Guest Post by David Middleton
In my first guest post (CO2: Ice Cores vs. Plant Stomata), we discussed the merits of ice cores vs. plant stomata as paleo-CO2 measurements. One of the key stomata papers I cited was Thomas van Hoof’s “Atmospheric CO2 during the 13th century AD: reconciliation of data from ice core measurements and stomatal frequency analysis.” Van Hoof and his coauthors demonstrated that the Antarctic ice cores only reflected the low-frequency component of the CO2 “signal”…
It is well known that diffusion processes within the firn layer and the gradual enclosure of the air in the lock-in-zone of the ice lead to a reduced signal of the original atmospheric variability and may obscure high-frequency variations (e.g. Trudinger et al., 2003).
This “diffusion process” is primarily a function of snow accumulation rate. The higher the accumulation rate, the less diffusion and the higher frequency resolution. Compaction effects due to burial can also add to the diffusion process. Dr. Van Hoof’s paper presented strong correlative evidence that the plant stomata chronologies were capable of recording a much higher frequency CO2 signal than the ice cores. I was curious about whether or not any evidence of the diffusion effect on CO2 resolution could be identified in the ice cores through the correlation of CO2 mixing ratios and snow accumulation rates. NOAA’s paleoclimatology library does not include any accumulation rate data for Antarctic ice cores with published CO2 chronologies; but the accumulation rate can by approximated by calculating a sample rate.
I used data from two Antarctic ice cores (Law and Taylor Domes) over most of the Holocene (11 kya to the early 20th century) to compare the sample rate to the CO2 mixing ratio. The sample rates were calculated by simply dividing the sample depth interval by the ice age interval:
sr = [(zn – zn+1) / (tn – tn+1)]
Where sr = sample rate (m/yr), z = sample depth (m) and t = ice age (yr)
I then plotted the CO2 mixing ratio against the sample rate. Not surprisingly, there is an extremely strong correlation between the sample rate and the CO2 mixing ratio throughout the Holocene…
This makes it very clear that the low CO2 values in the Antarctic ice cores during the Holocene could easily be the result of diffusion and do not constitute valid evidence of a stable pre-industrial atmospheric CO2 level of ~275 ppmv.
References
Etheridge, D.M., L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola, and V.I. Morgan. 1996. Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn. Journal of Geophysical Research 101:4115-4128.
Indermühle A., T.F. Stocker, F. Joos, H. Fischer, H.J. Smith, M. Wahlen, B. Deck, D. Mastroianni, J. Tschumi, T. Blunier, R. Meyer, B. Stauffer, 1999, Holocene carbon-cycle dynamics based on CO2 trapped in ice at Taylor Dome, Antarctica. Nature 398, 121-126.
Van Hoof, T.B., K.A. Kaspers, F. Wagner, R.S.W. van de Wal, W. Kürchner, H. Vissher, 2005. Atmospheric CO2 during the 13th century AD: reconciliation of data from ice core measurements and stomatal frequency analysis. Tellus (2005), 57B, 351–355.
David Middleton says:
January 4, 2011 at 6:31 am
This is exactly the relationship that should exist if van Hoof’s theory that diffusion in the ice cores acts like a low pass filter is valid.
Tom van Hoof was talking about the diffusion in the firn until bubble closing, not in the ice itself! The firn diffusion indeed acts as a low pass filter.
I made a fast plot of Vostok alone (76-252 kyr), as that was the only one I could find fast with ice depth available for CO2: no trend at all. The same in your plot: the trend probably is the result of using different ice cores with different sr ratio, but similar CO2 level ranges…
Hu McCulloch says:
January 4, 2011 at 8:56 am
The data, at least, is available for free via http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_data.html .
The assumptions behind the dating are crucial, but unfortunately are only discussed in the article.
Thanks Hu, I have looked there, but the “temperature” I did find was the reconstructed (hemispheric ocean) temperature, based on dD (or d18O). I am looking for the borehole temperature that the Russians have measured from the surface to the bottom of the drilling hole at Vostok (in preparation for an eventual drilling through the last ice to reach Lake Vostok).
“”””” Ferdinand Engelbeen says:
January 3, 2011 at 3:25 pm
George E. Smith says:
January 3, 2011 at 1:51 pm
So you don’t like my usage of terms; so let me redefine; For OUT diffusion read IN diffusion; as in CO2 diffusing INTO THE ICE; which just co-incidently is diffusion OUT of the trapped air samples, INTO the cie walls around the air bubble; where rumor has it the CO2 abundance is much lower than in the air; hence the reason why Fick’s Law would be applicable; and my comment stands.
Quite confusing, your use of IN and OUT, I must say… “””””
How so confusing ? Clearly if something can pass IN to something, it clearly must simultaneously pass OUT of something else; there must be an interface to cross. And yes in the diffusion case, Fick’s law would dictate that the (net) flow of diffusant, will be in the direction from higher concentration towardws lower concentration. In fact of course the flow is both ways, as in evaporation for example; but the concentration gradient, and statistical mechanics results in a net drive in one direction; from high to low.
You talk about diffusion as if there must be cracks or channels in the “solid” such as ice, for the CO2 or other gas to pass through; and passage will continue, until these defects are “corrected”, and sealed.
Such things can exist, along grain boundaries of course; but the Physics of solid state semiconductor technology, demonstrates that no such avenues are needed. Few Crystals or solids, are as pure and defect free, as modern semiconductors, and the diffusion of impurities into them is well demonstrated and understood, despite the lack of grain boundaries or other structural defects.
Impurity species diffuse interstitially, into a completely defect free structure; depending on the lattice and impurity species. The “intruders” may remain forever interstitial, or could in some cases be incorporated into the lattice itself.
Now as to Joel’s dsecription; I take a lot of notice of his writings; and in the case under discussion; he most likely is very much more up to speed than I am; on the methodology and theory of CO2 and air entrapment in ice cores.
Now basically, I pay little attention to those details, since I have never seen a complete blow by blow description of exactly where on earth the “air sample” and its CO2 was at the time it became incorporated into either liquid water of some ice crystal, or snow crystal, nor how it ultimately was transported to say, Antarctica, and Vostok in particular, and eventually deposited on the surface.
I can tell you that in an article in Scientific American Journal for March 2005, A William F. Ruddiman (unknown to me) postulates that humans have been causing global warming for 8,000 years, and as a result heve stopped an ice age; presumably the one we were supposed to get in 1976.
On page 52 of that issue, in that article, Ruddiman shows Carbon Dioxide data from two (Antarctic) ice cores, simply designated as ice core 1 and ice core 2; and they contain data going back for 2500 years. Well ice core 1 goes from 500 BC to about 1600 AD, while ice core 2 starts at 1000 AD and goes up to 1900 AD, so they overlap from 1000 AD up to 1600 AD; so 600 years of common proxy monitoring of atmospheric CO2 abundance (allegedly).
Well the problem is, that during the 600 year overlap period; the two ice core records of atmospheric CO2 are nothing at all like each other and in fact from 1000 AD to 1200 AD the go in exactly the opposite directions, with # 2 rising very steeply and #1 falling at a good clipand then from 1400 AD to a600 AD #1 is rapidly rising, while #2 begins a steep plunge.
Both are allegedly valid proxies for atmospheric CO2 and they tell two totally different stories with no possible meaningful commonality.
So no; I do not understand the whole process of air sample entombment; nor the core recovery and ice and included air pockets treatmetn and quantitative analysis; to the extent that I believe Joel Shore, and you too do; and wouldn’t claim to; which is why I read what you guys write.
But based on data from folks such as Dr Ruddiman; who I believe is well known for his work; I have about zero confidence, that these ice cores tell us much other than in the most gross manner, about the history of the earth atmosphere. I believe the Vostok and Dome C cores cover a history of seven or eight ice ages; or maybe that is eleven; and I suspect that earth orbit shifts are involved in those events; but I think it is specious to claim that recent decades or centuries of ice cores, are accurate depictions of the history of CO2 in the atmospehre.
Take the Vostok cores for example; aren’t they somewhere in the range of 10,000 feet or so of ice; or at leas a couple of km.
So that 800,000 year old ice from the vast depths. Was that laid down virtually on bedrock all those eons ago, while the atmospheric pressure was not too low; or was that laid down on km thickness of ice at 10,000 ft altitudes, and low atmospheric pressure, to be compressed and slowly sink as the bed rock contacting ice slowly melts, and runs away in never ending rivers.
Like I say, I take what both of you say quite seriously; and in no way meant that Joel’s picture is quite wrong; simply that it is not all that clear cut; and there are arguments that can be made either way.
Was it Einstein who said only a single experiment is required to falsify an incoreect theory; no matter how much supporting evidence there seems to be.
Well Ruddiman’s ice cores #1 and #2 present such a contrary experimental result; that says ice core CO2 proxies are not dependable records of the global atmospheric CO2 abundance; and likely not for Temperature either.
David Middleton says:
But, we already know that your plot is completely devoid of meaning. What you are seeing is that there is a relationship between accumulation rate (which you misname “sample rate”) and what the CO2 level was. However, the reason for that relationship is:
(1) For Law Dome vs. Taylor Dome, you are really just seeing the fact that Law Dome captures some of the modern industrial CO2 rise and Taylor Dome doesn’t.
(2) For the others, you are seeing the fact that accumulation rate for some ice cores is strongly dependent on climate, with the accumulation rate being higher when the temperature is higher because a higher temperature generally results in more precipitation (and temperature and CO2 level are, of course, strongly correlated).
So, your plot doesn’t really tell you anything about what you really want to know. It provides exactly zero evidence for what you claim to be showing evidence of. And, all the other available data (such as the paper that bubbagyro found) seems to imply that the effect of diffusion will range from completely negligible to maybe (in the very worst circumstances) about as large as the effects due to it taking time for the bubbles to close off.
Dear Dr Middleton?
Did you have a look on work by Dr Zbigniew Zaworowski (Poland) on CO2 Ice core analysis ; I believe he wrote several studies questioning the validity of this approach
George E. Smith says:
George,
Did you notice the scale on the vertical axis? The points from the two cores are **ALL** within 6ppm of each other! The majority are within about 2ppm of each other. Sure, if you are worried about measuring the CO2 concentration to a couple ppm that might be important…but I’ve never assumed that the ice core measurements are precise to that degree anyway! (After all, we know that there are some global and annual variations in CO2 levels on that scale.)
George E. Smith says:
January 4, 2011 at 12:16 pm
You talk about diffusion as if there must be cracks or channels in the “solid” such as ice, for the CO2 or other gas to pass through; and passage will continue, until these defects are “corrected”, and sealed.
Well, you understand the physics of diffusion of solids through solid matrices. You also know how much energy it costs to bring a few nanogram of a dope into the upper fraction of a mm from a silicon wafer. In the hope that the dope doesn’t migrate out of the matrix in the next decades (even not at elevated working temperature)… Thus how much CO2 do you think will migrate into the ice matrix at -40°C with a pressure equal for ice and gas? And how much will stay there (or migrate further)?
Well the problem is, that during the 600 year overlap period; the two ice core records of atmospheric CO2 are nothing at all like each other and in fact from 1000 AD to 1200 AD the go in exactly the opposite directions, with # 2 rising very steeply and #1 falling at a good clipand then from 1400 AD to a600 AD #1 is rapidly rising, while #2 begins a steep plunge.
Ruddiman’s thesis is well known, and was discussed at RC:
http://www.realclimate.org/index.php/archives/2005/12/early-anthropocene-hyppothesis/
I have no (free) access to the article, so can’t give much comment on the ice cores. But if you (or anyone) can send me the graphs, that will do. Or I will pay the reasonable $8.
Seems strange to me, as I have a graph of all archived ice cores from Antarctica over the past 1,000 years:
http://www.ferdinand-engelbeen.be/klimaat/klim_img/antarctic_cores_001kyr.jpg
If the difference in the CO2 levels of Ruddiman’s graphs is within +/- 5 ppmv, that is within the range you find in all Antarctic ice cores for the same gas age. Most ice cores have a resolution that isn’t even fine enough to detect the difference between the MWP and LIA. Only the Law Dome (and another one, Siple Dome?) have enough resolution to show the cooling around 1600. That gives the impression of contradictory results, but it is just a matter of accuracy and resolution.
George, my question is already answered by Joel…
daniel says:
January 4, 2011 at 1:52 pm
Did you have a look on work by Dr Zbigniew Zaworowski (Poland) on CO2 Ice core analysis ; I believe he wrote several studies questioning the validity of this approach
Sorry, what Jaworowski says (the same invariable) themes since 1992 is completely outdated (by the work of Etheridge e.a., 1996 on the Law Dome ice cores), physically impossible (migration of CO2 from low to high levels) or completely wrong (he takes the ice age instead of the gas age for CO2 to compare that to atmospheric data).
See further my comment on Jaworowski:
http://www.ferdinand-engelbeen.be/klimaat/jaworowski.html
“”””” Ferdinand Engelbeen says:
January 4, 2011 at 3:15 pm
George, my question is already answered by Joel… “””””
Ferdinand, I’ll take a look at what Joel had to say regarding your question.
As to the graphs Ruddiman presented in SA, don’t waste your $8.
The actual P-P CO2 range of the two ice cores over the 2500 years; with 600 years of overlap , was from a low of 275 ppm for ice core # 2 at the 1600 end point of core #1 going to about 288ppm at 1900 for the same core.
The #1 core ranged from 277 low 650 BC to a high of around 285 at 750 AD then dropped to 278 again around 1400 to top out at 283 at the 1600 end point.
So maybe as you say; not outside the expected noise levels of those ice cores.
But if so the question remains; where does Ruyddiman get off presneting this data to Scientific American, and its audience if it is all in fact phony baloney.
If I take your caution; which I pay a lot of heed to; there is nothing whatsoever in ruddiman’s graph that he should even be writing about. He even attaches all knids of historical significance to events; whcih may be nothing more than noise, such as blaming the black death on what those ice cores “were doing”around 1350.
Turns out that that is exactly the time period, when core #1 and core #2 tell their most totally different stories.
And a Roman era plague that killed 25-40% of Europeans circa 570 AD corresponds to a wild vertical transient for core #1, while amazingly the black death of 1350 happens at the low of core #1.
Evidently total BS of the highest order; but Ruddiman presents it as gospel truth for SA to swallow. I’m taking them at their word, that the december 2010 issue I received is the last one they will ever send me.
Oh for an encore, Ruddiman takes the very first data point for core #1 at 750 BC and draws a straight line to the end point of core #2 at 1900 and calls that straight line the long term deforestation trend. Only three of the several dozen data points happen to lie above that line, and those three just barely so. Of course the two end points are by design exactly on that line.
“”””” Joel Shore says:
January 4, 2011 at 2:08 pm
George E. Smith says:
George,
Did you notice the scale on the vertical axis? The points from the two cores are **ALL** within 6ppm of each other! The majority are within about 2ppm of each other. Sure, if you are worried about measuring the CO2 concentration to a couple ppm that might be important…but I’ve never assumed that the ice core measurements are precise to that degree anyway! (After all, we know that there are some global and annual variations in CO2 levels on that scale.)
Well Joel; I don’t disagree with you or Ferdinad, one bit on that score; but don’t you think it is highly disingenuous for Ruddiman to present that apparently quite noisy and totally meaningless data, as if it was significant. It is quite misleading for SA and their readership.
Now I trust you a whole lot more than that; to present meaningful input. I don’t have time or the energy to do my own independent ice core drilling; not am I that interested in the results; but I’m sure lots of folks are.
George E. Smith says:
George,
I agree with you that that sidebar in the Scientific American article is very weak…and some of the claims (like a drop in CO2 around 500 AD) are just weirdly wrong. To be fair though, that is not the most central part of Ruddiman’s hypothesis and it also seems like a part that was messed up quite a bit in the translation between his original peer-reviewed article and the Sci Am popularization. Here is the original one: http://bill.srnr.arizona.edu/classes/182h/Climate/Ruddiman.pdf Note that Fig. 7 is the corresponding figure and it and the discussion of it are considerably better than the Sci Am article (e.g., he tells you where the data is from, he is more clear about the timing of various plagues etc, …) Probably the most important clarification is that the “deforestation trend” line that you objected to is clearly explained as being from other longer term data not shown, not simply from connecting the two points at the two ends of this graph. (See Fig. 2 for the data that was presumably used.)
Anyway, I am not really trying to defend Ruddiman’s hypothesis (which I have no strong opinion either way on), and especially this particular aspect of it which really seems to be pushing the ice core data beyond its reliabability…but just wanted to point out that going to the original peer-reviewed source you do at least get a better picture of what he is trying to say.
“”””” Joel Shore says:
January 5, 2011 at 5:24 am
George E. Smith says:
As to the graphs Ruddiman presented in SA, don’t waste your $8. “””””
Joel; thanks for the elucidation. But for the SciAm article, I would have no knowledge of either Ruddiman or his thesis. It’s nice that others have access to more dependable sources.
But perhaps it is even more important that these authors get their story straight and square; when dealing with a more lay public such as the SciAm audience.
So I am hoping that SA keep to their promise and send me no more issues of their magazine following the Dec 2010 issue; which they assure me is my last issue, after 40 years of continuous subscription plus 35 years of continuous gift subscription, for my former Tarpon and Bone Fishing guide in the Florida Keys. His issues will continue to arrive on schedule; untill I (or maybe he) ends up getting recycled to the atmosphere and environs.