Pleistocene glacial variability and the integrated insolation forcing

Peter Huybers

Absract

Long-term variations in Northern Hemisphere summer insolation are generally thought to control glaciation. But while the intensity of summer insolation is primarily controlled by 20Ky cycles in the precession of the equinoxes, early Pleistocene glacial cycles occur at 40Ky intervals, matching the period of changes in Earth s obliquity. The resolution of this 40Ky problem appears to be that glaciers are sensitive to insolation integrated over the duration of the summer. The integrated 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.