CO2: Ice Cores vs. Plant Stomata


Anyone who has spent any amount of time reviewing climate science literature has probably seen variations of the following chart…  


A record of atmospheric CO2 over the last 1,000 years constructed from Antarctic ice cores and the modern instrumental data from the Mauna Loa Observatory suggest that the pre-industrial atmospheric CO2 concentration was a relatively stable ~275ppmv up until the mid 19th Century. Since then, CO2 levels have been climbing rapidly to levels that are often described as unprecedented in the last several hundred thousand to several million years.  

Ice core CO2 data are great.  Ice cores can yield continuous CO2 records from as far back as 800,000 years ago right on up to the 1970’s.  The ice cores also form one of the pillars of Enviromarxist Junk Science: A stable pre-industrial atmospheric CO2 level of ~275ppmv.  The Antarctic ice core-derived CO2 estimates are inconsistent with just about every other method of measuring pre-industrial CO2 levels.  

Three common ways to estimate pre-industrial atmospheric CO2 concentrations (before instrumental records began in 1959) are:   

1) Measuring CO2 content in air bubbles trapped in ice cores.   

2) Measuring the density of stomata  in plants.    

3) GEOCARB (Berner et al., 1991, 1999, 2004): A geological model for the evolution of atmospheric CO2 over the Phanerozoic Eon.  This model is derived from “geological, geochemical, biological, and climatological data.”  The main drivers being tectonic activity, organic matter burial and continental rock weathering.


The advantage to the ice core method is that it provides a continuous record of relative CO2 changes going back in time 800,000  years, with a resolution ranging from annual in the shallow section to multi-decadal in the deeper section.  Pleistocene-age ice core records seem to indicate a strong correlation between CO2 and temperature; although the delta-CO2 lags behind the delta-T by an average of 800 years…   



Stomata are microscopic pores found in leaves and the stem epidermis of plants.  They are used for gas exchange.  The stomatal density in some C3 plants will vary inversely with the concentration of atmospheric CO2.  Stomatal density can be empirically tested and calibrated to CO2 changes over the last 60 years in living plants.  The advantage to the stomatal data is that the relationship of the Stomatal Index and atmospheric CO2 can be empirically demonstrated…  


When stomata-derived CO2 (red) is compared to ice core-derived CO2 (blue), the stomata generally show much more variability in the atmospheric CO2 level and often show levels much higher than the ice cores…  


Plant stomata suggest that the pre-industrial CO2 levels were commonly in the 360 to 390ppmv range.  


GEOCARB provides a continuous long-term record of atmospheric CO2 changes; but it is a very low-frequency record…   


 The lack of a long-term correlation between CO2 and temperature is very apparent when GEOCARB is compared to Veizer’s d18O-derived Phanerozoic temperature reconstruction.  As can be seen in the figure above, plant stomata indicate a much greater range of CO2 variability; but are in general agreement with the lower frequency GEOCARB model.   


Ice cores  and GEOCARB provide continuous long-term records; while plant stomata records are discontinuous and limited to fossil stomata that can be accurately aged and calibrated to extant plant taxa.  GEOCARB yields a very low frequency record, ice cores have better resolution and stomata can yield very high frequency data.  Modern CO2 levels are unspectacular according to GEOCARB, unprecedented according to the ice cores and not anomalous according to plant stomata.  So which method provides the most accurate reconstruction of past atmospheric CO2?   

The problems with the ice core data are 1) the air-age vs. ice-age delta and 2) the effects of burial depth on gas concentrations.    

The age of the layers of ice can be fairly easily and accurately determined.  The age of the air trapped in the ice is not so easily or accurately determined.  Currently the most common method for aging the air is through the use of “firn densification models” (FDM).  Firn is more dense than snow; but less dense than ice.  As the layers of snow and ice are buried, they are compressed into firn and then ice.  The depth at which the pore space in the firn closes off and traps gas can vary greatly… So the delta between the age of the ice and the ago of the air can vary from as little as 30 years to more than 2,000 years.     

The EPICA C core has a delta of over 2,000 years.  The pores don’t close off until a depth of 99 m, where the ice is 2,424 years old.  According to the firn densification model, last year’s air is trapped at that depth in ice that was deposited over 2,000 years ago.    

I have a lot of doubts about the accuracy of the FDM method.  I somehow doubt that the air at a depth of 99 meters is last year’s air.  Gas doesn’t tend to migrate downward through sediment… Being less dense than rock and water, it migrates upward.  That’s why oil and gas are almost always a lot older than the rock formations in which they are trapped.  I do realize that the contemporaneous atmosphere will permeate down into the ice… But it seems to me that at depth, there would be a mixture of air permeating downward, in situ air, and older air that had migrated upward before the ice fully “lithified”.    

A recent study (Van Hoof et al., 2005) demonstrated that the ice core CO2 data essentially represent a low-frequency, century to multi-century moving average of past atmospheric CO2 levels.    

Van Hoof et al., 2005. Atmospheric CO2 during the 13th century AD: reconciliation of data from ice core measurements and stomatal frequency analysis. Tellus (2005), 57B, 351–355.

 It appears that the ice core data represent a long-term, low-frequency moving average of the atmospheric CO2 concentration; while the stomata yield a high frequency component.  

The stomata data routinely show that atmospheric CO2 levels were higher than the ice cores do.  Plant stomata data from the previous interglacial (Eemian/Sangamonian) were higher than the ice cores indicate…  


The GEOCARB data also suggest that ice core CO2 data are too low…  


The average CO2 level of the Pleistocene ice cores is 36ppmv less than GEOCARB…  


Recent satellite data (NASA AIRS) show that atmospheric CO2 levels in the polar regions are significantly less than in lower latitudes…  

"AIRS can observe the concentration of carbon dioxide in the mid-troposphere, with 15,000 daily observations, pole to pole, all over the globe, with an accuracy of 1 to 2 parts per million and a horizontal surface resolution of 1 by 1 degree. The monthly map at right allows researchers to better observe variations of carbon dioxide at different latitudes and during different seasons. Image credit: NASA"

"AIRS data show that carbon dioxide is not well mixed in Earth's atmosphere, results that have been validated by direct measurements. The belt of carbon dioxide concentration in the southern hemisphere, depicted in red, reaches maximum strength in July-August and minimum strength in December-January. There is a net transfer of carbon dioxide from the northern hemisphere to the southern hemisphere. The northern hemisphere produces three to four times more human produced carbon dioxide than the southern hemisphere. Image credit: NASA"

So… The ice core data should be yielding lower CO2 levels than the Mauna Loa Observatory and the plant stomata.  

Kouwenberg et al., 2005 found that a “stomatal frequency record based on buried Tsuga heterophylla needles reveals significant centennial-scale atmospheric CO2 fluctuations during the last millennium.”  

Plant stomata data show much greater variability of atmospheric CO2 over the last 1,000 years than the ice cores and that CO2 levels have often been between 300 and 340ppmv over the last millennium, including a 120ppmv rise from the late 12th Century through the mid 14th Century.  The stomata data also indicate higher CO2 levels than the Mauna Loa instrumental record; but a 5-point moving average ties into the instrumental record quite nicely… 


A survey of historical chemical analyses (Beck, 2007) shows even more variability in atmospheric CO2 levels than the plant stomata data since 1800…




The current “paradigm” says that atmospheric CO2 has risen from ~275ppmv to 388ppmv since the mid-1800’s as the result of fossil fuel combustion by humans. Increasing CO2 levels are supposedly warming the planet…

However, if we use Moberg’s (2005) non-Hockey Stick reconstruction, the correlation between CO2 and temperature changes a bit…

Moberg did a far better job in honoring the low frequency components of the climate signal.  Reconstructions like these indicate a far more variable climate over the last 2,000 years than the “Hockey Sticks” do.  Moberg also shows that the warm up from the Little Ice Age began in 1600, 260 years before CO2 levels started to rise. 

As can be seen below, geologically consistent reconstructions like Moberg and Esper are in far better agreement with “direct” paleotemperature measurements, like Alley’s ice core reconstruction for Central Greenland…

What happens if we use the plant stomata-derived CO2 instead of the ice core data?


We find that the ~250-year lag time is consistent. CO2 levels peaked 250 years after the Medieval Warm Period peaked and the Little Ice Age cooling began and CO2 bottomed out 240 years after the trough of the Little Ice Age. In a fashion similar to the glacial/interglacial lags in the ice cores, the plant stomata data indicate that CO2 has lagged behind temperature changes by about 250 years over the last millennium. The rise in CO2 that began in 1860 is most likely the result of warming oceans degassing.   

While we don’t have a continuous stomata record over the Holocene, it does appear that a lag time was also present in the early Holocene…


Once dissolved in the deep-ocean, the residence time for carbon atoms can be more than 500 years. So, a 150- to 200-year lag time between the ~1,500-year climate cycle and oceanic CO2 degassing should come as little surprise.


  • Ice core data provide a low-frequency estimate of atmospheric CO2 variations of the glacial/interglacial cycles of the Pleistocene.  However, the ice cores seriously underestimate the variability of interglacial CO2 levels.
  • GEOCARB shows that ice cores underestimate the long-term average Pleistocene CO2 level by 36ppmv.
  • Modern satellite data show that atmospheric CO2 levels in Antarctica are 20 to 30ppmv less than lower latitudes.
  • Plant stomata data show that ice cores do not resolve past decadal and century scale CO2 variations that were of comparable amplitude and frequency to the rise since 1860.

Thus it is concluded that:

  • CO2 levels from the Early Holocene through pre-industrial times were highly variable and not stable as the ice cores suggest.
  • The carbon and climate cycles are coupled in a consistent manner from the Early Holocene to the present day.
  • The carbon cycle lags behind the climate cycle and thus does not drive the climate cycle.
  • The lag time is consistent with the hypothesis of a temperature-driven carbon cycle.
  • The anthropogenic contribution to the carbon cycle since 1860 is minimal and inconsequential.


Note:  Unless otherwise indicated, all of the climate reconstructions used in this article are for the Northern Hemisphere.


Wagner et al., 1999. Century-Scale Shifts in Early Holocene Atmospheric CO2 Concentration. Science 18 June 1999: Vol. 284. no. 5422, pp. 1971 – 1973.

PHANEROZOIC TIME. American Journal of Science, Vol. 301, February, 2001, P. 182–204.

Kouwenberg et al., 2004. APPLICATION OF CONIFER NEEDLES IN THE RECONSTRUCTION OF HOLOCENE CO2 LEVELS. PhD Thesis. Laboratory of Palaeobotany and Palynology, University of Utrecht.

Esper et al., 2005. Climate: past ranges and future changes. Quaternary Science Reviews 24 (2005) 2164–2166.

Kouwenberg et al., 2005. Atmospheric CO2 fluctuations during the last millennium reconstructed by stomatal frequency analysis of Tsuga heterophylla needles. GEOLOGY, January 2005.

Van Hoof et al., 2005. Atmospheric CO2 during the 13th century AD: reconciliation of data from ice core measurements and stomatal frequency analysis. Tellus (2005), 57B, 351–355.

Rundgren et al., 2005. Last interglacial atmospheric CO2 changes from stomatal index data and their relation to climate variations. Global and Planetary Change 49 (2005) 47–62.

Jessen et al., 2005.  Abrupt climatic changes and an unstable transition into a late Holocene Thermal Decline: a multiproxy lacustrine record from southern Sweden. J. Quaternary Sci., Vol. 20(4) 349–362 (2005).

Beck, 2007.  180 Years of Atmospheric CO2 Gas Analysis by Chemical Methods. ENERGY & ENVIRONMENT. VOLUME 18 No. 2 2007.

Loulergue et al., 2007. New constraints on the gas age-ice age difference along the EPICA ice cores, 0–50 kyr. Clim. Past, 3, 527–540, 2007.



Etheridge et al., 1998. Historical CO2 record derived from a spline fit (75 year cutoff) of the Law Dome DSS, DE08, and DE08-2 ice cores.

NOAA-ESRL / Keeling.

Berner, R.A. and Z. Kothavala, 2001. GEOCARB III: A Revised Model of Atmospheric CO2 over Phanerozoic Time, IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series # 2002-051. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

Kouwenberg et al., 2005. Atmospheric CO2 fluctuations during the last millennium reconstructed by stomatal frequency analysis of Tsuga heterophylla needles. GEOLOGY, January 2005.

Lüthi, D., M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, U. Siegenthaler, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T.F. Stocker. 2008. High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, Vol. 453, pp. 379-382, 15 May 2008. doi:10.1038/nature06949.

Royer, D.L. 2006. CO2-forced climate thresholds during the Phanerozoic. Geochimica et Cosmochimica Acta, Vol. 70, pp. 5665-5675. doi:10.1016/j.gca.2005.11.031.


Moberg, A., et al. 2005. 2,000-Year Northern Hemisphere Temperature Reconstruction. IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series # 2005-019. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

Esper, J., et al., 2003, Northern Hemisphere Extratropical Temperature Reconstruction, IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series # 2003-036. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

Mann, M.E. and P.D. Jones, 2003, 2,000 Year Hemispheric Multi-proxy Temperature Reconstructions, IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series #2003-051. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

Alley, R.B.. 2004. GISP2 Ice Core Temperature and Accumulation Data. IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series #2004-013. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

VEIZER d18O% ISOTOPE DATA.  2004 Update.


12 Responses to “CO2: Ice Cores vs. Plant Stomata”

  1. ferdiegb Says:

    “Gas doesn’t tend to migrate downward through sediment… Being less dense than rock and water, it migrates upward.”

    David, one of the basic points is that gases mix in all directions. They don’t migrate only upward in firn. Only when the pores become smaller and smaller, that may stop the mixing at a certain density.

    From ice cores, it is known (and proven by Etheridge for the Law Dome ice cores) that they contain the same amount of CO2 in the firn as in the ice at bubble closing depth, which is only a few years younger than the atmospheric CO2. The ice cores CO2 and the atmospheric CO2 at the South Pole overlap each other with about 20 years. Thus ice core CO2 is reliable and accurate (1.2 ppmv – 1 sigma for Law Dome, within 5 ppmv for different cores with the same gas age). As yearly averages of CO2 levels from near the North Pole (Barrow) to the South Pole are within a few ppmv, we may say that the ice cores reflect quite accurately the historical global CO2 levels, but smoothed over (many) years, depending of accumulation speed.

    Stomata data are less accurate (+/- 10 ppmv) and only have a small useful range (about 50 ppmv), but a quite high resolution. They can be calibrated in modern times on ice core CO2.

    But there are severe problems with stomata data. The first is that they are (by definition) land based. CO2 levels on land seldom reflect global CO2 levels, as local/regional CO2 levels show large swings in diurnal, daily, monthly and yearly CO2 levels. That is visible in far more variability of CO2 levels derived from stomata data, which reflect the local/regional average CO2 level of the previous year (as I learned from the stomata people) and not the global variability in CO2.

    The second problem is that the regional CO2 levels can be quite different over time, due to changes in landscape, wind direction, temperature,…

    E.g. the Dutch landscape shows enormous changes over time from marshes to forests to agriculture over the centuries in the main wind direction (of today!), all influencing the local/regional CO2 levels. Temperature changes from the MWP to the LIA shows a decrease of about 6 ppmv in Law Dome ice core. But the long cold of the LIA did kill much food (and people), which may have had far more impact on local/regional CO2 levels.

    Thus sorry, stomata data still are in their infancy as reliable instrument to know global historical CO2 levels.

    I haven’t looked into Geocarb, thus no comment on that levels (which are anyway very coarse).

    AIRS data are completely in line with the surface data. You are looking at a one-day (month?) snapshot, which is influenced by the (mainly NH) seasonal use/release of CO2 by vegetation. Yearly averages only differ with a few ppmv.

    About Beck’s historical measurements: Same basic problem as with the stomata data: several of the historical measurements were made on land and are very variable and way too high. That is especially the problem for the 1942 “peak”, mainly based on two land based series: Giessen in Germany and Poona in India. That peak is not seen in high resolution (8 years) ice cores, stomata data (!) nor corraline sponges.
    Historical CO2 levels measured over the oceans or coastal places (with wind from the sea) are all around the ice core CO2 levels.
    See further:

  2. David Middleton Says:


    Once again, thank you for visiting my site and posting such a thoughtful and informed comment.

    A few points on the stomata…

    They are not “in their infancy.” Plant stomata data have been used far longer than ice core data to determine past levels of CO2.

    Stomata do not have a “small useful range;” although some taxa do have maximum CO2 levels, above which the stomatal density ceases to change. Stomata data from the Paleocene-Eocene Thermal Maximum show that CO2 levels were ~400ppmv in western North America and ~700ppmv in the Northeast Atlantic. This is not surprising because the massive flood basalt eruptions that formed the North Atlantic Igneous Province were underway at the time.

    The AIRS data that I presented were from a “snapshot.” It’s clearly shown in the snapshot that both polar areas had ~20ppmv lower CO2 levels than the mid and low latitudes. That’s not a seasonal variation. NASA clearly state that they were surprised at how “clumpy” the CO2 concentrations were and that it was not “well mixed” as previously thought.

    Unlike Jawaworski, I don’t think that the ice core data are fatally flawed; but I do think that, when integrated with all the other available data, the Antarctic ice cores present a very low frequency moving average of CO2 and that they do not capture the higher CO2 levels that would have occurred during the Pleistocene interglacials at lower latitudes.

    The best approach is to integrate all of the data.

  3. My Response to Skeptical Science « Debunk House Says:

    […] “When stomata-derived CO2 (red) is compared to ice core-derived CO2 (blue), the stomata generally show much more variability in the atmospheric CO2 level and often show levels much higher than the ice cores.” (David Middleton) […]

  4. ------ THE SKY IS FALLING ------ - Page 1239 - Discussion and Information Forums Says:

    […] […]

  5. Steve Gorehm Says:

    Hi David,

    May I give you a call? I have a question about the Kouwenberg data you showed. If so, at what number can you be reached?

    Or, please call me at 815-462-8924.

    Steve Goreham
    Executive Director
    Climate Science Coalition of America

  6. CO2 Past and Problems « Musings from the Chiefio Says:

    […] […]

  7. Toon Pepermans Says:

    while I agree that global warming is heavily exaggerated (and to a large extent caused by solar magnetic field fluctuations influencing cosmic ray influx), I can only laugh when I read things like “The GEOCARB data also suggest that ice core CO2 data are too low…”
    the GEOCARB data is meant to say something on a multimillion year time scale, any conclusions about finer time scales are totally rubbish

  8. walter mattson Says:

    I enjoy reading the article and also the comments to date. It is good to find comments that are civil and obviously intelligent. While I am not by any means an expert on climate and the various disciplines that are required to solve the complications of climate change, I learn more about the various studies that produce more understanding of how complex climate science really is. However to date, I still cling to the past evidence that the climate has changed for millions of years without influence from humans. The fact that CO2 concentrations are rising seems not to be in dispute. What still is in dispute is that CO2 is the driver of temperature. Just by seeing that CO2 and temperature have followed basically the same curve for a million years, indicates to me that CO2 is not the driver. How can it be? If it was,the rise during the previous cycles should have resulted in run away temperature similar to the planet Venus. Some thing other that CO2 is the driver. My feeling is that it is temperature and that the resulting more clouds and precipitation results in more rain and snow as the mechanism that brings nature back to another cooler point.

  9. john rowan Says:

    if u worked out the amount of fossel fuels 1 all coal driven power station 2 all aeroplanes flight patterns planes burn up tons of fuel on one flight 3 all motor cars burnt in 24 hrs in the whole world . then convert into one year. when u see the figure tell me it is tiny please give me an answer remember all the other gasses that are released from fossel fuel carbon monoxide etc

  10. walter mattson Says:

    There have been scientists who have already calculated the total CO2 emitted by all the sources you mentioned. They have concluded that the number is large but is small compared to the CO2 that is released by the ocean and other sources.
    It gets back to the same question. The above report shows that CO2 is not the driver of increasing temperature. Thus why would we want to control a factor that has so little influence.on temperature. If we truly need to slow the rate of temperature increases, we should attempt to control the largest factor. It is a known fact that clouds and water vapor are the major greenhouse gas at somewhere around 95%. Studies by other scientists show that cloud formation and temperature correlate. Those studies also indicate that the sun and solar activity are a factor in cloud formation and the earth temperature. To me it sounds like we are barking up the wrong tree that CO2 needs to be controlled. I also believe that controlling CO2 is now a political issue rather than a scientific issue. There are many scientists who believe the same . I just read a statement by Freeman Dyson, a greatly recognized scientist, that concluded this awhile ago but of course those that disagree have dismissed his opinion.

  11. Stomata and ice-core CO2 measurements – PLANT STOMATA Says:

    […] Plant stomata show higher and more variable CO2 levels “When stomata-derived CO2 (red) is compared to ice core-derived CO2 (blue), the stomata generally show much more variability in the atmospheric CO2 level and often show levels much higher than the ice cores.” (David Middleton) […]

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