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Shakun et al. Clarify the CO2-Temperature Lag

Posted on 10 April 2012 by dana1981

A popular myth amongst climate 'skeptics' is that historically atmospheric CO2 levels have risen after temperature increases began, and therefore it's actually temperature increases that cause CO2 increases, and not vice-versa as basic climate science and physics would have us believe.  To this point, the standard response to this myth has been that initial temperature increases have historically been caused by the Earth's orbital (Milankovitch) cycles, which in turn warm the oceans, causing them to release CO2, which in turn amplify the global warming.  Thus while the initial warming hasn't historically been caused by CO2, CO2 has amplified the warming for thousands of subsequent years, and thus is still the principal control knob governing Earth's temperature.

An intriguing new paper by Shakun et al. (2012) takes a more in-depth look at this particular myth.  Many headlines have declared that Shakun et al. have demonstrated that CO2 has historically led (rather than lagged) global warming - the reality is a little more nuanced than that, but that is the basic take-home message.  In Figure 1, the red line (Antarctic temperature) and yellow dots (atmospheric CO2) illustrate our previous unerstanding, while the blue line (global temperature) is the nuance added by Shakun et al.

Shakun Fig 2a 

Figure 1: The global proxy temperature stack (blue) as deviations from the early Holocene (11.5–6.5 kyr ago) mean, an Antarctic ice-core composite temperature record (red), and atmospheric CO2 concentration (yellow dots). The Holocene, Younger Dryas (YD), Bølling–Allerød (B–A), Oldest Dryas (OD) and Last Glacial Maximum (LGM) intervals are indicated. Error bars, 1-sigma; p.p.m.v. = parts per million by volume.  Shakun et al. Figure 2a.

What did Shakun et al. Do?

The key to this myth is that it's based on Antarctic ice core records, which are not necessarily an accurate representation of global temperatures.  In recent years there have been many studies collecting data from ice cores in Greenland, sediments drilled from the ocean floor and from continental lakes, and so forth.  Most of these proxies don't extend as far back in time as the Antarctic ice cores, but many do extend back to the last glacial-interglacial transition which began approximately 18,000 years ago, as Figure 1 shows.

Shakun et al. examined 80 such proxy records from around the globe (Figure 2), recording sea surface temperatures for the marine records and surface air temperatures.

Shakun Fig 1

Figure 2: Proxy temperature records. a) Location map. CBT = cyclization ratio of branched tetraethers; MBT = methylation index of branched tetraethers; TEX86 = tetraether index of tetraethers consisting of 86 carbon atoms; Uk37 = 37, alkenone unsaturation index. b) Distribution of the records by latitude (grey histogram) and areal fraction of the planet in 5° steps (blue line).  Shakun et al. Figure 1.

By comparing the atmospheric CO2 increase (note that since CO2 is well-mixed in the atmosphere, a single ice core record can be used as an accurate representation for CO2 - Shakun et al. used the Antarctic EPICA Dome C ice core for CO2 data) to these many different temperature records, Shakun et al. are able to discern whether the CO2 increase led or lagged temperature changes in various different geographic locations, and for the planet as a whole.

Does CO2 Lag or Lead?

This is where it really gets interesting, because the answer is yes - CO2 lags and leads.  In the Southern Hemisphere, Shakun et al. found that the temperature rise happened first, whereas in the Northern Hemisphere, the CO2 increase was first (Figures 3 and 4).

Shakun Fig 2b 

Figure 3:  The phasing of CO2 concentration and temperature for the global (grey), Northern Hemisphere (NH; blue) and Southern Hemisphere (SH; red) proxy stacks based on lag correlations from 20–10 kyr ago in 1,000 Monte Carlo simulations. The mean and 1-sigma of the histograms are given. CO2 concentration leads the global temperature stack in 90% of the simulations and lags it in 6%.  Shakun et al. Figure 2b.

Shakun Fig 5b

Figure 4:  Proxy temperature stacks for 30° latitude bands with 1-sigma uncertainties. The stacks have been normalized by the glacial–interglacial (G–IG) range in each time series to facilitate comparison.  Shakun et al. Figure 5b.

What's Going On?

What appears to have happened, based on global climate model simulations run by Shakun et al., is not all that different from our previous explanation of the supposed CO2 lag - just a bit more nuanced. 

  • As we already knew, the Earth's orbital cycles trigger the initial warming (starting approximately 19,000 years ago), which is first reflected at the highest latitudes (i.e. Greenland and the Arctic - see "Onset of seesaw" in Figure 4). 
  • This Arctic warming melted large quantities of ice, causing fresh water to flood into the oceans.
  • This influx of fresh water then disrupted the Atlantic meridional overturning circulation (AMOC), in turn causing a seesawing of heat between the hemispheres.  The Southern Hemisphere and its oceans warmed first, starting about 18,000 years ago.
  • The warming Southern Ocean then released CO2 into the atmosphere starting around 17,500 years ago, which in turn caused the entire planet to warm via the increased greenhouse effect.

In short, the initial warming was indeed triggered by the Milankovitch cycles, and that small amount of orbital cycle-caused warming eventually triggered the CO2 release, which caused most of the glacial-interglacial warming.  So while CO2 did lag behind a small initial temperature change (which mostly occurred in the Southern Hemisphere), it led and was the primary driver behind most of the glacial-interglacial warming.

According to the Shakun et al. data, approximately 7% of the overall glacial-interglacial global temperature increase occurred before the CO2 rise, whereas 93% of the global warming followed the CO2 increase.

What Does Shakun Say About Climate Sensitivity?

Shakun et al. did not address the question of climate sensitivity in their paper.  Readers may recall a paper by Schmittner et al. which used the glacial-interglacial transition to estimate climate sensitivity, and came up with an estimate (1.7–2.6°C with a best estimate of 2.3°C global surface warming in response to doubled CO2) towards the lower end of the IPCC range (2.0–4.5°C with a best estimate of 3°C).  However, this lower estimate could mostly be attributed to Schmittner et al.'s low estimate of the glacial-interglacial temperature change, of just 2.6°C globally (most previous estimates put the value at around 5°C).

Shakun et al. estimate the global surface temperature change at approximately 4°C during the transition.  A similar paper by many of the same authors recently published in the Proceedings of the National Academy of Science (Clark et al. 2012) estimates the greenhouse gas radiative forcing during the transition at approximately 2.5 Watts per square meter (W/m2), but neither Shakun nor Clark et al. estimated the total radiative forcing (including surface reflectivity changes, for example).  Overall, the estimated temperature changes and radiative forcings are both slightly lower than in Hansen and Sato (2011) (Figure 5).

Fig 2

Figure 5: Climate forcings during the ice age 20,000 years ago relative to the pre-industrial Holocene from Hansen and Sato (2011)

Similar to the implied sensitivity in Hansen and Sato, a ~4°C global temperature change caused by a ~6 W/m2 forcing corresponds to a ~2.5°C sensitivity to doubled CO2.  Note that this is a very rough estimate based on the Shakun and Clark results, but is within the IPCC climate sensitivity range.

Predictable Denial Reactions

Not surprisingly, since the 'CO2 lags temperature' myth is a climate denialist favorite (coming in at #12 on the most-used climate myths list), the reaction has predictably been one of, well, denial.  WattsUpWithThat in particular has devoted several posts denying the results in the Shakun et al. paper.  One such post quoting Don Easterbrook (of failed global cooling prediction fame) began with the following objection:

"1. They assume that CO2 is capable of causing climate changes..."

Need we continue?  We could devote an entire post to the glaring errors from Easterbrook in this WUWT post, but let's not. 

In the same post, Willis Eschenbach criticized the paper saying "My rule of thumb about these kinds of things is, no error bars … no science."  However, Shakun et al. were quite explicit about the associated uncertainties throughout the paper (for example, see the uncertainty ranges depicted in Figures 1 and 4 above).  Upon actually reading the paper, Eschenbach's criticism rings quite hollow.

Speaking of apparently not reading the paper before attacking it, a second WUWT post on Shakun et al. (also from Eschenbach) argued that their results are not valid because the glacial-interglacial warming occurred at different times in the different temperature proxies.  But that is of course the point - the Southern Hemisphere warmed before the CO2 increase, while the Northern Hemisphere warmed after (as Figures 3 and 4 show).  In looking for an excuse to reject this research, the denialists manage to miss the entire point of the paper.

In yet a third post, this one by Watts himself, Watts objects that Shakun et al. refuse to call orbital cycles the warming "trigger."  This is a little bit silly, since the authors titled an entire section of their paper The trigger for deglacial warming, discussing that the first warming (of the Arctic 19,000 years ago) was indeed triggered by orbital cycles.

An Intriguing Result

The knee-jerk denial rejections of the Shakun et al. results reflect the inconvenience of their results for the climate "skeptics."  The authors summarize their intriguing results, which expand upon our understanding of glacial-interglacial transitions.

"Lag correlations from 20–10 kyr ago suggest that the modelled global temperature lags CO2 concentration by 120 yr, which is within the uncertainty range of the proxy-based lag."


"Our global temperature stack and transient modelling point to CO2 as a key mechanism of global warming during the last deglaciation. Furthermore, our results support an interhemispheric seesawing of heat related to AMOC variability and suggest that these internal heat redistributions explain the lead of Antarctic temperature over CO2 while global temperature was in phase with or slightly lagged CO2."

Also see good coverage of this study by Climate Crocks, The Washington Post, and BBC.

Note: the Shakun et al. results have been incorporated into the CO2 lags temperature myth rebuttals.

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Comments 51 to 100 out of 161:

  1. Has a study been done on the amount of CO2 that was initially released and the amount of feedback temperature increase should have happened. Shakun et al. 2012 says 93% of the warming was caused by the CO2, could that value be confirmed with are |g^x| < |g^y| understanding of feedback's?
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  2. Michael Whittemore @51, from the article above I do not see where Shakun et al claim 93% of warming was caused by CO2. The article does say that " According to the Shakun et al. data, ... 93% of the global warming followed the CO2 increase." However, just because something follows something else does not mean it was caused by that thing. I suspect that Shakun et al believe the warming was caused by a combination of CO2, albedo changes due to the loss of continental ice sheets, methane, albedo changes due to change in vegetation, and changes in dust levels in the atmosphere among other factors. Of these, albedo changes are the largest factor, followed by CO2 and methane in that order. With respect to g, the formula is f = 1/(1-g), where f is the feedback response and g is the "gain", so named by analogy with electronic circuits. Again, the gain is not just the CO2 response, but the partial response to any warming. Any such response results in further feedbacks and hence further warming. So long as the partial response is less than 1, the total response approaches a limit equal to f (which is what we are interested in and can measure). For any study, g = 1 - 1/f, and f is approximately equal to the climate sensitivity for a doubling of CO2. Hence, base on the article above, f is probably about 2.5 and g is 0.6 for fast feedbacks; and f is probably about 6, and g 0.833 with slow feedbacks included (using Hansen's method of determination). The important thing here is not the numbers, because we cannot measure g directly. It is the understanding that the response to an initial warming does not happen all at once, but incrementally with initial responses causing further responses, and so on; and the understanding that this process is self damping provided that each incremental response is less than the temperature increase that caused that response.
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  3. Tom Curtis @52 Thanks for the explanation. As the paper says that 93% of the warming happened after the CO2 rise, is it fair to say that without the rise in CO2, there would had only been about 7% of the warming taking place?
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  4. Michael Whittemore @53, the paper is behind a paywall for me, so I cannot asses Shakun et al's claims on the subject. However, that seems unlikely to me. It is likely that there were changes in albedo concurrent with, and possibly preceding, the initial increase in CO2 levels. Based on the data from Clark et al quoted above, GHG "forcing" represented approx 40% of the total "forcings", and hence temperatures would have increased by about 2.4 C without the CO2 and methane feedback (based on Shakun et al's estimate of the temperature difference between LGM and the present). That estimate ignores the relative significance of different factors in triggering the transition which I am not able to assess. Perhaps Dana or somebody else who has read the paper can fill in with more details of Shakun et al's estimate of the relative importance of different factors in triggering the transition.
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  5. One of the perks of being enrolled in uni, I seem to be able to look at most papers for free :) You want me to email you the paper in pdf?
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  6. @Michael: Careful - copyright violation does get snooped... don't be blatant. @Tom, I will check it out, but the air is getting pretty thin for me - I am much more a geologist and jack-of-all-trades than a bona fide climate scientist/modeler. I specialize in tectonics and structural geology, with a strong interest in linked fields (of which this is one, to be sure)... but getting into the bones and guts of these models is very hard for me to do with confidence...
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  7. Tom Curtis @54 Just to add, thanks for your posts, it explained what happened much more clearly, I will also read the Clark et al. paper.
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  8. Shakun et al. do not comment on the relative importance of different factors. As noted in the Climate Sensitivity section in the above post, they do not comment on the size of radiative forcings other than greenhouse gases. I suspect they would not differ significantly from the estimates in Hansen and Sato (2011) - Figure 5 above.
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  9. It would still seem to me that without the release of CO2 you would not of had much of the ice sheet and vegetation forcing (Figure 5 in the post) occurring.
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  10. Michael @59 - I'd agree with that statement. The orbital forcing is too small to cause a lot of surface changes on its own. The CO2 feedback/forcing is critical in that respect, which is why it's the main control knob.
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  11. @Dana Critical point being that it is a control knob that will not move unless nudged... the nudging is key, and that is the Milankovitch forcing... Why is it so hard for deniers to grasp this very basic concept? (I know, I know, it's NOT hard, but it interferes with their ideologically (inflammatory snipped) driven requirements.
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    Moderator Response: TC: In compliance with the comments policy, please try to avoid suggestions of impropriety. Thankyou
  12. So it would be fair to say that when the (AMOC) stopped, the northern hemisphere cooling would have over powered the Milankovitch forcing and allowed the (AMOC) to start again?
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  13. By the look of figure 4 above, it would seem that CO2 might have lagged behind the temperature change in the regions 0-30N to 60-90S. Due to the extra cooling that is seen in the 30-90N zone, when averaged altogether with 0-90N proxy's, makes it look like the whole Northern Hemisphere lagged CO2. When this might not be the case.
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  14. Missing from the see saw discussion and from figure 5 is ice age dust. As warming commenced in the SH, dust values did not change immediately but dust sources started to become vegetated. As that became more prominent, the NH warmed. For dust by latitude see fig 1 here:
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  15. Really nice article. To enhance or continue global warming is atmospheric CO2 really necessary to explain it? What about H20? Particularily over ice fields, as ice melts the level of water vapor in the atmosphere increases dramatically and causes the atmosphere to warm because of green house heat trapping effects. The warmer the air the more water vapor it can hold. To cause significant increases in atmospheric water vapor only surface water needs to be warmed. A much larger volume of water needs to be warmed to cause significant increases in atmospheric CO2. I would think that starting with an ice aged earth, H2O would dominate any green house gas triggering effects.
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  16. William @65 - remember that water vapor is only a feedback, its atmospheric concentration dictated by the temperature of the atmosphere. The atmosphere has to warm first - that warming generally being caused by CO2 and other GHGs.
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  17. 65, William, To clarify Dana's point a little further... H2O responds very, very quickly to changes in temperature, as you yourself noted. As such, it is a fast feedback... raise air temperatures a little, water evaporates, and raises them even more. Cool the air a little, water condenses, and lowers them even more. But H2O never magically injects itself into the air on its own. Some temperature change is required to cause the H2O changes. Changes in solar insolation due to albedo changes (ice sheet growth or retreat), due to dimming aerosols (volcanic eruptions) or due to solar output changes (due to changes in solar activity or orbital configurations) represent one category of forcings. Changes in greenhouse gas concentrations (CH4, CO2, NO2) represent another forcing AND slow feedback -- i.e. something that need not be directly caused by temperature changes but can be, although in longer timeframes than the more rapidly changing H2O concentrations.
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  18. Perhaps I need something spelling out to me - but does this paper show why there is an 800 year lag just in the Antarctic proxies?
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  19. 68, Lazarus, If I'm reading the above analysis properly (I don't have access to the paper):
    1. Orbital changes cause a retreat in ice sheets
    2. Retreat causes warming and disruption of the the Atlantic meridional overturning circulation (AMOC)
    3. The disruption of the AMOC plus the warming causes a faster increase in temperatures in the Southern Hemisphere
    4. The increase in temperatures in the led to a rise in CO2
    5. The rise in CO2 caused further increases global temperatures, accelerating both the SH (Antarctic) and NH temperature increases, which releases more CO2.
    So the answer you're looking for isn't simple, but it's all there:
    • Initial trigger was orbital changes causing ice melt (albedo changes)
    • Ice melt added an unexpected factor in affecting the AMOC and a differential in warming in the two hemispheres
    • Warming in the Antarctic proceeded more quickly, starting before CO2 release -- CO2 was a feedback in response to the warming -- so CO2 lagged Antarctic temperatures by 800 years
    • Warming in the Northern Hemisphere proceeded more slowly, primarily driven by CO2, so CO2 (still, as a slow feedback) drove global temperatures higher and led temperature increases.
    Really, in the end, nothing has changed except to get a more detailed model of the interplay of factors and events.
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  20. Great post. It is maybe interesting to combine it with Schmitt et al 2012, which shows a drop in δ13Catm after 17.5 kyr BP explained as a result of upwelling old carbon-enriched waters in the southern ocean. They also report a small rise in δ13Catm before 17.5 kyr and, like Shakun, link it to rising temperatures: "After a very small increase in δ13Catm at the very end of the glacial, a sharp drop in δ13Catm starting at 17.5 kyr parallels the onset of increasing atmospheric CO2. Taken at face value, this would point to an early SST rise that preceded the onset of the CO2 increase." ... "Note, however, that this 0.06‰ excursion is within the uncertainties of our data and that other effects could also lead to this small enrichment in δ13Catm." @Sphaerica #69, the Shakun paper can be found here.
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  21. JosHag, Thanks!!!!!
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  22. 67 Sphaerica Thank you for your reply. But in the article I read things like: "The first warming was indeed triggered by orbital cycles". So it is not CO2 that started the warming that caused atmospheric H20 to increase because ice is melting, water surfaces are warming, and the atmosphere is warming, increasing its capacity to hold water vapor. It is really the increase in sun light that triggered the whole thing. As more water vapor enters the atmosphere the warming continues alowing even more water vapor to enter the atmosphere. Yes water does precipitate out but it is quickly replaced. Water is still the dominant green house gas in the atmosphere and was even more so during the ice age when even less CO2 was present. I do not understand how CO2 is needed to explain what happened.
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  23. William Haas @72, in the year 2000, CO2 contributed 31 W/m^2 of a total 152.6 W/m^2 greenhouse effect. Water vapour and clouds combined to contribute 112 W/m^2. At the Last Glacial Maximum, assuming a difference in temperatures of 7 degrees C, the total greenhouse effect dropped to about 120 W/m^2, but CO2 forcing dropped by only 4 W/m^2. If we assume a temperature difference of 4 degrees C as calculated by Shakun et al, the XXXXXXX total greenhouse effect XX dropped to 114 W/m^2. That leaves a change in greenhouse effect of between xxx 33 and xxx 40 W/m^2 to be explained, primarily by changes in Water Vapour and clouds. Even if we assume all other factors (which combined for a total 7 W/m^2 GHE in 2000) drop to zero contribution, that still leaves around xxx 26-43 W/m^2 of reduced GHE to be explained by reduction in the Water Vapour effect. These calculations do not take into account either albedo from dust, or the increased pole to equator temperature gradient during the LGM. If they are taken into account, they will explain a significant portion of that xxx 26-43 W/m^2. Never-the-less it is XXXXXXXXXXXXX highly probable that the reduction in water vapours contribution to the GHE in the LGM was greater than 15 W/m^2, and hence greater than the proportional reduction to the CO2 forcing. Therefore CO2 was a more important contributor to the total GHE at the LGM than was H2O. What is more, this is just what we would expect given the rapid decline in the saturation vapour pressure of H2O with temperature: Conversely, as temperatures continue to rise, water vapour will contribute proportionately even more to the total GHE than does CO2. Another was of saying this is that as temperature increases, so does the relative* strength of the water vapour feedback. Running that back to the LGM means that the water vapour feedback was weaker during the last glacial than it is now. That means it was even less able to trigger a self fueled runaway effect as you are suggesting than it is now. * added in edit, 10:17 AM Edited 3:25 PM of 14/4/2012 to correct error in calculation as detailed below.
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  24. Tom Curtis, thank you for your efforts and all the information you have supplied. I would think that CO2 forcing would be in proportion to the amount of it in the atmosphere. How much CO2 was in the atmosphere at LGM and how much today? I realize that the water vapor effect was less during the LGM then it is now but so was CO2. But that does not really matter. What matters is what changed because it is what changed that kept the temperture increase going. According to the article it was solar energy by means of orbital cycles that triggered the end of the ice age. According to the article CO2 levels at first did not rise because it took a while for the oceans to experieice significant warming. You state that "as temperatures continue to rise, the water vapour will contribute proportionately even more to the total GHE than does CO2" It seems to me that H2O levels alone are enough to explain the GHE effect part of triggering the end of the ice age.
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  25. @William Haas: The increase in sunlight would not have triggered anything *on its own without the prior pre-conditions being met* -- (there are many documented cases where orbital forcing/milankovitch cycles reached the same basic arrangement of insolation and heat delivery without producing a massive and global glacial meltdown... It's that the orbital forcing trigger was pulled at a time when the conditions in the Northern Hemisphere were ripe for a magnified positive feedback response to that trigger. Let's look at it this way: Do you think that over the last 30 or so million years that Antarctica has been ice-bound and covered with that sheet, there have or have not been orbital configurations (Tilt, Wobble, Precession, Ellipticity) that have delivered large and sustained highs in Southern Hemisphere insolation? I would bet that there have been such events, and that folks who model these things and try to push the orbital cycles back beyond 5 mybp have isolated just such events... And yet none of them triggered any kind of self-reinforcing feedback in anything like the way that the NH has responded to the orbital forcing over the last 3 my or so... (at least none that I know of ... and I am certainly willing to find out/admit ignorance/learn something new!!) So, even if it is the H2O(v) that is the big bad heavy hitter in the later stages of the meltdown, it's pretty clear from the available evidence and modeling that 1) these events require (or used to require!!) an orbital trigger. That the orbital trigger cannot set off the H2O(v) runaway by itself, it needs to "stage up" using more readily available, more readily releasable, GHGs to reach the threshold where there is continuous and sustainable increases year on year in GHG release and H2O(v) maintenance. I like to think about Thermite reactions when considering how this must work : 1) if you have a chunk of iron and a chunk of aluminum and a good fuse (magnesium), you have all the material you need to make thermite with not much effort.... but you cannot do it with the materials in their starting state! 2) Both the iron and the aluminum must be in the proper oxidation/reduction state, and the heat trigger must be arranged in such a way as to produce the proper amount of heat to overcome reaction barriers. 3) so only after you've powdered both metals and ensured that the Iron is in the Fe(III) oxide form and the Aluminum is pure metal (no oxides), and then mixed/homogenized the powders ... then you must trigger the reaction by attaining an ignition heat, where the magnesium comes in. So you light a match (low temperature, short lived, tiny reaction), and that lights the magnesium (much higher temperature, but slow burning, calm reaction). The lit magnesium melts the two metals at a high enough temperature that the molten aluminum (very hot!) reduces the oxidized iron in a massively exothermic (even hotter!) reaction that requires a lot of activation energy. The reaction is essentially self-sustaining, given enough reactants mixed appropriately - it supplies its own oxygen, it produces enough heat to keep the reaction going, and under water, it is hot enough to generate hydrogen, rather than quenching or smothering. So, with a little bit of sugar burning (in the muscles of your hand), you ignite a match that produces a bit of heat for a short time.... that triggers a much hotter but slow burn in the magnesium, which in turn triggers a massively hot, extremely exothermic, self-sustaining, un-smotherable reaction between Al and Fe2O3. If reactions like that can be easily envisioned and reproduced (I've done it in my backyard, and even did it with copper instead of iron, just for laughs), and involves multiple stages and triggers to climb the entropic ladder, then why would it be hard to imagine that a bit of change in insolation can trigger a bit of warming that can release a bit of CO2 that can thaw a bit of glacier... That releases a bit more CO2, thaws more glacier, and wham... up the ladder you go, releasing enough CO2 to trigger enough melting to trap enough warm water in the right place to release even more CO2... that heats the place up enough to get the H2O(v) reactions rolling, and suddenly... Some avalanches do start with a butterfly landing on a pebble...
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    Moderator Response: TC: Phrase between asterixes added by request; all caps turned to bold to comply with comments policy.
  26. William Haas, water vapour is a feedback, not a forcing. It cannot be a forcing because it precipitates out of our atmosphere in a few days. A greenhouse effect composed entirely of water vapour would collapse to an ice-house as the water vapour precipitates out and gets locked up in ice sheets (IIRC there's a paper describing this). Thus, the amount of water vapour in the atmosphere essentially depends on the temperature of the atmosphere. The rapid hydrological cycle leads to two things - water vapour easily gets into the atmosphere as a response to warming, and it easily precipitates out again as rain. [danielc - all you need is any kind of warming to add a bit to the water vapour feedback, not a large forcing to initiate it as in your analogy]. CO2 forcing provides the backbone for the other feedbacks to operate, as the CO2 cannot precipitate out quickly, lasting hundreds of years in the atmosphere. A feedback incorporating CO2 release, as occurred during deglaciation, leads to extended warming, long enough to melt some of the ice sheets, reinforcing the warming through a change in albedo. Our great CO2 release experiment is doing the same thing, only using a much larger and much quicker CO2 release, where the CO2 release does not depend on the behaviour of ocean ventilation... Water vapour feedback article on SkS
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    Moderator Response:

    [DB] Try:

    Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature, A. Lacis, G. Schmidt, D. Rind and R. Ruedy, 2010

  27. Thanks, skywatcher! And Thanks Moderators for both correcting my poorly formatted post and for the link to that article!! I like this site.
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  28. William Haas @74, the forcing of CO2 does not scale with the CO2 concentration but with the logarithm of the CO2 concentration. Consequently for a broad range of CO2 concentrations there is a constant change of CO2 forcing by 3.7 W/m^2 per doubling or halving of concentration. As the CO2 concentration at the LGM was 180 ppmv, and in the year 2000 it was 370 ppmv, the change in CO2 forcing is just over 3.7 W/m^2. The relevant formula is that deltaF = 5.35 * ln(C/C0) where deltaF is the change in forcing, ln is the natural log, C is the CO2 concentration and C0 is the initial CO2 concentration. This formula breaks down for very low and very high values of CO2 concentration, but is valid for the range of CO2 concentrations experienced over the last 600 million years of Earth's history. The water vapour contribution to the GHE also scales with the logarithm of concentration, although I do not know the exact formula. However, concentration increases exponentially with temperature so that the net effect is an approximately linear increase of GHE contribution with rising temperature. The major problem with assuming a water vapour driven emergence from the Ice Age is that it implies a very strong water vapour feedback. The total change in GHE between LGM and Holocene is somewhere between 50 and 100 W/m^2. If water vapour has driven that change from a small initial increase, then the climate sensitivity to a small perturbation in water vapour would be in multiple degrees. As ENSO causes fluctuations of plus or minus 5% in the water vapour content of the atmosphere, a strong El Nino such as 1998 would drive us into a new regime with radically increased temperatures. Clearly that does not happen. The reason, as others have pointed out, is that water vapour precipitates rapidly out of the atmosphere. Consequently it's concentration is strongly controlled by temperature. You should also recognize that the change in CO2 forcing between LGM and Holocene is very well established. If water vapour has a very strong self fueling feedback effect, it must have an equally strong feedback effect to the change in CO2 forcing, not to mention changes in albedo etc. So if you rely on a theory of water vapour driving the transition between glacials and interglacials you must then provide an ad hoc theory as to why the CO2 forcing did not result in an equally strong additional feedback.
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  29. Is there a sustained level of, or a sustained rate of change of CO2 concentration that is required to sustain consistently elevated H2O(v) levels? (Did I word that right? -- What I mean is: Is there a CO2 threshold beyond which water vapor in the atmosphere will provide strong and sustained positive feedback, or is there a particular CO2 input level relative to the starting level that is required to trigger and maintain the water vapor feedback?
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  30. danieic, Thank you for your efforts. I understand a triggered reaction but since the world is not thermite your analogy works just so far. I think that starting a simple camp fire would also be a good analogy. Modeling is not really evidence. Especially in an ice age environment, H2O is a more readily available green house gas than CO2 that can be released with just a little surface heating. It is the oceans that heat up so very slowly from year to year that help moderate the reaction but only the very surface of the ocean, or a glacier for that matter, has to be heated to release significant amounts of water vapor. Yes, not only does the water vapor need to be produced but the atmosphere needs to be warmed to contain a larger amount. There was probably a lot more going on than just green house gasses that caused the climate to change over a period of more than 10 thousand years and the evidence that we have only gives partial information on what really happened. CO2 may have played an important part but I do not see any evidence that it had to be CO2 that caused the ice age to end.
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  31. So perhaps you can name a plausible mechanism in contrast to the stated, and quite simple one (i.e. that orbital forcing drove up temperatures enough to release CO2 that then amplified the forcing signal)?
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  32. DB - thanks! That's exactly the paper I was thinking of. danielc #79, I think the answer to your question is 'no'. AFAIK there's no threshold level above or below which the WV feedback is particularly effective - the effect scales smoothly with increased temperature. Raise the temperature, add some WV. To go into more details - WV's impact is logarithmic with increasing temperature (see Tom's comment above), as with other greenhouse gases, as they are all 'bites' being taken out of the longwave IR spectrum. We would thus expect to see the impact of WV decreasing slightly with higher temperatures/concentrations of WV. However, there's so much WV in the atmosphere in relation to CO2 or CH4, that doubling the quantity is a lot harder, and so the feedback warming effect of WV will probably be effectively linearly correlated with temperature forcing. William Haas #80, you're not providing any evidence to support you ideas. Why would H2O be more readily available, given CO2 is more effective a GHG at lower concentrations? How would that WV last in the atmosphere long enough to melt glaciers and so sustain the warming (see the paper linked by DB to my above post)? "... probably a lot more going on ..." is just hand-waving, or, to be frank, appealing to leprechauns.
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  33. Shakun is a great paper and nuance is just the right word, but check out his curves for insolation, antiphase between hemispheres at lat 65.They are sooo slow. Why here, Why now? Thermite needeed. More nuances coming IMO.
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  34. The feedback effect of water vapor is not just a result of temperature but varies with climate regime. For example in glacial climates the storm tracks are weaker and further south, see and That leads to less latent heat transfer than the modern climate. Thus, the change in weather patterns from glacial to interglacial causes more warming than just from increase in WV (about 10% average) and other positive feedbacks. Another evening of water vapor (causing warming) comes from the increase in vegetation (reduction of deserts). Some of that is incorporated in fig 5 above. It is important to point out that we cannot predict the changes in weather patterns that will occur from modern CO2 warming. Some predictions are for less meridional flow (net global warming), some predictions are for more. That is obviously just one factor of many.
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  35. I was looking at figure 4 and could not help wonder if CO2 lagged the temperature rise when you left out the 30N-90N proxy's. It would seem to me the reduced cooling in the northern regions are so low, that they draw down the other proxy's. The basic idea is most of the planet may have lead CO2, but due to the large cooling in the far north, when averaged it makes it look like all of the northern hemisphere lead CO2. It might had only been the far north that lagged CO2. I think with more proxy's and a batter understanding of just how cold it was in the north might bring CO2 back into a lagging state. I graphed the 80 proxy's below.
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  36. Wow, so much response. Sorry for the delay in responding but I have been off doing other things. I really appreciate the effort that everyone has been putting in. Let me comment a little bit at a time here. 73. Tom Curtis. So you are telling me that in the year 2000 the CO2 contribution was 31 watts per square meter and at the last glacial minimum it was 27 watts per square meter. That does not seem to be much of a change. according to your chart at 22 below zero degrees f there will be essentially no water vapor in the atmosphere. Temperatures at places on the glacial surfaces may have averaged 22 below zero or less at the last glacial minimum. Of course that was not the value over the entire globe. So over the ice sheets at LGM, H2O in the atmosphere may have been almost zero yet there was always a non zero amount of CO2 providing a green house effect of 27 watts per square meter. I understant that if the input from the sun was less then than it is today then the 27 watts would be less From the discussion of the paper, the effect of the Milankovich cycle started 19,000 years ago but CO2 was not recorded to have started to rise until 2,500 years later after 7% of the temperature rise had taken place. Yes I think I understand the orbital effects. Because of all these orbital and geometry effects solar irradiance on parts of the earth gradually get more or less and the effects occour over centuries and mallena and not over just a few years. This is assuming that there are not any really long term changes in the sun's output that we do not know about. Apparently during this first 2,500 years some ice statred melting so I assume that there were ice sheet surface temperatures above -22 degrees f. Of course H2O will enter the atmosphere through sublimation so H2O can enter the atmsphere at temperatures even below freezing. At the last LGM a large portion of the earth was covered by either water or ice and that either are a source of atmospheric H2O. Weather effects are another factor effecting average levels of H2O It is hence plausable that during the first 2,500 years since the LGM as the ice sheets started to melt and as the atmosphere started to warm that average H2O in the atmosphere increased. Since H2O is a green house gas, more of it in the atmosphere would have added to warming. Of course at the LGM only part of the earth was covered with ice so the global level of H2O in the atmosphere was never zero but over the coldest parts of the ice sheets it was really probably quite low. CO2, on the ohterhand was most likely evenly distributed. So during the first 2,500 years after the LGM if not just the GHE but a change in the GHE contributed to global warming it would have been due to H2O because H2O levels most likely changed but according to the data CO2 did not. Where am I going with all of this? No, nothing sinister. So far we are talking about just the first 2,500 years since LGM I will comment further.
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  37. William Haas @86, I need to correct a major error in my post at 73. I initially, and correctly stated that the the total GHE at the LGM drops from 152.6 W/m^2 to approximately 120 W/m^2. I then had a "brain fart" and treated the 120 W/m^2 as the drop in the GHE, whereas the proper value is approximately 30-40 W/m^2. Fortunately the error effects nothing in the analysis. On the contrary, the figures make much more sense once the error is corrected. Regardless, I do apologize for the error. I will take advantage of my moderator status to correct the error so that readers of this thread are not confused by it. Turning to your major point (2nd paragraph) first, any increase in temperature will result in an increase in H2O concentration and hence a water vapour feedback. So you are absolutely correct that H2O contributed to the initial warming, and indeed to the total warming from glacial to interglacial. It contributed along with other feedbacks by strengthening a relatively weak forcing (GHG plus Ice albedo) that would have only caused a change in global temperature of about 2 degrees C so that the change was at double to three times that. The key point that other people are disputing is that it only contributed as a feedback. Absent the change in CO2 levels and ice sheet albedo, the water vapour feedback could not have prevented tempertures relapsing back to the LGM average once the Milankovitch forcing passed. So what you are looking at is the Minkovitch forcing causing the minor melting of NH ice sheets and a small rise in temperature, which leads to an increase in the water vapour (and other short term feedbacks) which leads to a stronger response in long term feedbacks such as GHG levels and Ice Sheet Melt which switches the Earth to a new stable climate state. Without the short term feedbacks, the long term feedbacks would not have been strong enough to make the switch. But without the long term feedbacks, the short term feedbacks would not have been stable enough to sustain the switch. Consequently climate scientists concentrate on the effects of the long term feedbacks, which can be treated as forcings for determining climate sensitivity when comparing the LGM to the Holocene. Note that it certainly is not the case that the initial water vapour response can feed back itself to bring about the glacial to inter-glacial transition. I, probably mistakenly, have understood you to be suggesting that it can. The suggestion that it can leads, however, to absurd conclusions. Specifically, it means the water vapour feedback is already strong enough to trigger a runaway global warming, which we know is not true. On minor points, the greenhouse effect is not constant at all latitudes, and in fact decreases in strength towards the poles. This is partly because the strength of the GHE is a function of the difference in temperature between the emitting GHG and the surface, and the fact that at the effective altitude of radiation for CO2, there is a much smaller temperature difference between equator and pole than there is at the surface; and partly because of the reduced H2O concentrations as you approach the pole. What does increase as you approach the poles is the ratio of greenhouse warming to insolation, which is why increasing GHG result in a reduced temperature differential between equator and pole. Note that the strength of the greenhouse effect does not depend directly on the total insolation, but only on the temperature difference between different levels of the atmosphere, and the GHG concentration at those altitudes. As to the approx 4 W/m^2 difference in CO2 forcing between LGM and late 20th century, it is small, but so is the percentage change in global temperature (1%). However,though a small change of temperature on the absolute scale, it was a massive change of temperature for human and ecological consequences.
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  38. William Haas @86 I am going to put it in a more simply way and see what you have to say. The orbital warming only happened in the far north at first, shown in figure 5. The initial warming is caused by the sun and all the feedback's that occur when there is an added forcing. With the onset of the seesaw, both 30-60N and 60-90N start to cool while all of the other latitudes start to warm. With H2O only being a short time GHG that does not mix as well as CO2, H2O can not explained the increased warming seen in the far north. Due to the north's temperature increase lagging CO2 and the fact that the (AMOC)was not causing any warming up there, H2O can only really be seen as a regional short term positive feedback that could not have caused a global warming as is seen. I personal think that if CO2 was not the powerful greenhouse gas the science community has shown it to be, then the (AMOC) would simply had come back on when the northern regions cooled enough. But due to them following CO2 increase (as expected) the (AMOC) could not be brought back on.
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  39. "The key to this myth is that it's based on Antarctic ice core records, which are not necessarily an accurate representation of global temperatures." I said about the unreliability of ice cores a while back and got ripped to pieces for it ... Glad to see the world is - slowly - coming round to my way of thinking.
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  40. "I said about the unreliability of ice cores a while back and got ripped to pieces for it" Actually, from your very first comment in this forum you made unsupported assertions that displayed an uneven level of understanding about a number of topics. You then attracted more attention to yourself by employing rhetoric to deride the comments of others in response to you, rather than responding with sourced supportive links (you attacked the commentators rather than the arguments). WRT the issues raised in the OP, you are repeating the same inadequate understandings of issues now as you did then. The ice core records were the best overall record we had. Shakun provides a new reconstruction that is even more global than the ice core records. This does not invalidate the ice core records in any fashion whatsoever. It improves our understanding of things, not detracts from the validity of the cores. That is how science works, as opposed to rhetoric. So the world of science will not be coming around to your way of thinking.
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  41. @ Daniel Bailey 90 I think this Comment sums up lurgee's view on proxy data: "So feel free to provide evidence that the experts regard the ice core record as 'clean' and relaibale, rather than a confused, torturous mess which, unfortunately, happens to the best we've got, and ever will get."
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  42. @lurgee: "Unreliable" is a very different animal from "not necessarily an accurate global estimate"... Climatologists, glaciologists, chemists, geologists and really any plain old geologist trying to understand large scale phenomena by piecing together evidence from individual locations knows that 1 site is not representative of a global system. The geological/climatological community has known from the very inception of polar research that the data collected there would not be fully representative of the global system - there is no one place on earth that is fully representative of the climate system. The polar data is extremely reliable: it is repeatable, you can find the same signals, trends, and local maxima/minima from core to core and from site to site across large distances, both arctic and antarctic. It is also extremely reliable in that there is strong, repeatable, and independently testable correlation and internal consistency between individual measurement systems: dust vs bubbles vs layer thicknesses vs isotopic records vs nearby seafloor sedimentary records. These have been put together and independently replicated numerous times. The ice core data is not unreliable... it is also not fully representative... that's why the Shakun paper is so good - because it incorporates records of similar extent and resolution from regions that encompass disparate subsystems in the global climate over relatively long periods of time. Shakun et al extend, rather than disprove or degrade or displace the existing polar records.
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  43. lurgee @89 - we never said that ice cores are unreliable. Quite the contrary. They are, however, only representative of a single location.
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  44. I know its not a big deal but I feel sort of short changed with these proxy's, due to the area-weighting. I do find it amusing that Willis Eschenbach at WUWT when talking about the Shakun paper never averaged the proxy data because he knew unless he area-weighed them he would not get the same data as in the paper.
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  45. "The geological/climatological community has known from the very inception of polar research that the data collected there would not be fully representative of the global system - there is no one place on earth that is fully representative of the climate system." That was rather my point, as I recall. Someone had made a comment about the vexacious lag issue, based on the Vostok (?) ice core. I pointed out to him that this was a single proxy, and you couldn't read too much into what it showed, unless it was confirmed by other sources. I suggested what an individual core showed might be global trends, local trends, or just 'noise.' This seemed to provoke an unwonted amount of ire, and it was assumed I was trying to dimish the utility of ice cores themselves. Not so. Now it turns out that 93% of the interglacial temperature increase followed the CO2 increase - something that had previosuly been obscured by focusing (understandably) on Antarctic ice cores, which were among the 7% where the temperature increase preceded the CO2 increase. CO2 as a potent GHG confirmed! Forgive me for feeling slightly vindicated. I'm only (semi) human. I might occasionally use language loosely - I accept, with hindsight, 'unreliable' has very different connotations to 'not necessarily an accurate global estimate' though my intented meaning was similar - but my contention seems to be borne out. But then, I did point out that I'm not a scientist.
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    Moderator Response: TC: The "Lurgee was right/wrong" debate is off topic on this thread. If you want to continue it, you can argue the science where the original comments where made. Future comments on Lurgee being right or wrong will be deleted. You are of course free to raise science issues discussed by Lurgee and Antagonists here provided they are relevant to the OP, and your discussion makes it clear how they are relevant.
  46. I again want to thank everyone for the effort they have been putting in here. I apologize that I cannot respond to everyone at once. I am trying to respont but just a little at a time. 75 danieic It seems to be the assumption here that the end of the last ice age was triggered by Milankovitch cycles and hence an increase in sunlight at least to certain parts of the earth at certain times of the year. The initial trigger started 19,000 years ago and the level of CO2 started to rise 17,500 years ago with 7% of the temperature rise associated with the ending of the ice age having taken place. The ends of ice ages have been occouring with regularity over the past half million years and I doubt that over that period any Milankovitch cycle triggering has been missed. Earlier possible Milankovitch cycle triggering misses is beyond the scope of this discussion. Other solar related causes may have been involved but we do not have any data on that. The ocean works as a giant, non-linear thermal capacitor that can keep the effect of additional heating on going for quite some time. What makes bodies of water such a different thermal capacitor than land or ice is internal convection. Melt water would add to this thermal capacity without adding appreciable CO2 to the atmosphere. It is only after larger volumes of existing ocean water are warmed, not just surface waters, that measurable global levels of CO2 would be observed. For 2,500 years the Milankovich triggered cycle triggered the ice sheets to start melting with an appreciable increase in CO2. Melting ice also adds H2O to the atmosphere. If a green house gas was involved to enhance the warming in the first 2,500 years then it had to be H2O which covered most of the globe in some form or another at that time. After 17,500 years ago it is a diffrent story because CO2 was being added to the atmosphere. Please be patient for I realize that I have other posts to reply to.
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    Moderator Response:

    [DB] "Melting ice also adds H2O to the atmosphere."

    Incorrect. Melting ice returns once-frozen water back to the water cycle. Only long-term upward perturbations in temperature can raise the moisture carrying capacity of air (humidity). As has been observed with the 4% increase in global humidity over the past 40 years.

  47. 76 skywatcher Again, thank you for your efforts. Feedbback and forcing are primarily modeling terms and are not laws of nature. Relative humidity varies with weather. As water precipitates out it is often quickly replaces if it is available like over bodies on water or ice or in the presense of clouds. It is the average H2O content of the atmosphere that increase with temperature. That idea is used in modeling CO2's effect on climate because more CO2 increases average temperature which in turn allows the average amount of H2O in the atmosphere to increase which in turn traps more heat. Just exhausting H2O into the atmosphere does little to add water vapor content because it quickly precipitates out but if the atmosphere is warmed it is another story. So at the end of the LGM as the atmosphere gradually warmed so the water vapor content would increase. The source of the heating does not matter. The earth as a green house contains a lot of other gases other than H2O and CO2. These gases are not thermally inert. The entire earth did not freeze over during the recent ice ages. The CO2 level never dropped to zero. According to this article, a significant increase in CO2 did not start to take place until 2,500 years after the start of the warm up had begun and 7% of the warmup had begun. There is no imperical data uncovered by this article to show that in the first 2,500 years since LGM that the green house gas, an increase in average atmospheric H2O did not have some roll in global warming. At this point, how much of a roll is speculation.
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    Moderator Response:

    [DB] "Feedbback and forcing are primarily modeling terms and are not laws of nature."

    Incorrect. These are terms used to describe physical, real-world processes and reactions.

    You would be advised to follow the advice you have been given, do more reading and less commenting, as your position contains many holes and inadequacies.

  48. lurgee @95 You have made it quite clear that one proxy temperature data is not a good global representative and you have also made clear what you think about proxy data when you said "So feel free to provide evidence that the experts regard the ice core record as 'clean' and relaibale, rather than a confused, torturous mess which, unfortunately, happens to the best we've got, and ever will get." You go on to say that “Now it turns out 93% of the interglacial temperature increase followed the CO2 increase something that had previosuly been obscured by focusing (understandably) on Antarctic ice cores, which were among the 7% where the temperature increase preceded the CO2 increase.”. The paper only says that 93% of the warming happened after CO2 rise, when I graph all the proxy’s I find that only the far north of the planet lagged behind CO2 increase, when the latitudes 30N to 90S are averaged out, which can be seen here . Also to say that a particular proxy lagged CO2 you would have to compare it to the CO2 record. Each proxy data used is individually graphed here here here here here here But due to area weighting done in the paper, it would be hard to determine what proxy data was averaged together, without knowing this you could not compare your results with the paper. Like I explained before, I think that the only reason why when all averaged out the proxy data shows a temperature lag behind the CO2 rise, is due to the increased cooling seen in the north which bring down the other proxy’s when averaged out. I also found a serious issue with one of the proxy data sets, the (TR163-22) Lea et al., 2006. It had over ten proxy data records that were -999 degrees Celsius !! You can find the excel data sheet for all the proxy’s used here William Haas @96 You are missing the basic points of the Shakun’s paper, you say “If a green house gas was involved to enhance the warming in the first 2,500 years then it had to be H2O which covered most of the globe in some form or another at that time.” There was only warming in the far north, look at figure 4. It is clear as day, no global warming.. The orbit warming only affected the far north which caused the (AMOC) to stop. H2O would have had played a part in this small warming up in the north, because in that region there was warming from extra sun hitting that area. More sun means higher temperatures, means more H2O in the atmosphere, but only in the far north because H2O is temperature dependent. Due to the (AMOC) stopping the north cooled and the south warmed, H20 would have also played a part in this initial warming in the south. But what you fail to understand is, the north regions of 30N-90N are cooling due to no warm water coming in from the (AMOC). But due to the rise in CO2, and the fact there is no way H2O being produced in the south could make its way up to the north where it is much colder, CO2 is the only GHG that could have done it.
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  49. I should add, thanks to Dana1981 for a very thorough, but easy to follow summary of the paper. Now that I've got my head around that, I might venture over to WUWT to see what the Other Side are saying.
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  50. William Haas #97
    There is no empirical data uncovered by this article to show that in the first 2,500 years since LGM that the green house gas, an increase in average atmospheric H2O did not have some role in global warming. At this point, how much of a role is speculation.
    I may be missing some subtlety here, but surely the figure of 7% proposed by Shakun et al for the initial, pre CO2 warming includes any feedback due to water vapour - as also does the 93% for the CO2 de-gassing ? Hence the percentages for warming can be equated with forcings, and considerations of water vapour can be ignored.
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