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mystery and raises the question of how much of the climate response is actually associated with orbital forcing.

Spectral analysis of the paleoclimate record shows that the maximum power lies in the approximately 100,000 year period, which is of the same order as the Earth's eccentricity variation. However, the changes in eccentricity, on the order of a few tenths of a percent over the past 5 million years, produce little change in net annual solar radiation, so that any possible effects on the seasonal distribution of radiation must be combined with variations in tilt and precession of the Earth's rotation axis, which are larger. Thus it is surprising that the about 100,000 year period dominates in the climate record. Examples of this mismatch can easily be found: the peak of the last ice age, about 20,000 years BP, coincides with a very weak minimum in Northern Hemisphere summer solar insolation, and the deglaciation Northern Hemisphere summer maximum at about 12,000 years BP is no larger than a similar feature at about 30,000 years BP, which did not lead to complete deglaciation. These facts suggest that processes other than direct solar forcing may be responsible for the observed climate record.

Even the timing of the insolation variations relative to the climatic response has been questioned. Winograd et al. (1988, 1992) analyzed the oxygen-18 variations found in a calcitic vein in the southern Great Basin. The uranium series age dates of the calcite vein indicated that major glacial/interglacial transitions occurred some 10,000 to 20,000 years before the solar insolation variations; for example, the peak interglacial in this record appears at 147,000 ± 3,000 years BP, significantly before the insolation peak. While the relevance of this local record to global temperature and precipitation changes may be in doubt, high sea level stands in the period 135,000 to 140,000 years BP have been found by various researchers (e.g., Moore, 1982). The absolute dating capability associated with the calcite vein is in contrast to the approximate dating techniques associated with the deep sea paleoclimate record, where assumptions about sedimentation rates are fundamental in matching the orbital periodicities.

When the orbital solar insolation variations are incorporated in general circulation climate models, the temperature changes are not sufficient to produce ice sheet growth, especially in regions of low altitude accumulation, as was apparently the case for the Laurentide ice sheet (Rind et al.,