A forcing function is derived from the insolation signal with a bipolar pulse modulation train that is a function of Earth's obliquity. From this obliquity-forced insolation, there is a calculated variation of about ¿9% in the energy flux at the top of the atmosphere. Pulsations in the incoming solar radiation, induced by the amplitude-frequency coupling effect of the obliquity, correlate well with continental biogenic silica data and the marine oxygen isotope record. The theory lends a new perspective to the physical mechanism involving climate change studies. Particularly, it provides a physical explanation of the 100-kyr cycle in glaciation, which is distinct from the commonly considered variations in orbital ecentricity. ¿ 1999 American Geophysical Union