Ice Age CO2 Cycles: Archer et al [2000]

I mentioned in connection with GCMs and Ice Ages, that the CO2 cycle was presently regarded by the leading paleoclimatologists as mysterious. This was contested by Lars Kamel, who observed that cold water dissolved more CO2 and did not see why there was a problem. Here I’m merely reporting what I’ve read and do not present any independent judgement. My source was Archer et al [Rev Geophys, 2000], an authoritative source who stated clearly that changing solubility did not account for changing levels as follows:

Because CO2 is more soluble in colder water, colder sea surface temperatures could lower pCO2. However, the magnitude of the glacial cooling can account for only a small fraction of the observed pCO2 drawdown.

They observed the following in respect to attempts to explain the CO2 cycles:

in spite of the clear importance of pCO2 as an amplifier or even a primary driver of the glacial cycles, and the additional motivation provided by the threat of future climate change, we remain ignorant of the mechanisms responsible for the glacial/interglacial CO2 cycles…. We conclude that in spite of the importance of understanding the natural carbon cycle, the solution to the mystery of the glacial/interglacial CO2 cycles still eludes us.

My surmise is that the mechanisms proposed in Archer et al [2000] are not included in GCMs – and if included, are all pretty speculative. Pause and reflect a little: here are effects which are said to have explanatory power for such small matters as whether we’re entering or exiting an Ice Age, but are (probably) not in GCMs. Here are some extended excerpts with no commentary.

Abstract. Fifteen years after the discovery of major glacial/interglacial cycles in the CO2 concentration of the atmosphere, it seems that all of the simple mechanisms for lowering pCO2 have been eliminated. We use a model of ocean and sediment geochemistry, which includes new developments of iron limitation of biological production at the sea surface and anoxic diagenesis and its effect on CaCO3 preservation in the sediments, to evaluate the current proposals for explaining the glacial/interglacial pCO2 cycles within the context of the ocean carbon cycle. After equilibration with CaCO3 the model is unable to generate glacial pCO2 by increasing ocean NO3 but predicts that a doubling of ocean H4SiO4 might suffice. However, the model is unable to generate a doubling of ocean H4SiO4 by any reasonable changes in SiO2 weathering or production. Our conclusions force us to challenge one or more of the assumptions at the foundations of chemical oceanography. We can abandon the stability of the “Redfield ratio” of nitrogen to phosphorus in living marine phytoplankton and the ultimate limitation of marine photosynthesis by phosphorus. We can challenge the idea that the pH of the deep ocean is held relatively invariant by equilibrium with CaCO3. A third possibility, which challenges physical oceanographers, is that diapycnal mixing in ocean circulation models exceeds the rate of mixing in the real ocean, diminishing the model pCO2 sensitivity to biological carbon uptake.

We are still unsure whether CO2 is a primary driver or a secondary amplifier of the glacial cycles. The Milankovitch orbital theory for the glacial cycles would seem to imply the latter, because there is a clear physical link between Northern Hemisphere summer heating and ice sheets but no easy link from orbital variations to pCO2. However, in the last two glacial terminations, the pCO2 rise clearly precedes the 18O of the atmosphere (an indicator of melted ice sheets) by several thousand years [Sowers and Bender, 1995; Broecker and Henderson, 1998], implying that pCO2 is a primary driver (Figure 2). Alternatively, pCO2 could be driven by changes in meteorological forcing, such as dust delivery of trace metals to the ocean surface, resulting in an acausal correlation between Northern Hemisphere summer insolation and ice volume….

Instead, the d13C from deep-sea CaCO3, more 12C rich during glacial time, tells us that if anything, the terrestrial biosphere released carbon during glacial time [Shackleton, 1977], the wrong direction to explain lower glacial pCO2.

The first proposed mechanisms to lower glacial pCO2 were to increase the rate of biological productivity in surface waters of the ocean, exporting carbon from the surface ocean to the deep sea in the form of sinking particles. Either an increase in the ocean inventory of nutrients (A second class of mechanisms to lower glacial pCO2 is to change the pH of the whole ocean, converting seawater CO2 into HCO3 and CO3 , which are unable to evaporate into the atmosphere.

The model incorporates a new sedimentary diagenesis component which simulates the role of CaCO3 compensation in determining atmospheric pCO2, and a new iron cycle component, which predicts the effect of atmospheric iron deposition on carbon uptake by photosynthesis. Many constraints on the cause of the glacial/interglacial atmospheric pCO2 come from ice core and sedimentary records.

First, the deglacial increase in pCO2 leads ice volume, eliminating sea-level-driven explanations such as those based on the submersion of the continental shelves [Sowers and Bender, 1995; Broecker and Henderson, 1998].

Second, the deglacial pCO2 transition was slow: 6–7 kyr in one case and as much as 14 kyr in the other. The pCO2 response time to the biological pump in the ocean is much faster than that, and so biological pump explanations would require some external pacer to slow the pCO2 transition.

Third, the glacial rate of weathering and global deep-sea CaCO3 burial was probably not much higher than today [Henderson et al., 1994; Catubig et al., 1998; Oxburgh, 1998]. Additional constraints come from isotopic signatures of carbon, nitrogen, and boron, the concentrations of the trace metal species cadmium and barium, and the distribution ofCaCO3 and SiO2 on the seafloor.

The ingredients of the glacial ocean that control CO2, simulated to the best of our knowledge in a numerical model, are as follows:

1. The first is the circulation of the ocean. The direct effect of the glacial circulation and surface ocean cooling on pCO2 appears to be small, especially after CaCO3 compensation.

2. The second is iron fertilization. Even if sea surface nutrients were completely depleted by a stronger biological cycle in the ocean (a much stronger response than is inferred for the glacial ocean), pCO2 would not reach the glacial concentration of 200 matm, especially if we release 40 3 1015 mol C from the terrestrial biosphere.Perhaps this is an artifact of excessive diapycnal diffusion in the z-coordinate circulation model.

3. The third is sediment geochemistry. Changes in biological production and water chemistry affect the diagenesis and burial of CaCO3, and hence pCO2. A new sediment model is able to simulate the transition to anaerobic conditions, efficiently enough to implement at every grid point in a global ocean model. The most striking effect of the anaerobic chemistry is to greatly enhance CaCO3 burial with decreasing oxygen concentration in the deep sea. This effect drives CaCO3 compensation to completely override the pCO2 drawdown from a proposed increase in ocean nutrients.

Three possibilities emerge as front-runners to address the discrepancy between observations and our present state of knowledge, although none of them fit the data well.

The first is that the ocean circulation models now in use for simulating the ocean carbon cycle are more diffusive than the real ocean and for this reason underestimate the pCO2 sensitivity to the biological pump.The verdict on this possibility awaits reconciliation of a suite of discordant field and modeling estimates of diffusivity in the real ocean [Archer et al., 2000].

The second possibility is to increase the glacial NO3 inventory of the ocean beyond PO4 32 limitation. Here we would assume that the Redfield ratio of N/P in phytoplankton was different during glacial time. This hypothesis is consistent with carbon isotopic data but generates anoxic conditions in the Pacific thermocline and a general decrease in the oxygen content of the deep sea as a whole. Decreasing oxygen in turn promotes CaCO3 burial, requiring an eventual decrease in ocean pH to compensate, which raises pCO2. Perhaps a source of ventilation to the intermediate Pacific [Kennett and Ingram, 1995], missed by our adjoint fit to the glacial ocean circulation, could alleviate some of these coupled problems.

The third possibility is to double the inventory of H4SiO4 in the ocean, thereby shifting the ratio of organic carbon to CaCO3 production and raising the pH of the deep ocean. This hypothesis is consistent with boron isotopes (a paleo-pH tracer), and if a benthic pH artifact on d13C is found such as has been found for planktonic foraminifera, then this scenario might be consistent with carbon isotope data as well. However, the predicted distribution of CaCO3 is not a good match for observations. Here the difficulty is explaining why the ocean H4SiO4 should have increased; the model global SiO2 burial rate scales with the H4SiO4 inventory squared, implying that H4SiO4 is relatively insensitive to SiO2 weathering or details of burial mechanism. Perhaps the dynamics of SiO2 production in surface waters and dissolution in sediments is not captured by the model.

We conclude that in spite of the importance of understanding the natural carbon cycle, the solution to the mystery of the glacial/interglacial CO2 cycles still eludes us.

Reference: David Archer and Arne Winguth1, David Lea and Natalie Mahowald, 2000, WHAT CAUSED THE GLACIAL/INTERGLACIAL ATMOSPHERIC pCO2 CYCLES? Reviews of Geophysics, 38, 159–189.


  1. Posted Dec 19, 2005 at 4:09 PM | Permalink

    It seems as the obvious explanations are contradicted by measurements. Strange.

  2. Paul Penrose
    Posted Dec 19, 2005 at 8:46 PM | Permalink

    Not strange at all. It just shows how little we really know about the subject.

  3. Murray Duffin
    Posted Dec 20, 2005 at 9:36 PM | Permalink

    I find that Archer tries to dazzle with vocabulary and presentation of hypotheses as something better than SWAGs while being internally inconsistent at best. It would take an impossible effort to check his references, find which have since been invalidated, and determine how selective he is in what he presents as rigorous research, but I, for one, find him less than credible. Getting into Archer is not the same as the work you have done to qualify ( or disqualify) statistical methodology Steve, but “caveat emptor”. Murray

  4. Posted Dec 23, 2005 at 6:14 PM | Permalink

    Regardless of the mechanism behind the CO2 changes during glacial transitions (I agree with Lars Kamél), the lag of CO2 changes after (ocean) temperature changes can be seen at all time scales (even in the current industrial era, as a change in CO2 increase speeds up ~6 months after the onset of El Niño’s).

    At the onset of a deglaciation (initiated by solar insolation changes), the lag is some 600 +/- 400 years. As there is a huge overlap between temperature and CO2 changes (over some 3,000 years warming), it is impossible to know the real attribution of the forcings to the temperature change. After the start of a glaciation, CO2 levels still remain high for many millennia, probably as result of abundant (starving) biomass at one side and a smaller ocean surface due to growing ice sheets. There is one such a start of a glaciation (the Eemian, the previous interglacial), where CO2 level remains high during near all of the cooling and start to decline after temperatures nearly reached their minimum. This is an interesting one, as that makes it possible to separate the influence of solar (insolation) changes and CO2 changes. Even a recent correction of the (proximated) temperature curve doesn’t change the timing of these events, only the amplitude of the temperature change.

    Fischer ea. comment in Science on this lag as follows:
    “Despite strongly decreasing temperatures, high carbon dioxide concentrations can be sustained for thousands of years during glaciations; the size of this phase lag is probably connected to the duration of the preceding warm period, which controls the change in land ice coverage and the buildup of the terrestrial biosphere.”
    “During further glaciation in MIS 5.4, CO2 concentrations remain constant, although temperatures strongly decline. We suggest that this reflects the combination of the increased oceanic uptake of CO2 expected for colder climate conditions and CO2 release caused by the net decline of the terrestrial biosphere during the glaciation and possibly by respiration of organic carbon deposited on increasingly exposed shelf areas. These processes, however, should terminate (with some delay) after the lowest temperatures are reached in MIS 5.4 and ice volume is at its maximum at 111 ky B.P. (22). In agreement with this hypothesis, CO2 concentrations start to decrease in the Vostok record at about 111 ky B.P.”
    See the enlarged temperature/CO2/CH4 trends of the Eemian here.

    The interesting part further is that one climate model could mimic the cooling without the help of CO2, in a model used by Kaspar and Cubasch. Unfortunately the simulation ended in 115 ky BP, thus didn’t cover the decrease of CO2. So it is not possible to see if the modeled CO2 decrease matches the temperature trend…

  5. Steve Sadlov
    Posted Feb 1, 2006 at 7:17 PM | Permalink

    Sadly, the ideologues of the global climate dabate seem to steer well clear of any substantive discussion regarding the geochemical and other mechanisms affecting the partial pressure of atmospheric CO2. And yet, such a discussion may hold one of the keys to understanding what the relationship is between pp(CO2) and the climate. This site is one of the few realms of sanity, may it live long and prosper.

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