Change In Ice Ages Not Caused By CO2

Around 1.2 million years ago, a shift in global climate began that caused a change in the timing of the alternating warm and cold periods—called interglacials and glacials—that have persisted during the Pleistocene Ice Age. Prior to that time, ice age glacial periods lasted about 40,000 years but since ~700,000 years ago ice-age cycles have lasted for around 100,000 years. Orbital variations, called the Croll-Milankovitch cycles, do exert some forcing on the 100,000 year time scale, but it is relatively weak. Orbital cycles seem to many too feeble an explanation for the change in glacial-interglacial timing. Some scientists have attempted to attribute the timing shift to a drop in CO2 but a new study confirms that carbon dioxide levels were not the cause of the climate shift.

The dominant period of Pleistocene glacial cycles changed during the mid-Pleistocene from 40,000 years to 100,000 years, for reasons unknown to science. A new paper in the June 19, 2009, edition of Science investigates whether that shift could be attributed to changes in atmospheric CO2 levels. A group of researchers, led by Bärbel Hönisch, examined the factors that influenced the mid-Pleistocene transition (MPT) around 1250 to 700 thousand years ago. Here is the published abstract of the paper:

The dominant period of Pleistocene glacial cycles changed during the mid-Pleistocene from 40,000 years to 100,000 years, for as yet unknown reasons. Here we present a 2.1-million-year record of sea surface partial pressure of CO2 (PCO2), based on boron isotopes in planktic foraminifer shells, which suggests that the atmospheric partial pressure of CO2 (pCO2) was relatively stable before the mid-Pleistocene climate transition. Glacial PCO2 was ~31 microatmospheres higher before the transition (more than 1 million years ago), but interglacial PCO2 was similar to that of late Pleistocene interglacial cycles (<450,000 years ago). These estimates are consistent with a close linkage between atmospheric CO2 concentration and global climate, but the lack of a gradual decrease in interglacial PCO2 does not support the suggestion that a long-term drawdown of atmospheric CO2 was the main cause of the climate transition.

The atmosphere is a mixture of gases and each gas has a partial pressure, which is the pressure that the gas would exert on a container if it alone occupied the container's volume. In other words, take a bottle full of air, remove all the gases other than CO2 and the pressure inside the bottle would be the partial pressure of carbon dioxide. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture. The world’s oceans readily exchange CO2 gas molecules with the atmosphere. The extent to which CO2 dissolves in water is determined by its partial pressure and the chemical reactions of the dissolved carbon dioxide with other chemicals in the water. The sea surface partial pressure of carbon dioxide (PCO2) is the pressure of carbon dioxide in the air above the water which would be in equilibrium with the dissolved carbon dioxide.

The most accurate archive for atmospheric pCO2 comes from ancient air bubbles trapped in polar ice. Hönisch et al., report: “Contrary to the suggestion that pCO2 decreased toward the late Pleistocene, the earlier pCO2 amplitude and average were lower than in the more recent past.” Unfortunately, ice core bubble data only goes back some 800,000 years and cannot be used as a source of data for the entire MPT. Instead of direct gas samples, the chemical composition of the shells from tiny sea creatures, which have died and become part of the ocean floor sediment, can be used to approximate the CO2 content of the ancient atmosphere.

Phase-contrast photomicrograph by Scott Fay, UC Berkeley.

Using these proxy records of past sea surface carbonic chemistry, constraints can placed on past atmospheric pCO2. Planktonic foraminifera, forams for short, are small floating animals with shells made of calcium carbonate (CaCO3). The boron isotopic composition of foram shells acts as a proxy for past seawater pH. This proxy is based on the equilibrium reaction between the two dominant isotopes of dissolved boron in seawater and the isotope fractionation or ratio between the two types. For explanations of how air bubbles are trapped, samples are analyzed and proxies work see chapter 13 of The Resilient Earth.

The researchers extend the existing 400-ky boron isotope record from Ocean Drilling Program (ODP) site 668B beyond the MPT to 2.1 million years ago. From this sediment core, they constructed a high-resolution oxygen isotope (δ18O) record from shells of the surface-dwelling forams. From these and other proxy data, the researchers were able to construct a 2.1 million year history of pCO2, sea surface temperature (SST) and other factors shown in the figure below.

2.1-million-year estimation of atmospheric pCO2 from marine proxies recorded at ODP site 668B in the eastern equatorial Atlantic. Source Hönisch et al. Science, 19 June 2009.

From these data, Hönisch et al., were able to analyze the CO2 content of the atmosphere during the transition from 40,000 year to 100,000 year glacial periods. They found that CO2 levels were higher during glacial periods prior to the transition, which they considered consistent with warmer glacial SSTs, ice extents, and deep-sea temperatures. But since interglacial pCO2 was similar before and after the transition they concluded that changes in CO2 levels did not cause the change in ice age cycles. “We therefore conclude that CO2 was unlikely to have been the main driver of the MPT,” the paper concludes.

What is the meaning of all this? It could be that human CO2 emissions will alter the existing rhythm of glacial-interglacial periods, but that seems unlikely. Once again the impact of CO2 as a driver of climate change is shown to be minimal, and that it is more likely an indicator that change has taken place than as the instigator of change. The Croll-Milankovitch cycles remain widely accepted as the forcing that decides the timing of glacial-interglacial periods. Data from the glaciation record are in strong agreement with this theory. In particular, during the last 700,000 years, the dominant period of glacial-interglacial oscillation has been 100,000 years, which corresponds to changes in Earth's eccentricity and orbital inclination. Global warming alarmists continue to stress the dangers of carbon dioxide while mounting scientific evidence indicates that CO2 plays only a minor roll in climate change.

Be safe, enjoy the interglacial and stay skeptical.

The real cause of Ice Age cycles?

(see Ice Ages & CO2, Part II – Rising Sea-levels in Tahiti for more)

Global Cooling

Actually, the world has been cooling for more than 35 million years, but that is just the latest long term trend. When I say “for reasons unknown to science” I mean that scientists do not know the proximate cause for the change in periodicity of the glacial cycles. Certainly something changed the sensitivity of the Earth system to the Milankovitch orbital forcings, but exactly what confluence of factors are to blame is still unclear. For some of the contributing factors see my post “Erroneous Lessons from Earth's Past.” You might also be interested in my review of a Science review article on the causes of climate change, “Trends, Rhythms & Aberrations: The Mechanisms of Climate Change.” Thanks for your comment.


Is methane really as powerful a greenhouse as is sometimes believed?

Go to:

and click on Methane or download:

This appears to show that methane only absorbs long-wave radiation over a narrow band (which it shares with N2O).


Yes, methane is a more potent GHG than carbon dioxide

According to the US EPA, methane (CH4) is a greenhouse gas that remains in the atmosphere for approximately 9-15 years. Methane is over 20 times more effective in trapping heat in the atmosphere than carbon dioxide (CO2) over a 100-year period and is emitted from a variety of natural and human-influenced sources. Human-influenced sources include landfills, natural gas and petroleum systems, agricultural activities, coal mining, stationary and mobile combustion, waste-water treatment, and certain industrial process.

In 1971, the first survey of the possible inadvertent human modification of climate stated that “methane has no direct effects on the climate or the biosphere [and] it is considered to be of no importance.” This illustrates how young a science climate change really is. In 1996, the IPCC published the following table of Global Warming Potentials for various gases in the atmosphere. I deleted most of the chlorofluorocarbons (CFHs) and hydrochlorofluorocarbons (HCFCs) except HFC-23, which I left for comparison with the natural greenhouse gases.

CO2 1
HFC-23 11,700
Table ES-1: Global Warming Potentials (100-Year Time Horizon)

The methane (CH4) GWP includes the direct effects and those indirect effects due to the production of tropospheric ozone and stratospheric water vapor. The indirect effect due to the production of CO2 is not included. There are also several gases that do not have a direct global warming effect but indirectly affect terrestrial and/or solar radiation absorption by influencing the formation or destruction of greenhouse gases, including tropospheric and stratospheric ozone. These gases include carbon monoxide (CO), oxides of nitrogen (NOx), and non-CH4 volatile organic compounds (NMVOCs). Aerosols, which are extremely small particles or liquid droplets, such as those produced by sulfur dioxide (SO2) or elemental carbon emissions, can also affect the absorptive characteristics of the atmosphere.

As you can see methane is, indeed, a much more potent greenhouse gas than carbon dioxide, and nitrous oxide (N2O) even more so—two orders of magnitude more potent that that devil, carbon dioxide. The IPCC rates methane as 21 times as potent and I have seen estimates of 25, 27 and even 30 times potency from other sources.

Yes, methane is a more potent GHG than carbon dioxide


I have looked at the IR spectrum chart and see that methane,is a minimal absorber of IR,far less than CO2 which is also a minor absorber of IR.

The idea that Methane is 21 times or whatever does not explain the TRUE absorbing potential of IR.

Approximately 90% of all OUTGOING IR never gets absorbed by the trace "greenhouse" gases.

That is why I am not concerned about Methane's putative powers of the mistakenly stated "heat trapping" capability.

More on methane

To me, your reply rather than answering my question simply restates it.

The classic (simplified) explanation of anthropogenic climate change is that as levels of CO2 increase, its absorption band becomes saturated and this causes an increase in temperature relative to pre-industrial levels of up to 1 to 2 degrees C. Warming beyond the 1 to 2 degrees cannot come from CO2 but comes from increased water vapour which absorbs long-wave radiation over a much wider range of wave-lengths. This effect can be seen in the next diagram from the same site:
which compares absorption in cloudy and clear sky conditions.

It seems to me that the factor of 21 you mention refers to the concentration of methane necessary to saturate its absorption band. The fact that N2O and HFC-23 have such high factors in your table suggests that this interpretation is correct. The units of the graph mean that energy absorption is proportional to area and it is clear that NH4 and N2O between them have much less potential than CO2.

I may be wrong, but I will need to see an explanation of how methane can be a major problem given its relatively narrow absorption band to be convinced.


It's not that simple

I appreciate your skepticism, Julius, but it is not as simple as looking at the radiative energy present in a GHG's absorption spectrum. Different gas molecules absorb light with varying efficiency. This is partly because a molecule of CO2 or CH4 doesn't just absorb light by having electrons hop orbitals like individual atoms do. For molecules there are vibrational and rotational modes as well. These additional ways of absorbing photonic energy widen the absorption spectra of the molecules.

Vibrational modes involve the bonds between the molecule's constituent atoms. The bonds can vibrate like springs, varying in length or flexing from side to side. Rotation is just what it says, the entire molecule spins. If you look at diagrams of carbon dioxide and methane molecules you can see that the number of bonds and fundamental shapes are different, implying that their enthusiasm for absorbing radiation is also different. But that isn't all there is to measuring the effectiveness of a GHG, there are other physical processes in action within the atmosphere that come into play, affecting the way energy is retained.

The Earth's surface temperature is 35°K warmer than its effective blackbody temperature because of the presence of clouds and GHGs and what is called the natural greenhouse effect. To account for these other factors scientists invented the radiative-convective equilibrium and forcings. The concept of radiative forcing is based on the hypothesis that the change in global annual mean surface temperature is proportional to the imposed global annual mean forcing, and that the nature of the applied forcing is not important. The fundamental assumption underlying the radiative forcing concept is that the surface and the troposphere are strongly coupled by convective heat transfer processes; that is, the earth-troposphere system is in a state of radiative-convective equilibrium (RCE).

According to the Committee on Radiative Forcing Effects on Climate's Climate Research Committee, part of the National Research Council, the radiative-convective equilibrium concept, the equation for determining global average surface temperature of the planet is


is the heat content of the land-ocean-atmosphere system with ρ the density, Cp the specific heat, T the temperature, and zb the depth to which the heating penetrates. The equations above describe the change in the heat content where f is the radiative forcing at the tropopause, T' is the change in surface temperature in response to a change in heat content, and λ is the climate feedback parameter. The climate feedback parameter also known as the climate sensitivity parameter, which denotes the rate at which the climate system returns the added forcing to space as infrared radiation or as reflected solar radiation (by changes in clouds, ice and snow, etc.). It is used to set the “sensitivity” of a model to changes in GHG levels.

In essence, λ accounts for how feedbacks modify the surface temperature response to the forcing. In principle, T′ should account for changes in the temperature of the surface and the troposphere, and since the lapse rate is assumed to be known or is assumed to be a function of surface temperature, T′ can be approximated by the surface temperature. For steady state, the solution yields

The implication of this equation, with fixed λ, is that surface temperature change is uniquely determined by the radiative forcing at the tropopause (or at the top of the atmosphere if the stratosphere is adjusted for radiative equilibrium). The next step is to calculate total radiative forcing and then the portion of this forcing attributable to each of the major greenhouse gases.

Where do the values so often quoted come from? From empirical measurements taken by scientists in the field. One interesting expedition took measurements for all the major gases on a long sea voyage. The radiative forcing due to all well-mixed greenhouse gases since pre-industrial times was estimated to be 2.43 watts per square meter in the IPCC TAR with an uncertainty of 15% (They also give a number of simplified equations for the calculation of f, called ΔF in the IPCC documents). Non-CO2 GHGs have added 1.17 Wm-2 (44.5% of total from all GHGs) of climate forcing from 1750 to 2000. Unfortunately, these values are hard for a non-scientist who is not a specialist in the field to interpret. The desire to make things easier to understand leads us to global warming potential and the concept of carbon dioxide equivalence.

Global warming potential (GWP) is a measure of how much a greenhouse gas contributes to global warming. It compares the gas in question to that of the same mass of carbon dioxide (by definition, CO2 has a GWP of 1). A GWP is calculated over a specific time interval and starting from a specific reference temperature. Remember, The relationship between a GHG and radiative forcing is logarithmic so that increased concentrations have a progressively smaller warming effect. This is why you need to know the initial conditions the calculated values are based on. These values are hardly ever stated when a GWP is quoted so beware of comparisons among values given in different papers. You can go digging through the IPCC reports to find the values they use.

Carbon dioxide equivalence is a quantity that describes, for a given mixture and amount of greenhouse gas, the amount of CO2 that would have the same GWP, when measured over a specified timescale (generally, 100 years). The carbon dioxide equivalence for a gas is obtained by multiplying the mass and the GWP of the gas. For example, the GWP for methane over 100 years is 25 and for nitrous oxide 298. This means that emissions of 1 million metric tonnes of methane and nitrous oxide respectively are equivalent to emissions of 25 and 298 million metric tonnes of carbon dioxide.

On a more simplistic level, consider that 20% of the GHG forcing is attributed to methane. Carbon dioxide accounts for 56%. In the atmosphere CO2 is present at 380 ppm while CH4 only 1.74 ppm. From these measured values we can directly calculate that methane is 78 times more potent than carbon dioxide, fairly close to the IPCC 20 year number of 62. Since methane has a lifetime of only around 12 years in the atmosphere its impact falls off rather rapidly: from 62 at 20 years, to 23 at 100 years and only 7 after 500 years. Any way you calculate it CH4 trumps CO2, irrespective of their absorption spectra.

Joe Doctor

I think the simple answer to the comment that once an absorbance band reaches saturation, that it can no longer have an effect is simply that the band continues to get wider. CO2 is a prodigious absorber of IR light at its primary frequency, but just because that frequency is opaque after a few inches doesn't mean that the band won't absorb at both higher and lower frequencies as the concentration increases. Or, the isotopes carbon-13 or oxygen-15 will also cause shifts in the absorbance and these become significant at the gigaton levels.

Remember too that there are a lot of smaller absorbance bands - many are overtones. These usually weak bands become significant as the concentration increases. Interestingly, too, this is why such gases as HFC-23 and SF6 have such huge CO2 equivalencies. They are certainly strong absorbers, but more importantly they show up in regions that are not affected by either CO2 or water vapor, so the infrared light in those regions, which would ordinarily be reflected into space unencumbered, is now being absorbed.

Think of water. Water in a glass is clear and colorless, right? Put it into a swimming pool (with a white-painted bottom) and it becomes blue. This is because the primary absorbances of water at 3 and 6 microns have overtones down into the red region of the visible spectrum. When the pathlength of liquid water gets into the regime of a couple of feet, you can actually see where the red light is being absorbed, leaving behind only the blue portion of the visible spectrum. This is also why glaciers have an aquamarine color. I'd love to tell you that this also explains why the sky is blue, but that is due to scattering, not water vapor.

As for the skeptics that question the anthropogenic contribution - when you deal in numbers of 1.17 watts / meter, that really starts to add up when you are talking about a cross section the size of a planet. The world is about 8,000 miles in diameter - that is 130 trillion square meters facing the sun at any time. Yeah, it adds up fast. Plus, it will be here for a long, long time. Start addressing the problem now for the sake of your great, great grandchildren.

I respectfully decline to abandon my skepticism

Thank you for verifying the accuracy of my presentation of your paper for our readers, at least until my last paragraph interpretation of the greater meaning of your results. Your paper showed that CO2 levels vary with climate change but do not drive the change. Our current conditions, where mankind has artificially increased atmospheric CO2, has not occurred before excluding events like the PETM, the cause of which remains uncertain, so you really don't know what the effect will be.

For those who aren't up on the details of Earth's climate history since the dinosaurs went extinct, the boundary between the Paleocene and Eocene periods, 55.8 million years ago, was marked by the most rapid and significant climatic disturbance of the Cenozoic Era. A sudden global warming event that resulted in the Paleocene–Eocene Thermal Maximum or PETM. There has been much speculation as to the cause of this sudden jump in world wide temperature but the leading explanation centers on a massive release of methane gas, probably from marine clathrate deposits.

For a number of reasons, primarily centering on changes in the ratio of carbon isotopes 13C and 12C (δ13C) in sediments of carbonate and organic matter, a sudden methane release from sea floor clathrates is the leading candidate for this, and several other bouts of true “global warming.” During the Phanerozoic, these times include the Permian Triassic boundary, 250 million years ago, and multiple episodes of the Mesozoic, particularly 183 and 120 million years ago. For each time period, researchers suggest that massive release of methane from marine gas hydrates was an important factor in climate change.

At the end of the Paleocene, global temperatures rose by around 6°C in the span of 20,000 years, with a corresponding rise in sea level as the whole of the oceans warmed. There was a commensurate rise of CO2 levels but, since methane (CH4) oxidizes in both sea water and the atmosphere releasing carbon dioxide, a massive methane release could account for that as well. Further more, since CH4 is a far more effective greenhouse gas than CO2 , a rapid release of methane seems to fit the evidence of rapid temperature increase.

Interestingly, the recovery from the PETM requires a much more rapid rate of CO2 removal from the atmosphere than is typical currently, hinting that there is a feedback mechanism at work to reduce excessive levels of that gas. This might be taken as good news by those like Dr. Hönisch, who fear rampant warming from anthropogenic CO2 .

By the way, please don't confuse the terms skeptic and denier. A denier is someone who refuses to accept reality when confronted with unassailable fact; a skeptic is one who questions the voracity of an assertion until unambiguous proof is presented. I am a skeptic regarding anthropogenic global warming, at least with respect to the importance of CO2 as a driver of climate change. I do not deny that Earth's climate has warmed slightly over the past 150 years or so—I am skeptical that human CO2 emissions are responsible for all or even most of that change.

I'm sceptical too....

Quote: "I do not deny that Earth's climate has warmed slightly over the past 150 years or so—I am skeptical that human CO2 emissions are responsible for all or even most of that change."

Given the consequences of our failing to make changes if our CO2 emissions are responsible for the shifting climate, it behooves you to explain how such a dramatic release of fossil carbon isn't affecting the biosphere!

Note that the PETM has been at least partly attributed to the flood basalt eruption in what was to become the Nth Atlantic between Greenland and Europe. Over 1500 Gigatonnes of carbon released over a 1000 year period, an approximate average of 1.5Gt per year. Humanity is currently pumping out around 10Gt per year.


The points you raise regarding the PETM require a more detailed answer than I can fit into a quick reply so I have a proposal. Usually I write my blog posts for the week over the weekend and then, after review for accuracy, spelling, etc., I post them during the week (I have a day job :-) Currently I have two posts in process, but the next post after those will be an in depth exploration of the PETM and other similar paleo-events. So keep monitoring the site (or subscribe to the rss feed) for my take on the PETM and why the current rise in carbon dioxide is not analogous to what happened 55 million years ago.

Baerbal Hoenisch

Is MrHoenisch aware that Bangladesh has actually gained 1000sq km of land in the last 50 years, and that they expect to gain another 1000 sq km in the next 50yrs due to sediment buildup in the Bay of Bengal ? Is he aware that sea levels around Tuvalu and the Maldives have not risen ,but fallen over the last 20years ? Or does he believe the propaganda machine of which he is a part? Does he agree with the IPCC that the Medieval Warm Period, the Roman Warm period, the Bronze age and the little ice age never occurred, and that the climatewasunchanginging for 10,000years before the modern warm period? Yet he calls skeptics deniers... I guess rationalisation is not completely rational. Thanks very much for this excellent website. Mr Hoffman ,you are a gem.

ian hilliar


Dear mister Anonymous,

Have you actually read the IPCC report yourself? They don't deny the Medieval Warm Period and the rest you are talking about! So I would like to know were you get your information from?


Medieval Warm Period Denied

The IPCC didn't say the MWP did not exist, they simply published revisionist temperature histories for the past 2000 years or so which eliminated that well documented historical period, along with several others. This was evidently in an effort to show how anomalous the temperature rise over the past 150 years or so has been (it really isn't). You have to be at least scientifically literate enough to be able to interpret their numbers and compare them with what is in the scientific literature. Google for "hockey stick graph."