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100,000-year problem
The 100,000 year problem is a discrepancy between past temperatures and the amount of incoming solar radiation, or insolation. The latter rises and falls according to the strength of radiation given off by the sun, the distance from the earth to the sun, and the tilt of the Earth's axis of rotation. However, the recent change between glacial and inter-glacial states that occurs on a circa 100,000 year (100 ka) timescale, does not correlate well with these factors.
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. 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. Isotope analysis shows the dominant periodicity of the climate response to be around 100,000 years, but the orbital forcing at this period is small.
== Reconstructing past climate ==
Past climate data—especially temperature—can be readily inferred from sedimentary evidence, although not with the accuracy that instruments can measure current temperatures. Perhaps the most useful indicator of past climate is the fractionation of oxygen isotopes, denoted . 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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