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. 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 Mid-Pleistocene 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 transition in periodicity from 41,000 years to 100,000 years can now be reproduced in numerical simulations that include a decreasing trend in carbon dioxide and glacially induced removal of regolith, as explained in more detail in the article Mid-Pleistocene Transition. 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. By the late 1990s, δ18O records of air (in the Vostok ice core) and marine sediments was available and was compared with estimates of insolation, which should affect both temperature and ice volume. As described by Shackleton (2000), the deep-sea sediment record of δ18O 'is dominated by a 100,000-year cyclicity that is universally interpreted as the main ice-age rhythm'. Shackleton (2000) adjusted the time scale of the Vostok ice core δ18O record to fit the assumed orbital forcing and used spectral analysis to identify and subtract the component of the record that in this interpretation could be attributed to a linear (directly proportional) response to the orbital forcing. The residual signal (the remainder), when compared with the residual from a similarly retuned marine core isotope record, was used to estimate the proportion of the signal that was attributable to ice volume, with the rest (having attempted to allow for the Dole effect) being attributed to temperature changes in the deep water. The 100,000-year component of ice volume variation was found to match sea level records based on coral age determinations, and to lag orbital eccentricity by several thousand years, as would be expected if orbital eccentricity were the pacing mechanism. Strong non-linear 'jumps' in the record appear at deglaciations, although the 100,000-year periodicity was not the strongest periodicity in this 'pure' ice volume record. The separate deep sea temperature record was found to vary directly in phase with orbital eccentricity, as did Antarctic temperature and CO2; so eccentricity appears to exert a geologically immediate effect on air temperatures, deep sea temperatures, and atmospheric carbon dioxide concentrations. Shackleton (2000) concluded: 'The effect of orbital eccentricity probably enters the paleoclimatic record through an influence on the concentration of atmospheric CO2'.