From the University of Bristol
Carbon cycling was much smaller during last ice age than in today’s climate
Atmospheric carbon dioxide (CO2) is one of the most important greenhouse gases and the increase of its abundance in the atmosphere by fossil fuel burning is the main cause of future global warming. In past times, during the transition between an ice age and a warm period, atmospheric CO2 concentrations changed by some 100 parts per million (ppm) – from an ice age value of 180 ppm to about 280 ppm during warm periods.
Scientists can reconstruct these changes in the atmospheric carbon stock using direct measurements of atmospheric CO2 trapped in air bubbles in the depth of Antarctica’s ice sheets. However explaining the cause of these 100ppm changes in atmospheric CO2 concentrations between glacial and interglacial climate states – as well as estimating the carbon stored on land and in the ocean – is far more difficult.
The researchers, led by Dr Philippe Ciais of the Laboratoire des Sciences du Climat et l’Environnement near Paris, ingeniously combined measurements of isotopes of atmospheric oxygen (18O) and carbon (13C) in marine sediments and ice cores with results from dynamic global vegetation models, the latter being driven by estimates of glacial climate using climate models.
Dr Marko Scholze of the University of Bristol’s School of Earth Sciences, co-author on the paper said: “The difference between glacial and pre-industrial carbon stored in the terrestrial biosphere is only about 330 petagrams of carbon, which is much smaller than previously thought. The uptake of carbon by vegetation and soil, that is the terrestrial productivity during the ice age, was only about 40 petagrams of carbon per year and thus much smaller: roughly one third of present-day terrestrial productivity and roughly half of pre-industrial productivity.”
From these results, the authors conclude that the cycling of carbon in the terrestrial biosphere – that is, the time between uptake by photosynthesis and release by decomposition of dead plant material – must have been much smaller than in the current, warmer climate.
Furthermore there must have been a much larger size of non-decomposable carbon on land during the Last Glacial Maximum (the period in the Earth’s history when ice sheets were at their maximum extension, between 26,500 and 19,000 years ago).
The authors suggest that this inert carbon should have been buried in the permanently frozen soils and large amounts of peat of the northern tundra regions.
Paper
‘Large inert carbon pool in the terrestrial biosphere during the Last Glacial Maximum’ by P. Ciais, A. Tagliabue, M. Cuntz, L. Bopp, M. Scholze, G. Hoffmann, A. Lourantou, S. P. Harrison, I. C. Prentice, D. I. Kelley, C. Koven and S. L. Piao in Nature Geoscience
“Atmospheric carbon dioxide (CO2) is one of the most important greenhouse gases… ” la de da da…
how convenient they never state the actual truth:
“Water vapor (H2O) is the most important greenhouse gas found in Earth’s atmosphere by far, by a factor approaching 50:1 over it’s nearest cousin greenhouse gas, carbon dioxide (CO2) due to it’s absorption of infrared across the entire IR spectrum. Water vapor’s influence, being just as much a greenhouse gas as carbon dioxide, simply dwarfs any other GHG’s influence found within Earth’s climate system.”
How convenient to ignore it in scientific papers, of all places, with nary a mention. Gail, you are right, these are not scientists, they are puppets.
Very interesting post. Good luck with the publication
Ed Mertin says:
November 21, 2011 at 1:16 am
Ferdinand, What about the way CO2 levels dropped after Pinatubo & Cerro Hudson. Couldn’t the same ash fall on oceans have happened back then?
The Pinatubo caused a cooling of about 0.6 C. That was more than enough to compensate for the CO2 emissions of the Pinatubo itself and a large part of the human emissions… The average ratio between temperature variations and CO2 increase variations around the trend are around 4 ppmv/C
John Marshall says:
November 21, 2011 at 1:36 am
According to estimates by the US Department of Energy, and others, burning fossil fuels provides 3% of the global atmospheric CO2 budget. I repeat 3%. Not an earth shattering, or warming, quantity. there is also much disagreement on atmospheric CO2 residence time between Dr. Lindzen, 5 years, to the IPCC, 200 years.
John, the 3% is additional, the other 97% is only going in ánd out within a year, that is simply circulating. In fact 97% in and 98.5% out, thus taking more out than pushing in. Thus leaving 101.5% in the next year. That means that the 3% is entirely responsible for the measured increase. It is the same as when you are eating 2000 kcal a day and using it by living, walking, working, etc… Your weight doesn’t go up or down. Just start eating 2100 kcal a day, that is only 5% more, and watch the result after a year or so…
The 5% residence time is residence time: 20% of all CO2 is replaced every year by CO2 from other reservoirs. But that doesn’t change the total amount of CO2. The real removal of extra CO2 is the above 1,5% more sink than source. That gives a removal rate of about 38 years half life time. Less than the IPCC says, more than Dr. Lindzen says, see:
http://www.john-daly.com/carbon.htm
pofarmer says:
November 21, 2011 at 6:44 am
What I would like to see, is whatever they use to “baseline” or “proof” the ice core information. Any links would be helpful, because I’ve certainly looked, and can’t find it.
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Here are your links
http://www.co2web.info/stoten92.pdf
http://www.warwickhughes.com/icecore/zjmar07.pdf
For more info Try the CO2 web http://www.co2web.info/
Ian W says:
November 21, 2011 at 4:02 am
Firstly, you have to show that sudden depressurization as the cores are brought to the surface does not alter the various gases in the bubbles found. Perhaps there are only bubbles from such deep ice under pressure after the ice is depressurized?
After drilling, the ice may relax under cold conditions (-20°C) for a year and more. Normally long enough to expand all ice (+50% in volume) and decompose all formed clathrates and reforming the bubbles. Any remaining clathrates decompose at measuring time, as that is done under vacuum.
Then you have to confirm that the carbon dioxide does not diffuse through the ice over the thousands and thousands of years
The reference you did provide shows that there may be a (theoretical) diffusion of CO2 through the ice, which results in a broadening of the averaging from 20 to 22 years at medium depth and up to 40 years at full depth (70 kyr old). Thus the resolution gets worse, but that doesn’t alter the average over that period, only increases the smoothing of peaks to both sides. That is the case for the relative “warm” Siple Dome ice core (-22°C). For the much colder (-40°C) 420 kyr Vostok and the 800 kyr Dome C ice cores, there is obviously not such a problem: if there was even the slightest migration, the ratio of CO2 vs. temperature (dD or d18O) proxy would fade over time. For every interglacial/glacial period, the ratio between CO2 and temperature is about 8 ppmv/°C. This remains the same for each period 100,000 years back in time. If there was migration, the high (280-300 ppmv) levels over 10% of the time would spread over the 90% period where the levels were low (around 180 ppmv).
there is a leap of faith involved in stating that this was the atmospheric carbon dioxide concentration at the age of the layer of ice in which the bubble was found.
There concentration found is the average over the resolution, which is several years: from 8 years (over the last 150 years) for two Law Dome cores to 560 years (over the last 800 kyrs) for the Dome C core. The resolution depends of the local snow accumulation, which governs the speed at which the bubbles reach closing depth. And as there still are exchanges possible between the atmosphere and open pores in the firn, the average gas age is (much) younger than the ice where the gas is enclosed. That again is governed by the accumulation rate: for Law Dome the ice age – gas age difference is 30 years, for Vostok several thousands of years.
kadaka (KD Knoebel) says:
November 21, 2011 at 7:52 am
From Gail Combs on November 21, 2011 at 4:57 am, quoted in “Hydroponic shop” section:
Because 300 PPM is the atmospheric CO content, this amount is chosen as the 100% growth point.
Been awhile since it was only 300ppm, that value isn’t even on the “official” Mauna Loa graph, now we’re almost at 400ppm. Is that a very old reference, an “unmixed” local reading, or just mistaken?
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The use of CO2 in green houses has been around for quite awhile. Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy. Remember CO2 was being measured by chemical methods since the early 1800’s and fairly accurately by the 1880’s. [ http://www.biomind.de/realCO2/ ] By 1851, the French chemist, Jean Boussingault, had done experiments on plants. “His conclusions: plants require water and obtain hydrogen from it; plant dry matter contains hydrogen plus carbon and oxygen which comes from the air; plants also contain nitrogen and other elements. http://ag.arizona.edu/ceac/sites/ag.arizona.edu.ceac/files/Copy%20of%20Copy%20of%20pls217nbCH1.pdf
The move to “soilless” agriculture, hydroponics, that did not use composting manure to provide CO2 prompted the use of other sources of CO2. (They were experimenting with Hydroponics as early as 1600!) The first commercial greenhouse in the USA was built in 1820. By the 1920’s CO2 enrichment was used in Germany and Sweden. “Modern” research in hydroponics was carried out from the 1860’s to the 1940’s. The U. S. army even used hydroponics to supply the troops
stationed on islands in the Pacific during WWII.
So yes all of this information was very well known for almost a century!
Gail Combs says:
November 21, 2011 at 4:57 am
Now when will Engelbeen come to valiantly defend this very important pillar of CAGW? After all low CO2 is the KEY to CAGW.
Yes it is one of the pillars, reason why it must be defeated like any other multiheaded dragon…
I think we had this discussion before: plants grow on land (plants at sea have plenty of CO2 for their synthesis, no problem there). But the average near-ground CO2 level is 40-50 ppmv higher than in the rest of the atmosphere. Much higher at night (soil bacteria at work and plant respiration) and in the first hours of the day. So at least during a few hours in early sunlight, plants can grow even when the “background” CO2 levels were 180 ppmv.
JJ says:
November 21, 2011 at 6:28 am
That are real data that don’t say what they are claimed to say. The alleged pre-industrial CO2 cap of 280 ppm is a convenient ‘err’ in the assessment of ice core data. The temporal resolution of those data is not sufficient to support such claims.
The resolution of 2 out of 3 Law Dome ice cores is less than 10 years, the third is at some 30 years years. The cores overlap with the direct measurements at the South Pole for the period 1960-1980:
http://www.ferdinand-engelbeen.be/klimaat/klim_img/law_dome_sp_co2.jpg
and the CO2 levels over the past 1000 years were:
http://www.ferdinand-engelbeen.be/klimaat/klim_img/antarctic_cores_001kyr_large.jpg
Ice cores with quite different accumulation rates, dust/salt inclusions, average temperature,… show the same CO2 levels +/- 5 ppmv for the same gas age periods…
pofarmer says:
November 21, 2011 at 6:44 am
What I would like to see, is whatever they use to “baseline” or “proof” the ice core information. Any links would be helpful, because I’ve certainly looked, and can’t find it.
As shown before, there is an overlap of some 20 years between the Law Dome ice cores and direct measurements at the South Pole. That is in Etheridge e.a. 1996, unfortunately behind a paywall:
http://www.agu.org/pubs/crossref/1996/95JD03410.shtml. Etheridge also measured CO2 in firn, top-down until bubble closing depth. Over the 72 m depth, there was a gradual change in CO2 level, where at closing depth the level was about the same as 7 years before at the South Pole. Thus at closing depth, the average gas age was 7 years older than at the surface. There was no difference in CO2 levels measured at closing depth between still open pores in the firn and already closed bubbles in the ice. Thus the composition didn’t change for CO2 in the bubbles (it does for smaller molecules like O2 and Ar).
Further, the consecutive overlappings between the different ice cores show a kind of cross-correlation for different periods, ranging from the last 150 years (Law Dome) to 800,000 years for the longest one (Dome C)
In how many places are samples taken for the various proxies to determine global CO2 levels pre-industry times? There’s ice cores from Greenland and Antarctica and, if I recall correctly, a couple of glaciers, and then there’s a handful of sediment sites.
From the plotted data I’ve seen the data points were all over the place, not one neat dot at 280. If the CAGW major rubber duck CO2 weapon were a skeptical tool of reason instead, from how many sites would the skeptics need the various proxies have been needed to be taken from to cover the globe with an average of 280 to the point of satisfying the raging CAGW alarmist that it was a more or less accurate number?
:p
higley7 says:
November 21, 2011 at 8:18 am
It is completely wrong to assume that CO2 measurements from ice cores have anything to do with absolute values. They can show trends and highs and lows, but they are NOT quantitative in any way. Jaworowski, the leading expert has stated that there is 30–50% losses of CO2 during the traumatic process of core extraction. If you back calculate the losses, you end up with CO2 concentrations the same or higher than now.
This opinion of the late Prof. Jaworowski was based on his experiences which ended before 1992, when he wrote that. In 1996, Etheridge e.a. published their findings of the drilling of 3 cores at Law Dome, which answered a lot of allegations made by Jaworowski.
Here is a challenge to everybody who thinks that Jaworowski is right:
With what mechanism can CO2 levels enclosed in air bubbles of the ice be reduced to 180-300 ppmv, while the air surrounding the core is at 360 ppmv (drilling period) to 395 ppmv nowadays?
I am waiting…
More Soylent Green! says:
November 21, 2011 at 9:02 am
They’re in denial that warming proceeds increases by CO2 levels by an average of 800 years. The two hardly go hand-in-hand. The data shows first it warms and then CO2 goes up.
Be careful, it is not because temperature preceeds CO2 (and is the base for the CO2 levels of the past), that CO2 can’t have an influence on temperature. The influence can’t be huge, or one will have a runaway effect. But a small effect of huge CO2 changes on temperature still is possible. According to Hansen, even 50% of the glacial-interglacial transition is caused by the increase of CO2. But that is a far too much. If you look at the end of the Eemian (the previous warmer interglacial), there cooling was already maximal, while CO2 levels remained high. When CO2 levels dropped with 40 ppmv, there was no visible effect of CO2 on temperature, neither in ice sheet formation. Thus the effect of CO2 on temperature is anyway small. See:
http://www.ferdinand-engelbeen.be/klimaat/eemian.html
1DandyTroll says:
November 21, 2011 at 2:14 pm
From the plotted data I’ve seen the data points were all over the place, not one neat dot at 280.
One series of measurements at one suitable place would have been sufficient. But the problems with the historical measurements are plenty: the treatment of the sample, the reagens, the skill of the people involved, the accuracy and repeatability of the reagens, the limited number of samples over time, the place where was sampled…
The latter is the main problem. Far too many samples were taken at places where huge sources (natural as well as human) and sinks (vegetation) were at work: over land, in towns, between leaves of growing crops… These have very little value to know the “background” levels of that time. But measurements taken over the oceans on seaships and coastal with wind from the seaside show values around the ice cores for the same period. As good as they do today…
Ferdinand Engelbeen says:
The resolution of 2 out of 3 Law Dome ice cores is less than 10 years, the third is at some 30 years years.
Law Dome only goes back ~1,000 yrs, which is absolutely uninteresting when discussing the peaks of a cycle with period ~120,000 years.
The resolution of the ice cores used to claim that 280-300 has never been exceeded (400K-800k YBP to pre-industrial) have resolutions of several hundred to several thousand years near the CO2 peaks. We are at one of those peaks now. What is the current CO2 concentration, averaged over several hundred to several thousand years?
JJ says:
November 21, 2011 at 2:49 pm
Law Dome only goes back ~1,000 yrs, which is absolutely uninteresting when discussing the peaks of a cycle with period ~120,000 years.
It still is interesting, as we may say that any one-year peak of 20 ppmv or an increase of 2 ppmv sustained over 20 years would be measurable in the short, high resolution Law Dome ice cores (accuracy of the measurements at 1.2 ppmv – 1 sigma). For the 1000 years Law Dome core, a one-year shot of 80 ppmw would be visible or 2 ppmv sustained over 80 years. Taylor Dome goes back just over two interglacials (140 kyr) with a resolution of 150 years, good for the detection of 200 ppmv in one year, 20 ppmv over 30 years or 2 ppmv over 300 years. Even a one-way 100 ppmv increase as we have now over 150 years, would be detected in the 560-year resolution of the Dome C ice core…
The only way for non-detection of huge changes is if there is a high frequency of the changes. But that assumes that the low-levels of 180 ppmv sustained over 90,000 years would show much higher and much lower levels over short time (decades to centuries), with land plants already at the edge of extinction at 180 ppmv.
Ferdinand Engelbeen,
I had not previously seen the .pdf that Gail Combs posted:
http://www.co2web.info/stoten92.pdf
Now that I’ve read it, it appears that CO2 measurements from glaciers and ice cores is highly uncertain and questionable. If you wouldn’t mind, what are your thoughts on that paper and its conclusions?
Ferdinand Engelbeen says:
It still is interesting, as we may say that any one-year peak of 20 ppmv or an increase of 2 ppmv sustained over 20 years would be measurable in the short, high resolution Law Dome ice cores (accuracy of the measurements at 1.2 ppmv – 1 sigma).
Which is applicable only to the short, high resolution Law Dome ice cores. Assuming that the accuracy of short, high resolution ice cores persists with long, low resolution ice cores is, well, an assumption. BTW – 1 sigma? That is a wide net with large holes.
Taylor Dome goes back just over two interglacials (140 kyr) with a resolution of 150 years …
My copy of Taylor Dome CO2 for 18-63K YBP has maximum resolution of 150 yrs. That is for only 2 of 72 intervals. The range of resolution extends to a low of more than 4,000 years, and averages more than 600 years. Near CO2 peaks, it can be more than 1,000 years. And TD is just for the last two interglacials, which is ~1 CO2 cycle length. The claim is made for up to 800k years – which is itself barely representative at ~6.5 cycles.
The only way …
Never say never. Or by corallary, “only”. 🙂
“ … for non-detection of huge changes is if there is a high frequency of the changes.
Or, for the accuracy of the ice cores to be less than you assume it is. Or …
But that assumes that the low-levels of 180 ppmv sustained over 90,000 years would show much higher and much lower levels over short time (decades to centuries), with land plants already at the edge of extinction at 180 ppmv.
No, it is not necessary to assume that. It is not necessary to assume that CO2 concentrations in the atmosphere have the same variability at 180 ppm as they do at 300 ppm, let alone that CO2 concentrations in ice cores would behave that way. And what was the CO2 threshhold for land plants, say 400K years ago? We aren’t assuming that it is the same as it is currently, are we? That is 200K years before the currently postulated advent of homo sapiens …
So much of this larger topic is rests upon what is assumed, and what is not …
Everyone seems to be focused on CO2 as a driver of temperature. Of all greenhouse gases, it represents, only, 3.5%. I would think water vapor ( 95% of all greenhouse gases ) would be of greater importance and interest; yet it’s hardly ever considered.
Smokey says:
November 21, 2011 at 3:51 pm
Now that I’ve read it, it appears that CO2 measurements from glaciers and ice cores is highly uncertain and questionable. If you wouldn’t mind, what are your thoughts on that paper and its conclusions?
That is where Jaworowski/Segalstad have put their main objections about ice cores. Most of these objections were already refuted by the investigations of Etheridge e.a. in 1996. Despite that, Jaworowski and Segalstad insist(ed) on their interpretation of the data (high outliers are right, low values are wrong), while the newest analytical techniques (total sublimation of the sample and cryogenic separation of the gases) show the opposite…
JJ says:
November 21, 2011 at 4:48 pm
Or, for the accuracy of the ice cores to be less than you assume it is. Or …
If there are no huge variations in the past 150 years (with 8 years resolution), neither in the past 1000 years (with 30 years resolution), neither in the past 10,000 years (with 150 years resolution), then it is a huge leap of faith that the variability over previous interglacials were much higher.
What is sure is that the current increase of CO2 in all known proxies (stomata data, sediments, coralline sponges) and ice cores is unprecedented in the previous 10,000 years over the full Holocene. Moreover, there is a quite good correlation (with a lag) between temperature proxies and CO2 levels over all periods, within all resolutions, except for the past 160 years, where CO2 starts to lead temperature, be it that still a temperature influence of about 4 ppmv/C is detectable around the CO2 trend. Thus any huge variation in CO2 levels in the far past should be preceded with huge variations in temperature (which has a much better resolution than CO2), but that is not the case.
BTW, you need to make a distinction between the sample distribition and the resolution. While the resolution still may be similar (depending of the accumulation speed) even for the deepest parts of the ice core, the size of the sample needed, together with the resolution is what gives the time period which is measured. With the same sample size, the resolution broadens with depth as the number of layers per length increases. With smaller samples, as is the case for the Dome C ice core, even the deepest samples at 800 kyr have a resolution of 500-600 years.
Ferdinand Engelbeen says: November 22, 2011 at 1:33 am:
“What is sure is that the current increase of CO2 in all known proxies (stomata data, sediments, coralline sponges) and ice cores is unprecedented in the previous 10,000 years over the full Holocene.”
This is not correct. There was a debate over on Bishop Hill recently where stomata data were discussed, and I easily “cherry picked” the following from one of the papers referenced (I could have picked several others): “…therefore call into question the concept of the Intergovernmental Panel on Climate Change, which assumes an insignificant role of CO2 as a preindustrial climate-forcing factor”
Also, the 2007 paper by Beck reported on thousands of chemical measurements of atmospheric CO2 concentrations during the 19th and 20th centuries, that showed that concentrations were sometimes greater than those measured today. My own conclusion is that I would place greater reliance on the chemical and stomata data than the ice core data.
Ferdinand,
Could you please post a link to Etheridge? Because the many points raised by Jarowowski and Segalstad appear to be damning. If there are other convincing non-ice core proxies that confirm the ice core CO2 measurementts, I’ll be persuaded that the ice core measurements are accurate.
Jarowowski et. al make many arguments that seem to be very hard to refute. I used to accept ice core evidence regarding CO2 levels as being very strong. Now I’m not so sure. But if you have contrary evidence I’ll consider it with an open mind.
Ferdinand Engelbeen says:
November 21, 2011 at 1:25 pm
Gail Combs says:
November 21, 2011 at 4:57 am
Now when will Engelbeen come to valiantly defend this very important pillar of CAGW? After all low CO2 is the KEY to CAGW.
Yes it is one of the pillars, reason why it must be defeated like any other multiheaded dragon…
I think we had this discussion before: plants grow on land (plants at sea have plenty of CO2 for their synthesis, no problem there). But the average near-ground CO2 level is 40-50 ppmv higher than in the rest of the atmosphere. Much higher at night (soil bacteria at work and plant respiration) and in the first hours of the day. So at least during a few hours in early sunlight, plants can grow even when the “background” CO2 levels were 180 ppmv.
__________________________________________
Yes we have had the discussion before.
Statements like:
” But the average near-ground CO2 level is 40-50 ppmv higher than in the rest of the atmosphere. Much higher at night (soil bacteria at work and plant respiration) and in the first hours of the day. So at least during a few hours in early sunlight, plants can grow even when the “background” CO2 levels were 180 ppmv.”
I find defy logic.
We are told that the CO2 in the atmosphere is “well mixed” so a reading at Mauna Loa or in the Antarctic represent the CO2 of the entire world. Now you are saying “the average near-ground CO2 level is 40-50 ppmv higher than in the rest of the atmosphere.”
So if I follow your logic the CO2 in the atmosphere is 40-50 ppmv LOWER than the readings at Mauna Loa or in the Antarctic because the last time I checked volcanoes and glaciers didn’t fly.
Also according to the guy who was checking CO2 in green house tomatoes the “extra” 50ppm of CO2 was not available for a “few hours” in the morning. The author stated quite clearly:
So at dawn the plants sucked down 50 ppm in a just a few minutes not in a few hours. – BIG DIFFERENCE. (Efficient little buggers aren’t they.)
Also the wheat field maintained a 305 ppm +/- 5ppm level of CO2 during the day and only at night did it return to the higher levels of (estimating here) 315 ppm to 505 ppm from “soil activity” Anyone who has run a compost pile will understand the difference would be because of day to day differences in moisture levels and heat. The two things a dry cool glaciation would have in short supply. Therefore your 40-50 ppm higher at dawn would only apply to warm moist days although it certainly could shoot up to as much as 200 ppm more on some days in the summer after a rain.
I think the really big key is this:
Smokey, here is the reference:
http://www.agu.org/pubs/crossref/1996/95JD03410.shtml unfortunately behind a paywall, even if the publication is already from 1996. Etheridge e.a. drilled with three different methods, wet and dry (which didn’t give any difference), they measured the CO2 in firn top down, which shows that the CO2 levels at closing depth were average 10 ppmv less than in the atmosphere and thus about 7 years older, while the ice layers are already 40 years old. CO2 levels in firn and already closed bubbles in the ice were the same, no measurable fractionation. There is an overlap of some 20 years between the ice core measurements and the direct measurements at the South Pole.
Recently the ice core measurements were confirmed by sediment proxies, independent of the ice core measurements (confirmation of the findings in carbonate shells of foramins only against direct atmospheric CO2 levels), be it with a worse resolution, but extended to over 2 million years:
http://www.sciencemag.org/content/324/5934/1551.abstract
The full article again behind a paywall… More explanation of the findings at:
http://www.sciencedaily.com/releases/2009/06/090618143950.htm
If you look at the objections against ice cores of Jaworowski and Segalstad, all objections are about ice cores from (long) before 1992 and most references are from… Jaworowski. Not the slightest new allegations or references after 1992. And two main impossibilities: there is no physical possibility for CO2 to migrate from lower levels to higher levels (and where cracks/drilling fluid are found, higher levels occur). And he didn’t accept that there is a difference between the age of the ice layer and the average age of the enclosed air. The latter is incredible for an ice core specialist, especially as Etheridge already in 1996 had measured the difference in-situ for the Law Dome ice cores.