FREQUENCY VARIATIONS OF THE EARTHS OBLIQUITY AND THE 100-KYR ICE-AGE CYCLES

VARIATIONS in the Earth's orbital parameters modulate the seasonal distribution of solar radiation and thereby induce changes in the Earth's climate1. Periodicities in the geological climate record with cycles of 100, 41 and 23 kyr have been linked with changes in eccentricity, obliquity and precession of the equinoxes, respectively2,3. But although the eccentricity does vary with a 100-kyr period, the effect on the incoming solar radiation is rather weak relative to the signals from obliquity and precession variations. The 100-kyr signal in the climate record should therefore be of negligible intensity, yet it is observed instead to dominate the record. Internal, nonlinear processes within the climate system have been proposed to account for this fact4-14. In contrast, I show here that variations in the frequency of the obliquity cycle can give rise to strong 100-kyr forcing of climate.