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Scripps Institution of Oceanography, University of California, San
Diego, La Jolla, CA 92093-0244
Contributed by Charles D. Keeling, February 2, 2000
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.
Geophysics
The 1,800-year oceanic tidal cycle: A possible cause of rapid
climate change
*
To whom reprint requests should be addressed. Email:
cdkeeling{at}ucsd.edu.
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