100,000-year problem

The 100,000-year problem ("100 ky problem", "100 ka problem") of the Milankovitch theory of orbital forcing refers to a discrepancy between the reconstructed geologic temperature record and the reconstructed amount of incoming solar radiation, or insolation over the past 800,000 years. Due to variations in the Earth's orbit, the amount of insolation varies with periods of around 21,000, 40,000, 100,000, and 400,000 years (Milankovitch cycles). Variations in the amount of incident solar energy drive changes in the climate of the Earth, and are recognised as a key factor in the timing of initiation and termination of glaciations.

While there is a Milankovitch cycle in the range of 100,000 years, related to Earth's orbital eccentricity, its contribution to variation in insolation is much smaller than those of precession and obliquity. The 100,000-year-problem refers to the lack of an obvious explanation for the periodicity of ice ages at roughly 100,000 years for the past million years, but not before, when the dominant periodicity corresponded to 41,000 years. The unexplained transition between the two periodicity regimes is known as the transition, dated to some 800,000 years ago.

The related "400,000-year-problem" refers to the absence of a 400,000-year periodicity due to orbital eccentricity in the geological temperature record over the past 1.2 million years.

The geologic temperature record can be reconstructed from sedimentary evidence. Perhaps the most useful indicator of past climate is the fractionation of oxygen isotopes, denoted δ¹⁸O. This fractionation is controlled mainly by the amount of water locked up in ice and the absolute temperature of the planet, and has allowed a timescale of marine isotope stages to be constructed.

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