The 1,800-year oceanic tidal cycle: A possible cause of rapid climate change
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Contributed by Charles D. Keeling

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.
Footnotes
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↵* To whom reprint requests should be addressed. Email: cdkeeling{at}ucsd.edu.
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Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073/pnas.070047197.
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Article and publication date are at www.pnas.org/cgi/doi/10.1073/pnas.070047197
Abbreviations
- kyr,
- 1,000 years;
- IRD,
- ice-rafted debris
- Accepted February 2, 2000.
- Copyright © The National Academy of Sciences
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- Abstract
- The 1- to 2-kyr Ice-Rafted Debris (IRD) Cycle.
- A Proposed Tidal Mechanism for Periodic Oceanic Cooling.
- The 1,800-Year Tidal Cycle.
- A 5,000-Year Modulation of the 1,800-Year Cycle.
- Observational Tests of Millennial Tidal Climatic Forcing.
- Secular Variations in Tidal Forcing.
- Ramifications of the Tidal Hypothesis.
- Acknowledgments
- Footnotes
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