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The 1,800-year oceanic tidal cycle: A possible cause of rapid climate change
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Charles D. Keeling* and
Timothy P. Whorf


EXCERPT:

Abstract

Variations in solar irradiance are widely believed to explain climatic change on 20,000- to 100,000-year time-scales in accordance with the Milankovitch theory of the ice ages, but there is no conclusive evidence that variable irradiance can be the cause of abrupt fluctuations in climate on time-scales as short as 1,000 years. We propose that such abrupt millennial changes, seen in ice and sedimentary core records, were produced in part by well characterized, almost periodic variations in the strength of the global oceanic tide-raising forces caused by resonances in the periodic motions of the earth and moon. A well defined 1,800-year tidal cycle is associated with gradually shifting lunar declination from one episode of maximum tidal forcing on the centennial time-scale to the next. An amplitude modulation of this cycle occurs with an average period of about 5,000 years, associated with gradually shifting separation-intervals between perihelion and syzygy at maxima of the 1,800-year cycle. We propose that strong tidal forcing causes cooling at the sea surface by increasing vertical mixing in the oceans. On the millennial time-scale, this tidal hypothesis is supported by findings, from sedimentary records of ice-rafting debris, that ocean waters cooled close to the times predicted for strong tidal forcing.

High resolution ice-core and deep-sea sediment-core records over the past million years show evidence of abrupt changes in climate superimposed on slow alternations of ice-ages and interglacial warm periods. In general these abrupt changes are spaced irregularly, but a distinct subset of recurring cold periods, on the millennial time-scale, appears to be almost periodic. Such events, however, are not clearly apparent in ice-core data after the termination of the most recent glaciation, about eleven thousand years (11 kyr) BP (kyr before A.D. 2000). This absence of recent events has led to the hypothesis that their underlying cause is related to internal ice-sheet dynamics (ref. 1, p. 35). Interpretations of sediment-cores by Bond et al. (1, 2) indicate, however, that a 1- to 2-kyr periodicity persisted almost to the present, characterized by distinct cooling events, including the Little Ice Age that climaxed near A.D. 1600. Although evidence that cooling was more intense during glacial times may be explained by some aspect of ice-dynamics, a continuation of cooling events throughout the postglacial Holocene era suggests an alternative underlying mechanism.

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