Wednesday, August 7, 2013

New paper finds ice ages explained even with constant levels of CO2

A paper published today in Nature finds that ice ages in the Northern Hemisphere can be explained by Milankovitch cycles and the unique geography of the North American ice sheet, finding, "the crucial mechanism for the 100,000 year cycles is the delayed glacial isostatic rebound which keeps the ice elevation low, and, therefore,  the ice ablation high, while the ice sheet retreats." The authors are able to reproduce the saw-tooth pattern of ice ages using a model that assumes a constant level of atmospheric CO2. The prior 'consensus' was that Milankovitch cycles and CO2 were the primary drivers of ice age cycles, but this paper finds CO2 played a minor role, stating, "Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles." Still unexplained, however, is that ice ages were approximately synchronous between the Northern and Southern hemispheres, even though by Milankovitch theory an increase in solar radiation in the Northern Hemisphere is offset by an equal decrease of solar radiation in the Southern Hemisphere.

Excerpts:


The 100-kyr periodicity, the sawtooth pattern and the timing of the terminations are reproduced with constant CO2 levels 20,24 (for example  220 p.p.m.; Fig. 1e), and are robust for a range of model parameters (Supplementary Fig. 4).  

By contrast, the spectral peak of ,100-kyr cycles is greatly reduced,  and permanent large ice sheets remain, with the imposition of instantaneous isostatic rebound (Fig. 1f). This result supports the idea that the  crucial mechanism for the 100-kyr cycles is the delayed glacial isostatic rebound 14,15, which keeps the ice elevation low, and, therefore,  the ice ablation high, while the ice sheet retreats. We note, however,  that CO2 variations can result in amplification of the full magnitude of ice-volume changes during the 100-kyr cycles, but do not drive the cycles. Ice-sheet changes may induce variations in CO2 through changing sea surface temperature, affecting the solubility of CO2 (ref. 25), and through changing sea level, affecting the stratification of and CO2 storage in the Southern Ocean18. During deglaciation, the melt water may affect ocean circulation, leading to an increase in atmospheric  CO2 (refs 23, 26, 27). 

A remarkable conclusion from our model results is therefore that  the 100,000 year glacial cycle exists only because of the unique geographic  and climatological setting of the North American ice sheet with respect to received insolation. Only for the North American ice sheet is the  upper hysteresis branch moderately inclined; that is, there is a gradual  change between large and small equilibrium ice-sheet volumes over a large range of insolation forcings. For this reason, as demonstrated in  Fig. 2b, the amplitude modulation of summer insolation variation in  the precessional cycle, due primarily to eccentricity, is able to generate the 100-kyr cycles with large amplitude, gradual growth and rapid  terminations.


Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume


Nature
 
500,
 
190–193
 
 
doi:10.1038/nature12374
Received
 
Accepted
 
Published online
 

 
 
 
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14 comments:

  1. In Defence of Milankovitch
    Gerard Roe 2006
    Covers a lot of this ground

    ReplyDelete
    Replies
    1. Thanks tallbloke, that is a classic

      Delete
  2. Noted above"Still unexplained, however, is that ice ages were approximately synchronous between the Northern and Southern hemispheres, even though by Milankovitch theory an increase in solar radiation in the Northern Hemisphere is offset by an equal decrease of solar radiation in the Southern Hemisphere." Is this true of all three Milankovitch cycles? I can see how it would be true of the tilt and wobble cycles. But I suspect the orbital distance cycle, circular to ellipse transitioning, would affect both north and south similarly.

    ReplyDelete
    Replies
    1. You are correct that eccentricity [orbit] does affect both the NH & SH similarly [although not exactly the same since there is more NH land mass than SH land mass], but eccentricity has the least influence of the 3 cycles. This has led to the so-called 100,000 year problem:

      http://en.wikipedia.org/wiki/100,000-year_problem

      Delete
  3. The 100,000yr 'problem' goes way once you correctly integrate longterm effect of insolation difference on oceans. Under-estimating depth and timescale of ocean response to changing solar forcing is part of the AGW rationale.

    Rog Tallbloke

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  4. "New paper finds ice ages explained even with constant levels of CO2"

    I would like to refer you to figure 1 g of the article;
    there you will see that different CO2 concentrations lead to differen levels of glaciation, measured in sea level equivalents;
    so you can explain the 100 k periodicity and the saw-tooth without CO2, not the amplitude of the ice age;

    ReplyDelete
    Replies
    1. I disagree in this respect:

      1. Fig 1g black line shows CO2 220 ppm but allowed to vary with out-gassing & in-gassing. Fig 1e has an almost identical amplitude of the glacial cycles with CO2 held constant. Therefore, the prior 'consensus' that positive-feedbacks from CO2 levels following temperature are necessary to explain the amplitude of glacial cycles is disproven.

      2. The modeling assumptions of CO2 radiative forcing made in this paper also suffer from the same assumptions that have caused all 73 IPCC AR5 climate models to exaggerate warming - namely over-sensitivity to CO2.

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  5. summary of this paper published today

    http://www.terradaily.com/reports/Ice_ages_only_thanks_to_feedback_999.html

    with no mention of CO2/greenhouse gases as a feedback

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  6. "We note, however, that CO2 variations can result in amplification of the full magnitude of ice-volume changes during the 100-kyr cycles, but do not drive the cycles."

    ReplyDelete
  7. http://www.sciencedaily.com/releases/2013/08/130814132228.htm

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  8. http://wattsupwiththat.com/2013/08/14/claim-co2-ended-the-last-ice-age/

    http://wattsupwiththat.com/2013/08/14/claim-co2-ended-the-last-ice-age/#comment-1390496

    http://wattsupwiththat.com/2013/08/14/claim-co2-ended-the-last-ice-age/#comment-1390760

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  9. http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12376.html

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  10. more on the 100,000 yr problem

    http://www.clim-past.net/9/2003/2013/cp-9-2003-2013.pdf

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  11. http://ars.els-cdn.com/content/image/1-s2.0-S0031018213005063-gr9.jpg

    http://www.sciencedirect.com/science/article/pii/S0031018213005063

    ReplyDelete