CO2 levels rose to ~280 ppm (from a low of ~180 ppm during the previous glaciation). They then stayed at about that level (+/- 15 ppm) for thousands of years... until, starting around 1850, they began growing at a rate orders of magnitude faster than anything in the Vostok chart above. We are now at 400 ppm.

    In short, we were at the peak of a natural cycle which plays out every ~100,000 years... and then in ~150 years humans drove up the atmospheric CO2 level by an amount greater than the entire range of variation over the course of that natural cycle, with more still to come. That's how we know that we aren't "just" at the peak of the natural cycle... we started there, but are now far far above the range that cycle has ever experienced.

  23. TGU

    Try a simple calculation. CO2 concentrations vary over a glacial cycle between around 180 to 280 parts per million (ppm). The fastest rate of change is during the warming phase when they vary by that much over perhaps 10,000 years. Thats 1 ppm/century.

    Today CO2 levels are rising at around 1 ppm every 22 weeks!

  24. "How can this be refuted when all we have is relatively miniscule time slices of human impact to compare?"

    Well the usual way - decades of hard work doing measurements and examining the evidence.

    A couple of things to consider. The natural glacial cycle is driven by regular cycles in the earth's orbital parameters. The change in solar radiation at around 65N is tightly correlated with the glacial cycle because of feedbacks in albedo and GHGs that ensue. Eg see Hansen and Sato 2012. 

    The maximum milankovich forcing per century at 65N at 0.25W/m2. Compare that 1.66W/m2 from CO2 alone operating not just at one region of the earth but over the whole globe. If the natural cycle was dominant, then we would be cooling slowly now.

    Secondly, the orbital cycle have been around a long time but they can only induce the glacial cycle when global temperatures are low enough for the albedo feedback to cut in. The last time we had 400ppm CO2 in atmosphere was in the Pliocene and there were no glacial cycles then.

  25. TGU,

     You are talking about the uncertainty principle where there is never enough measurement to satisfy all consumers of science. Science, in the end, is about consensus and it starts with nomenclature.

     Do you know what 'error value' is?

  26. The article states that CO2 is a feedback that assists Milankovitch cycle warming / cooling.  Many commentaters here have also stated that Milankovitch influences are weak, and need a feedback to have any effect on climate.  Can someone explain why?

    The following graph is of Milankovitch forceing changes in the all-important high latitudes, and they are up to 100 wm2, or 25% of the average insolation strength.  In what way is this large change in insolation 'insignificant', requiring a feedback to assist it?

     

    https://www.cabrillo.edu/~rnolthenius/climate/Denialists/D-Ruddiman/milak&temp.jpg

     

    Response:

    [Rob P] - See this post: Milankovitch Cycles, by Chris Colose.

  27. Let me see if I can make that graph display properly:

    [img]https://www.cabrillo.edu/~rnolthenius/climate/Denialists/D-Ruddiman/milak&temp.jpg[/img]

  28. And other question, if I may.

    If the world warms, and increasing CO2 acts as a positive feedback, why does the temperature eventually stop increasing, and reverse? Surely more CO2 means ever increasing temperatures.

    Especially as Milanokovitch cycles are not very influential, and need a feedback, as many commentaters have said here.

    Conversely, when the world cools and reducing CO2 is less able to warm the world, why does the temperature not continue cooling and end in a snow-ball Earth?

    Surely a strongly positive feedback CO2 would be very unstable, leading to runaway temperatures both ways. But this is not what we see, because temperatures are tightly controlled withing 10ºc.

    Thanks

    Response:

    [Rob P] - The flaw in your reasoning is the assumption that CO2 would keep increasing. The leading hypothesis is that carbon stored in the Southern Ocean is released by warming. Once that ocean carbon store is exhausted, no more extra CO2 is reaching the atmosphere. So no more push in the direction of warming. 

    There is a recent paper that seems to be making a stronger case for this hypothesis: Southern Ocean buoyancy forcing of ocean ventilation and glacial atmospheric CO2. Here's some slides from a lecture given by the papers authors. 

  29. The graphic which tatelyle was trying to post @477 is below.Milankovitch temperature graph

  30. tatelyle, you're talking about a runaway warming or cooling, which can happen only if the feedbacks are greater than 1. It is obviously not the case. See the "positive feedback means runaway warming" post, use the search engine.

  31. tatelyle @476, the Earth is effectively a sphere.  No amount of rotations, changes of tilt or axial wobbles will change the total amount of energy recieved by a sphere by radiation from a distant source.  All that will change is the location on the surface that recieves the insolation.  It follows that while changes in obliquity and precession may change the insolation in July at 65 degrees North substantially, it cannot change the total amount of energy recieved by the Earth anymore than the day night cycle can.  Thinking that it can is just a more subtle version of the error that thinking that just because it happens to by night time at my current location, therefore the global average energy recieved from the Sun at the moment is zero.

    In contrast, changes in eccentricity can change the annual average insolation.  That is because with a more eccentric orbit, the Earth spends more time at a greater distance from the Sun.  That more than compensates for the briefer time spent closer to the Sun.  This effect, which is the only Milankovitch cycle that actually changes global, annually averaged insolation, is very small.  If my calculations are correct, the maximum change is of the order of 0.2% or about 0.5 W/m^2, and is typically much smaller than that.

    That being the case, milankovitch cycles can only significantly effect global temperatures if there are, not just feedbacks, but differences in th feedbacks that depend on location and season.  If the winter feedback to the change in insolation in the NH was as strong as the summer feedback, milankovitch cycles could not result in glaciations.  Likewise, if the NH feedback was as weak as the SH feedback, the milankovitch cycles could not result in glaciations.

    And of course, if there were no feedbacks, there could be no variation in the strength of feedbacks based on latitude and season.

    As it happens, the feedback that shows the most seasonal and latitudinal difference is the albedo feedback.  This is, primarilly because in the NH, lower latitude snow falls on land and can accumulate, wheras in the SH it falls on water and melts.  As a result, the NH albedo feedback is stronger.  Further, because of seasonal variation in insolation, the albedo feedback is also seasonal, being stronger in the summer than in the winter.  That means that an albedo feedback by itself could, in principle, induce glaciations from milankovitch cycles.  In practice, however, the size of the albedo feedback is fairly well known from knowledge of the size of glaciation.  It turns out to be about half of what is needed to actually explain the temperature variations.  The total temperature variations can be fully explained only by assuming an additional, greenhouse feedback.  As that feedback is predicted by radiative theory in any event, it would be obtuse to insist that it does not exist, thereby requireing a new radiative theory (which has not been supplied by the "deniers") and that we add significant complications to our theory of milankovitch cycles to make room for this non-existent new radiative theory.

  32. tatelyle @478, from the wording of your comment, I assume you are making the error discussed here.  For what it is worth, a feedback is positive if its incremental gain (g) is positive.  Because feedbacks are iterative, ie, they respond to temperature increases resulting from feedbacks just as much as they respond to those from forcings, the final response of a feedback (f) is given by the formula, f=F/(1-g), where F is the Forcing, and f and g have already been defined.  A simple look at that formula tells you that feedbacks do not "runaway" unless g is greater than or equal to 1, which is not the case.

    You may, however, by raising the issue that atmospheric CO2 concentration can be a feedback on temperature, and temperature is a feedback on atmospheric CO2.  Independent of the factors discussed by Rob P, CO2 concentration increases as a linear function of temperature, all else being equal.  In contrast, global temperature increases with the log of CO2.  That means that, all else being equal, the CO2 increase loop will quickly self damp.  You would require the CO2 concentration to increase exponentially with temperature to get a runawy effect (until oceanic CO2 was effectively exhausted). 

  33.  

     

    Tom Curtis @481.

    The figures I have read elsewhere do conform with your calculated 0.2% for the global annual insolation variation (resulting from obliquity which is varying from roughly zero to a little above 0.06). Mind, the graphic tatelyle was linking to above (as @479) originates from the web-page here that for some reason gives the figure for global annual insolation variation a lot higher at 0.7% although the graphic they show for this (their fig 3) does look to be the right shape. Still, they aren't the only folk to get their numbers wrong on this matter. This Washington Edu slide show manages to miss off the decimal point in their insolation figure, twice, giving a 0.18% change in insolation from obliquity as a rather toasty 5Wm-2.

  34. MA Rodger @483, thank you for drawing my attention to this graph:

    That shows an approximate 2 W/m^2 total range of fluctuation, but is calculated for TOA insolation.  Converting for current albedo brings that down to 1.4 W/m^2, or approx three times my calculated range.  The key point is that is still a tiny forcing.  If total anthropogenic forcing since the pre-industrial could be limited to 1.4 W/m^2, global warming would not be a problem.  Conversely, if we asssume that the temperature differences between glacial and interglacial were due to a feedback on the global annual average value (calculated as 1.4 W/m^2), then the climate sensitivity would be approximately 13 C per doubling of CO2.

    Setting that aside, however, I notice that Chis Colose calculates a similar value to mine:

    "Eccentricity is the only Milankovitch cycle that alters the annual-mean global solar insolation (i.e., the total energy the planet receives from the sun at the top of the atmosphere). For the mathematically inclined, the annually-averaged insolation changes in proportion to 1/(1-e2)0.5, so the solar insolation increases with higher eccentricity. This is a very small effect though, amounting to less than 0.2% change in solar insolation, equivalent to a radiative forcing of ~0.45 W/m2 (assuming present-day albedo). This is much less than the total anthropogenic forcing over the 20th century. However, eccentircity does modulate the precessional cycle, as we shall see."

    John Baez finds only a 0.167% range (or 0.4 W/m^2 with current albedo):

    "Now, the first important thing to realize is this: it's not obvious that Milankovitch cycles can cause glacial cycles. During a glacial period, the Earth is about 5°C cooler than it is now. But the Milankovitch cycles barely affect the overall annual amount of solar radiation hitting the Earth!

    This fact is clear for precession or changes in obliquity, since these just involve the tilt of the Earth's axis, and the Earth is nearly a sphere. The amount of Sun hitting a sphere doesn't depend on how the sphere is 'tilted'.

    For changes in the eccentricity of the Earth's orbit, this fact is a bit less obvious. After all, when the orbit is more eccentric, the Earth gets closer to the Sun sometimes, but farther at other times. So you need to actually sit down and do some math to figure out the net effect. Luckily, Greg Egan did this for us—I'll show you his calculation at the end of this article. It turns out that when the Earth's orbit is at its most eccentric, it gets very, very slightly more energy from the Sun each year: 0.167% more than when its orbit is at its least eccentric."

    Baez also shows the derivation of the result.

    Even the Washington Edu slide you link to gets the percentage change about right (0.18%), but messes up in calculating the change in forcing that results.

  35.  

    Tom Curtis @484.

    I would say your 0.2% is sound. Another person calculating this value is Tamino who arrives at 0.18% and 0.61Wm-2 for obliquity ranging "between a minimum of near zero, and a maximum of slightly less than 0.06" and prior to albedo considerations.  My take on the www.climatedata.info is that they calculated the discal forcing (as graphed in their fig 3 that you show) but then they forgot to divide by 4 when they applied that as a global forcing to calculate the percentage, which should therefore be 0.175%.

  36. It is usual, when discussing Milankovitch cycles, to dismiss any effect of obliquity on global, annual mean insolation based on the fact that the Earth is a sphere.  I did it myself @481.  As the Earth is an oblate spheroid, however, it is not strictly accurate.  In particular, on the equinoxes, the Earth presents its minimum aspect to the Sun, showing an eclipsed area of 1.2737 x 1014 m2 to the Sun.  As the Earth moves to the solstice (either winter or summer) it presents its maximum aspect, showing an eclipsed area of 1.2744 x 1014 m2.  That represents a difference of 0.05% in recieved sunlight, or approximately 0.12 W/m2.  That seasonal variation is much less than the 6.8% (16.2 W/m2) seasonal variation due to the eccentricity of the Earth's orbit.

    As obliquity changes, the maximum eclipsed area (ie the insolation at the solstice) also changes, although the minimum eclipsed area (ie, insolation at the equinoxes) does not.  Just as the variation due to the Milankovitch cycle for eccentricity (~0.175%) is much smaller than the seasonal variation, so also is the variation due to the Milankovitch cycle in obliquity much smaller than the seasonal variation.  Specifically, it represents only a 0.01% variation in the solstice insolation.  As the equinoctial insolation does not change, the variation in global, annually averaged insolation due to changes in obliquity will be much less than 0.01%.

    For perspective, these factors are also less than the difference of treating the Earth as a perfect oblate spheroid, or allowing for the additional interception of sunlight due to continents, mountains and, of course, the atmosphere.

  37. Thank you for your explanations. However:

    >>Milankovitch cycles require feedbacks.

    I understand all your arguments, that overall insolation cannot change much. However, the critical season and region for Ice Age modulation is the NH summer at northerly latitudes, because of the large NH landmass, as you say. It is this insolation that will decide whether the winter ice sheets melt, and so it is this region that will modulate Ice Ages. 

    Thus obliquity, precession and eccentricity must ALL effect ice sheet extent.  Obliquity will obviously increase summer warming at high latitudes. Precession will also increase summer warming at high latitudes, when synchronised with obliquity.  And it is the NH summer melt that is important, rather than the global energy budget. And therefore it is high latitude NH insolation and albedo that are important.

    And as that Milkanovitch graph shows, the critical summer NH insolation can change by up to 25% during the Milankovitch cycle.  Not a few wm2 here or there, but a whopping 90 wm2.  So why would the 4 wm2 provided by CO2 be significant, in comparison to the 90 wm2 of the Milankovitch cycle? 

    >>Once that ocean carbon store is exhausted, no

    >>more extra CO2 is reaching the atmosphere. 

    Thank you, but I do not follow that argument.  CO2 solubility can proceed all the way up to 60oc, so why would the 'CO2 store' be exhausted at say 20oc?  If the average southern ocean temperature in an Ice Age is 10oc, then the reducing solubility of CO2 in the ocean should continue at almost the same rate all the way to 20oc or more. So why would the outgassing of CO2 stop during an Interglacial, just because the southern ocean has reached its current 15oc temperature?

    http://www.rocketscientistsjournal.com/2006/10/_res/CO2-06.jpg

    Thank you,

    Response:

    [Rob P] - "Thank you, but I do not follow that argument."

    Read the links I provided to you earlier, particularly the recent Watson et al (2015) paper. Here's the abstract:

    "Atmospheric CO2 concentrations over glacial–interglacial cycles closely correspond to Antarctic temperature patterns. These are distinct from temperature variations in the mid to northern latitudes, so this suggests that the Southern Ocean is pivotal in controlling natural CO2 concentrations. Here we assess the sensitivity of atmospheric CO2 concentrations to glacial–interglacial changes in the ocean’s meridional overturning circulation using a circulation model for upwelling and eddy transport in the Southern Ocean coupled with a simple biogeochemical description. Under glacial conditions, a broader region of surface buoyancy loss results in upwelling farther to the north, relative to interglacials. The northern location of upwelling results in reduced CO2 outgassing and stronger carbon sequestration in the deep ocean: we calculate that the shift to this glacial-style circulation can draw down 30 to 60 ppm of atmospheric CO2. We therefore suggest that the direct effect of temperatures on Southern Ocean buoyancy forcing, and hence the residual overturning circulation, explains much of the strong correlation between Antarctic temperature variations and atmospheric CO2 concentrations over glacial–interglacial cycles."

  38. Citing denier blogs lends the opposite of credibility.

    Response:

    [PS] debunking such nonsense however is however largely what this site is about.

  39. tatelyle @487:


    "I understand all your arguments, that overall insolation cannot change much. However, the critical season and region for Ice Age modulation is the NH summer at northerly latitudes, because of the large NH landmass, as you say. It is this insolation that will decide whether the winter ice sheets melt, and so it is this region that will modulate Ice Ages."


    Causing "winter ice sheets" to melt is a feedback.  Further, it is a seaonal and regional feedback.  If the seasonal and regional feedbacks in all areas  and seasons were equally strong, then that feedback would be balanced by other, opposite feedbacks because the changes in global annually averaged insolation balance out (for all intents and purposes).  It would follow that there would be no net change in GMST.  That is the point of the arguments above.  Ergo, when you say "I understand all your arguments" but "feedback", which is essentially what you have said, you clearly do not understand the arguments at all.


    "And as that Milkanovitch graph shows, the critical summer NH insolation can change by up to 25% during the Milankovitch cycle. Not a few wm2 here or there, but a whopping 90 wm2. So why would the 4 wm2 provided by CO2 be significant, in comparison to the 90 wm2 of the Milankovitch cycle?"


    The Earth has 46 million kilometers squared of area North of 55 North, out of a total of 510 million kilometers squared.  That represents 9% of the Earth's total surface area.   Further, Summer constitutes just 25% of the year.  Ergo, your 90 W/m^2 at 65 North in Summer is equivalent to 2 W/m^2, globally averaged.  So why would the 3.7 W/m^2 globally averaged forcing from a doubling of CO2 by more significant than the 2 W/m^2 globally averaged milankovitch forcing when we ignore everything but the milankovitch forcing in Summer in the high NH?  Or the less than 0.4 W/m^2 globally averaged forcing if we include the full milankovitch forcing?

    Your original question was why do we need to infer strong feedbacks given that the seasonal/regional milankovitch forcing was strong.  Even only considering that forcing (ie, ignoring the opposite milankovitch forcings in other regions or seasons), the globally averaged mean annual temperature would only change by 0.65 C without feedbacks.  In practise it changed by about 5 C.  Further, the SH temperature changes have the same sign, and approximately match the size of the NH temperature changes.  That would be impossible unless at least one of the feedbacks was necessarilly global in nature, and quite strong (to counteract the opposite signed local milankovitch forcings in the SH). 

    Response:

    [PS] rate matters too. The change of forcing at 65N only per century is about 2 orders of a magnitude smaller than rate of change of in CO2 forcing globally.

  40. Thank you again for your replies, but they are still not making complete sense. You said:

    >> feedback would be balanced by opposite feedbacks because the
    >>changes in global annually averaged insolation balance out.

    But that is not true is it? - because as you said the NH and SH are very different, and so there is no 'balance' or equivalence between them. 

    And I can prove this to you because in the graph in post 479, the Interglacial temperature rise closely follows northern hemisphere Milankovitch forcing, and not the southern hemisphere forcing (which would happen 12,800 years later). So although the Ice Age becomes a global phenomina, it is triggered and forced by northern hemisphere Milankovitch forcing and only by northern hemisphere forcing. That is an indisputable fact. So Interglacial forcing and warming is asymmetric and regional, not global.

    So there is no 'balance' as you call it. Ice Ages are caused by NH conditions, and so your spreading out the NH forcing across the globe, and thereby reducing it to just 2 wm2 is completely erroneous. Firstly, the NH summer melt is 40% of the year, as you know. And secondly, the SH is completely irrelevant, as we has just been demonstrated. So the Milankovitch forcing is actually 6.5 wm2, before any feedbacks have been added. 

    So why would CO2 be important in this process?

    Your assertion that CO2's influence is significant is based upon a doubling of CO2, or 3.7 wm2. But that assertion is completely erroneous and misleading. The initial CO2 increase at the beginning of the Interglacial warming is about 20 ppm, and this equates to less than 1 wm2.  And why would less than 1 wm2 have any influence on world temperature?  So what will be the primary driver - 6.5 wm2 of Milankovitch warming, or  less than 1 wm2 of CO2? And since a warmer NH summer during an Ice Age is going to produce melt, exposing dirty old ice, the primary feedback is more likely to be reducing albedo than increasing CO2.

    >>Citing denier blogs lends the opposite of credibility.

    What a silly comment. Why are comments like this tolerated on a serious blog?   I did not cite anything, I borrowed a standard graph that is present on hundreds of web pages.  So do you have an issue with that graph?  Do you consider it incorrect or misleading in any way?  And if not, then why the silly comment?

  41. Actually, I would like to take back that 6.5 wm2 that I calculated above.

    Spreading the summer Milankovitch forcing all over the winter period makes no sense whatsoever. The ice-sheets are in perpetual lock-down for the winter, and nothing happens, so the ambient temperature is irrellevant.  It is only the summer melt that is meaningful.  And when I was doing research on the Himalayan glaciers, melting only occurred during sunny days, while during cloudy days and nights the glacier remained frozen.

    And if we erase the winter dilution that was suggested, the Milankovitch forcing on Ice Ages is more like 16 wm2.  And it may well be more than that, for I see no reason for spreading this forcing around the whole northern hemisphere. It is sunny days that cause most melting on a glacier, rather than raised ambient temperatures. We were in shorts and singlets during sunny days, even at 18,000 ft. So I think that this 16 wm2 should be doubled to something more like 32 wm2, to take into account the local or regional aspect of this melting process.  That is the true increase in forcing of Milankovitch cycles on northern ice-sheets over the whole Interglacial warming cycle - 30 to 40 wm2.

  42. tatelyle.

    You ask difficult questions but that is because you appear to seek answers that are simpler than the situation merits. Do also bear in mind that the full mix of forcings that result in ice age cycles is not yet fully determined which makes an informed answer a tad harder still.

    Examine that graphic @479. At first glance, the last ice age shows a reasonable match between Antarctic temperature and July insolation for 65ºN. But look again. There is a good wobble-for-wobble match but there is also a ramping down of temperature over the ice age. This ramping shows there is something else at work other than NH insolation. And look at the graph for the ice age previous to the last one. The wobbles are a far poorer match but the ramping down remains a strong feature. Further, if the ice age cycles were solely about NH insolation, why didn't the insolation maximum 170ky bp, a maximum larger than that which saw the last ice age replaced by the holocene, why did it not have the power to create an inter-glacial warming?

    Things get more interesting when we look at the Greenland temperature record. The graphic below is Figure 1 from EPICA (2006) which compares Antarctic temperatures with NGRIP from Greenland.

    NGRIP & EPICAThere are things afoot in the NH that are certainly zip to do with insolation.

    As well as insolation, we have ice albedo, CO2, CH4 and dust and added to that climatic effects. A quick back-of-the-fag-packet calculation suggests the annual energy flux arriving at 65ºN from insolation, something like 100Wm-2 ±7Wm-2, is about the same size as the net energy from the South warming latutides at 65ºN, mainly through the atmosphere.

    Answers on how ice ages happen are not simple. So are you then still happy to assert "So although the Ice Age becomes a global phenomina, it is triggered and forced by northern hemisphere Milankovitch forcing and only by northern hemisphere forcing. That is an indisputable fact."? (My bold)

  43. Rodger,

    Thank you for your considered reply and the additional datasheet.

    When I said the trigger for Interglacials was NH Milankovitch (and an indespitable fact), I think this is a correct statement.  The forceing trigger is Milankovitch, but this does not exclude feedback assistance following the initial trigger.

    I was trying to counter the previous statement by Tom that 'Causing ice sheets to melt is a feedback'.  Sorry, that is wrong.  There needs to be a trigger, before feedbacks can assist in that process.  Tom was trying to undermine the value of NH Milankovitch forcing, and I think I have successfuly countered that argument.

    As to the subsequent feedback enhancement, you name a few possibilities, but the question then becomes 'which is the primary feedback'? And I think the obvious answer is albedo. A reduction of albedo from ice (say 50%) to soil (say 10%) is highly significant, giving a local summer daytime absorption increase of about 180 wm2. That is significant, especially if we now know the primary forcing-driver is a regional NH event (melting ice sheets), which subsequently allows worldwide warming. Remembering that all net warming is tropical, which spreads out over the globe, with the higher latitudes being constant net losers of energy. 

    And if albedo is the primary feedback for Interglacial warming, then the reason for the lack of response at 170k bp is equally obvious.  In this scenario, higher latitude NH Milankovitch warming requires an albedo response to be successful, but the major dust storms did not occur until 150k bp.  So the brilliant white 170k bp ice sheets could not melt at any insolation value; while the 140k bp ice sheets were dirty, primed and ready to go. 

    Tate

     

    Response:

    [PS] Can you please reference your "obvious answer"? Hansen & Sato 2012 by contrast calculate albedo and GHG forcings to be approximately equal and match observed temperature well (Fig3). Furthermore it is not obvious how albedo can create a NH-SH teleconnection.

  44. >>Can you please reference your "obvious answer"?

    Seriously? Are we not allowed to think for ourselves?
    Can you see a flaw in my logic or argument?

     

     

    >> Hansen & Sato 2012 by contrast calculate albedo
    >>and GHG forcings to be approximately equal.

    Yes, but Hansen has made a very basic error in this paper, that should have been picked up in peer review. Why was it not?

    Hansen claims an equivalence between albedo feedback and sea level, and this is the basis for his albedo feedback calculation. So Hansen has smeared the albedo feedbacks out across the whole globe, much as Tom tried to do in post 489. But as I demonstrated in posts 490 and 493, the Interglacial initiation or trigger is SOLELY a function of increased northern hemisphere (NH) Milankovitch insolation, and so the feedbacks that assist this initiation are likely to be NH feedbacks as well. The southern hemisphere (SH) has nothing to do with the Interglacial warming process, as the Ice Age record in post 479 clearly demonstrates. Increased insolation in the SH never triggers an Interglacial, only the NH can do that.

    So Hansen's smearing of albedo feedbacks across the entire globe is incorrect, and we can therefore double his 'insolation equivalent' calculation from the outset. So Hansen's 4 wm2 figure for albedo feedbacks suddenly becomes 8 wm2. But as I pointed out previously, the albedo trigger effect is mainly a local ice sheet condition in the higher latitudes, rather than a hemispherical response, mainly caused by surface dust and dirt, so there is no point adding the entire NH into this initial calculation. Ambient temperature is not the key to ice-sheet melting, direct insolation onto the ice sheet is, as was patently obvious on the Himalayan glaciers. But if that is so, then we can restrict the albedo feedback to the ice sheets themselves, and therefore multiply the albedo forcing-feedback by 5 (the area above 55º N). And this results in a true albedo feedback-forcing of 40 wm2 (about half the Milankovitch forcing value, over the complete Interglacial warming). Which is exactly what I said in post 491, but we have approached this new calculation from a different direction - Hansen's erroneous paper.

     

     

    >>Furthermore it is not obvious how albedo
    >>can create a NH-SH teleconnection

    Because of the hemispherical heat transport process, as in the following (simplistic) diagram, with tropical heat being exported to the poles. NH ice sheet melting will cause a large net increase in net energy absorption, not only on the NH ice sheet itself but also at the retreating margins. Each annual km of ice retreat at the terminus (that is about the rate of ice retreat), will have its albedo changed from about 60% to about 30% with sandy soils, and then to about 15% as plants reestablish themselves. I make that a 4 x 10(12) watts increase in absorption at 50ºN, each year. That is a large increase in the whole-earth energy radiation budget, that will be transported to the SH pole as easily as to the NH pole.

    And once the world has begun to warm, there may then be some minor assistance to the process from CO2 and CH4. Correct me if I am wrong but my back of a fag-packet calculation for a 5,000 year ice retreat gives 4 x 10(12) w increase for albedo changes at 50ºN, annually, while CO2 influences averaged over 5,000 years are only 4 x 10(11) w. So CO2 on its own, without CH4 or H2O, is order of magnitude less in influence.

    http://www.goes-r.gov/users/comet/tropical/textbook_2nd_edition/media/graphics/tropics_surplus_heat2.jpg

     

     

    Response:

    [PS] Seriously, yes. Handwaving away a paper that is doing the hard maths is little better than sloganeering. If you can fault Hansen and Sato, then publish it. Other commentators are giving you guidance on your logic. Since I am moderating, I will try to stay out of it.

  45. tatelyle @493.

    Can we then be clear then - Ice ages are triggered. Following this they are forced by feedbacks.

    Concerning the trigger, the evidence suggests the trigger is NH insolation, in the late pleistocene this being synchronised to the 100ky orbital eccentricity cycle while in the early pleistocene this was synchronised to the 41ky orbital obliquity cycle. The trigger mechnanism is thus probably more a question of what primes the trigger than what the trigger happens to be.

    Nothing about ice ages is straightforward. Even the events that began the ice age cycles at the end of the pliocene, events bound up with the closing of the Panama isthmus, events which caused both warming and cooling, even those events are not simple and obvious.

    So to say "the trigger for Interglacials was NH Milankovitch ..., I think this is a correct statement." is not an end to the 'trigger' story. (Note you yourself @493 are speculating that dust was not present 170kybp preventing an interglacial occurring but was 150kybp for the creation of the Eemian.)

    Then concerning feedbacks, once the ice age is in progress, the idea that feedbacks will drag the climate through the cycle is perhaps simple to grasp. But, while feedbacks can be listed out, the mix of feedbacks cannot be determined accurately enough to be certain of that mix. Thus your question @493 'which is the primary feedback'? is a bit presumptive, even as a genuine question. Why should there be a single primary feedback? All the evidence points to there being no single primary feedback. So the answer you yourself give to a misplaced rhetorical question "And I think the obvious answer is albedo."is not speculative. It is wrong.

    Thus the moderator response pointing the need for reference (that you protest about @494) is no great surprise to me.

    And the remainder of #494 reminds me so much of why I wrote the first paragraph @492. I would humbly suggest you revisit it.

  46. tatelyle @493:


    "I was trying to counter the previous statement by Tom that 'Causing ice sheets to melt is a feedback'. Sorry, that is wrong. There needs to be a trigger, before feedbacks can assist in that process. Tom was trying to undermine the value of NH Milankovitch forcing, and I think I have successfuly countered that argument."


    The brazen assertion that "Causing ice sheets to melt is a feedback" is wrong evidently needs some clarification.  Specifically, you commented on the origian articles claim that:


    "In other words, increasing CO2 levels become both the cause and effect of further warming. This positive feedback is necessary to trigger the shifts between glacials and interglacials as the effect of orbital changes is too weak to cause such variation."

    (My emphasis)


    The inclusion of the two sentences is necessary to establish context, specifically that the articles claim about feedbacks related not to icesheet melt directly, but to the warming of the climate, ie, the increase in Global Mean Surface Temperature.

    With regard to the claim in the article, you asked the question:


    "The article states that CO2 is a feedback that assists Milankovitch cycle warming / cooling. Many commentaters here have also stated that Milankovitch influences are weak, and need a feedback to have any effect on climate. Can someone explain why?"

    (My emphasis)


    Given the clear context of the claim you were discussing, you were asking for an explanation as to why feedbacks must be inferred to explain the "... warming of the climate", ie changes in GMST.  At least, if you were not, you were instead indulging in the rhetorically odious, and vacuous tactic of bait and switch.  But, just to reinforce it, you yourself clearly reference effects on climate.  Once again, ice sheet melt is not a change in climate, still less the more precise change in GMST required by context.

    Formally, A is a feedback on B if and only if:

    1)  A is not B;

    2)  Changes in B cause changes in A; and

    3)  Changes in A cause changes in B.

    As such, it seems straightforward that melting of icesheets are a feedback on global GMST.  Specically, melting of ice sheets are not GMST (clause 1); changes in GMST can cause icesheets to melt (clause 2); and melting of ice sheets changes GMST by changing albedo (clause 3).  Ergo, the only way you can deny that the melting of ice sheets is a feedback in the context of this discussion is to deny that we are talking about GMST (ie, genuinely be indulging in the odious tactic of bait and switch), or deny that either the melting of ice sheets can effect GMST, or that changes in GMST can effect the melting of icesheets.  Neither of the later two seems at all probable.  The first (denying that melting icesheets effects GMST flies in the face of all the ice core evidence), while denying the second amounts to denying that the changes of state of water are controlled by temperature.

    I require urgent clarrification of what claim exactly you are basing your denial that melting of ice sheets is a feedback.  Are you employing bait and switch tactics (ie, discussing feedbacks on the melting of ice sheets rather than as required by context and your prior statements, feedbacks on changes in GMST) - or are you making extraordinary claims without evidence (ie, denying either of the two relevant cause/effect relationships).

    I also request that other commentors not engage with Tate until he has clarified what is the exact basis of his absurd rejection that ice sheet melt is a feedback.  Prima facie, and absent clear clarification such, engagement is futile in that he is merely using rhetoric to mask confused thinking rather than honestly engaging in the discussion.

  47. If a gun is fired it doesn't mean the target will be hit.

    Many things happened to make the gun fire and many things happened after the gun was fired.

    This is complexity: now measure it!

    ("Um, What should we measure first and how?")

  48. >>The trigger is synchronised to the 100ky orbital eccentricity.

    No it is not.  Count the years.  The trigger is synchronised in the later Ice Ages with increasing NH Milankovitch insolation, which is directly linked to the 25,700-year precessionay cycle.  So the cycle length is either 102,000 years or 128,000 years, as you can see.  

    Big difference. Wrong cycle equalls wrong explanation.

    >>'which is the primary feedback' is presumptive.

    Not sure why. There are always greater and lesser feedbacks. But now we have identified the correct cycle that regulates Interglacials, we are in a better position to identify the primary feedback.  And since Interglacials are SOLELY triggered by increased NH insolation events we can be pretty sure that CO2 is not the primary feedback.  If Co2 was thenorimary feedback, Interglacials could also be triggered by SH warming.  But they are not.

    The exclusivity of NH insolation triggers strongly suggests that albedo is the key factor and the key feedback regulating Interglacials. And as we know the actual high latitude insolation and the actual ice sheet insolation increase is an order of magnitude greater that CO2 can manage.

    Actual insolation increase 90 wm2 on the ice sheets (over whole Interglacial).

    Actual albedo insolation increase 170 wm2 in northern latitudes (over the whole interglacial)

    (Assumptions.  NH insolation 460 wm2.  Cloud albedo -80 wm2. Increase in albedo between ice and vegetation 50% or 170 wm2.)

    So which is going to be the primary feedback?  CO2's 4 wm2, or albedo's regional 170 wm2?

  49. Tom Curtis @496.

    I think you are asking for more than tatelyle is capable of providing. Consider his track record down this thread & make your own judgement. Below I collate the interchange that led to your comment @496. It is interesting how often tatelyle actually argues against what is his own misunderstanding. Even tatelyle's attack on Hansen & Sato (2012) leads him to his back of a fag-packet calculation @494 which looks to me as though he perhaps also mistakenly multiplied by the number of cigarettes in the packet, because with the model tatelyle tries to use the albedo effect seems to confirm Hansen & Sato. But, of course, that would never do because with tatelyle it is ABC - anything but carbon.

    TC@481 "(M)ilankovitch cycles can only significantly effect global temperatures if there are, not just feedbacks, but differences in th feedbacks that depend on location and season."

    ☻ tl@487 to TC "(You say ) Milankovitch cycles require feedbacks?

    I understand all your arguments, that overall insolation cannot change much. However, the critical season and region for Ice Age modulation is the NH summer at northerly latitudes, because of the large NH landmass, as you say. It is this insolation that will decide whether the winter ice sheets melt, and so it is this region that will modulate Ice Ages."

    ☻ TC@489 to tl "Causing "winter ice sheets" to melt is a feedback. Further, it is a seaonal and regional feedback. If the seasonal and regional feedbacks in all areas and seasons were equally strong, then that feedback would be balanced by other, opposite feedbacks."

    ☻ tl@490 to TC "(you say) feedback would be balanced by opposite feedbacks because the changes in global annually averaged insolation balance out? But that is not true is it? - because as you said the NH and SH are very different, and so there is no 'balance' or equivalence between them.

    So although the Ice Age becomes a global phenomina, it is triggered and forced by northern hemisphere Milankovitch forcing and only by northern hemisphere forcing. That is an indisputable fact."

    ☻ MAR@492 to tl "Answers on how ice ages happen are not simple. So are you then still happy to assert "So although the Ice Age becomes a global phenomina, it is triggered and forced by northern hemisphere Milankovitch forcing and only by northern hemisphere forcing. That is an indisputable fact."? (My bold)

    ☻ tl@493 to MAR "I was trying to counter the previous statement by Tom that 'Causing ice sheets to melt is a feedback'. Sorry, that is wrong. There needs to be a trigger, before feedbacks can assist in that process. Tom was trying to undermine the value of NH Milankovitch forcing, and I think I have successfuly countered that argument."

    ☻ TC@496 to tl "The brazen assertion that "Causing ice sheets to melt is a feedback" is wrong evidently needs some clarification."

  50. tatelyle @498.

    Thank you for your response to #495. It is always refreshing to meet new ideas.

    I am assuming that you have not yourself "counted the years." For my own part, I have in the past been content to take the word of others for the recent ice age cycles being 100,000 years long, this being a well-known and unchallenged finding. Thus your contrary assertion would certainly tumble much scientific work, for example, Abe-Ouchi et al (2013) which is titled "Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume." That is to say, your assertion would certainly tumble such work if your assertion were based on a shred of truth. So I did some measuring. I see no evidence for a 102ky/128ky cycle. My measurements are cursory but I feel they are adequate. They show cycles in orbital eccentricity to average 97ky(s.d. 7ky) and the ice age cycles to average 100ky(s.d. 10ky). I conclude that your bold assertion "equalls" wrong presumption.

    I would be interested to learn why you assert "There are always greater and lesser feedbacks." Certainly, as a general rule systems are driven by simple mechanisms rather than complex ones but it is well known that 'general rules' do not apply "always".

    Sadly, the remainder of your comment tumbles into incoherence. I am, for instance at a loss with what you mean by the phrases "key factor" and "key feeback" or what you mean by 'regulating interglacials.' You rely on a finding of your own calculation (presented less than confidently "correct me if I am wrong" @494 ) but you may have already noted @499 that I refute that "back of a fag-packet calculation"@494. Indeed (and here adding substance to the moderation Response@494), within the ice age dynamics it would be a surprise if any realistic calculation of Δ(NH ice albedo forcing) were greatly different to the Δ(global ice albedo forcing), which is the value presented by Hansen & Sato (2012): a surprise as all here do appear in agreement that these two quantities are effectively equivalent. The other 'back of a fag-packet calculation' you present (@498) is more of a mystery despite the list of assumptions provided. It is not at all clear by what means you convert peak 65ºN Milankovitch insolation figures for July into a figure for "over the whole interglacial" or indeed how you define that term "over the whole interglacial". I think these exemplars of the error & the lack of precision that you have managed to entwine together @498 adequately illustrate the level of incoherence @498.

    To paraphrase my comment @492 - You create bizarre answers but that is because you appear set on answers that are simpler than the situation merits. Do also bear in mind that the full mix of forcings that result in ice age cycles is not yet fully determined which makes any informed response to you a tad harder still.

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