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Originally published in Science Express on 22 June 2006
Science 28 July 2006:
Vol. 313. no. 5786, pp. 508 - 511
DOI: 10.1126/science.1125249
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Reports

Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing

Peter Huybers

Long-term variations in Northern Hemisphere summer insolation are generally thought to control glaciation. But the intensity of summer insolation is primarily controlled by 20,000-year cycles in the precession of the equinoxes, whereas early Pleistocene glacial cycles occur at 40,000-year intervals, matching the period of changes in Earth's obliquity. The resolution of this 40,000-year problem is that glaciers are sensitive to insolation integrated over the duration of the summer. The integrated summer insolation is primarily controlled by obliquity and not precession because, by Kepler's second law, the duration of the summer is inversely proportional to Earth's distance from the Sun.

Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA.

E-mail: phuybers{at}fas.harvard.edu

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