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  • 04-Apr-09 On Causes and Mechanisms of the 1500-Year Climate Cycles
  • SEPP Science Editorial #11-2009
    (in TWTW Apr 4, 2009)

    S. Fred Singer, Chairman and President , Science and Environmental Policy Project (SEPP)

    On Causes and Mechanisms of the 1500-Year Climate Cycles

    Apr 4, 2009

    The existence of a (roughly) 1500-year climate cycle of abrupt warming and cooling, first noted in Greenland ice cores by Dansgaard and Oeschger, is well established from a multitude of geological data [Singer and Avery. Unstoppable Global Warming: Every 1500 Years. Rowman & Littlefield Publ. 2007]. The cycle appears to extend into the Holocene and can account for the Medieval Warm Period (MWP) and Little Ice Age (LIA) [Loehle and Singer 2009]. Its synchronicity seems to be preserved. Early on, Bond [2001] suggested a solar cause; but we do not know of any solar phenomenon with such a period. Nor do we know the mechanism by which the Sun could cause such abrupt climate changes.

    In view of the fairly precise timing of the cycle, we speculate that purely internal oscillations of the oceanatmosphere system are unlikely to be the principal or only cause. We also note that the amplitude of D-O events was much larger during the period of glaciation than during the current warm period of the Holocene. We therefore favor a quasi-periodic internal oscillation synchronized by a solar trigger; the mechanism resembles "stochastic resonance." This would also account for the occurrence of missing cycles. As to the actual mechanism, we favor changes in solar activity modulating the energy spectrum of Galactic Cosmic Rays [Singer 1958] and thereby the flux of GCR impinging on the Earth's atmosphere. The most reasonable way this could affect the climate is by changes in cloudiness [Svensmark 2007]. The large amplitude of the D-O events suggests a positive feedback, perhaps a greenhouse effect, ultimately limited by a negative feedback inherent in the atmosphere-ocean system. Although many puzzles still remain, the observations suggest that large-amplitude abrupt changes become less likely in a warmer climate [NRC 2002].

    Why is that?

    (1) One reason might be that a colder ocean mixed-layer contains more dissolved CO2 and therefore releases more CO2 into the atmosphere when warmed - compared to a warmer Holocene ocean.

    (2) Further, this released CO2 produces a stronger GH forcing when added to the low CO2 levels of the ice ages -- because of the well-accepted logarithmic dependence of CO2 forcing on CO2 concentration.

    (3) Even more important, the ice-age atmosphere is extremely dry; there is little evaporation from the cold ocean surface and ice cover. Hence there will be little or no 'negative feedback' from WV or from clouds. (I am assuming here that such a feedback exists now -- reducing the GH effects of CO2). So one can see the full GH effect of CO2.

    (4) Finally, we don't see "run-away warming" because the amount of CO2 released from the mixed layer is limited. (At much higher temperatures, of course, the 'thermostat' effect of Ramanathan would operate.)

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