100,000-year problem
The 100,000-year problem (also 100 ky problem or 100 ka problem) of the
While there is a Milankovitch cycle in the range of 100,000 years, related to Earth's
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.[2]
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.[3]
Recognition of the 100,000-year cycle
The
By the late 1990s, δ18O records of air (in the
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 the 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".[4]
Elkibbi and Rial (2001) identified the 100 ka cycle as one of five main challenges met by the Milankovitch model of orbital forcing of the ice ages.[5]
Hypotheses to explain the problem
As the 100,000-year periodicity only dominates the climate of the past million years, there is insufficient information to separate the component frequencies of eccentricity using spectral analysis, making the reliable detection of significant longer-term trends more difficult, although the spectral analysis of much longer palaeoclimate records, such as the Lisiecki and Raymo stack of marine cores[6] and James Zachos' composite isotopic record, helps to put the last million years in a longer-term context. Hence there is still no clear proof of the mechanism responsible for the 100 ka periodicity—but there are several credible hypotheses.
Climatic resonance
The mechanism may be internal to the Earth system. The Earth's climate system may have a natural
Orbital inclination
Orbital inclination has a 100 ka periodicity, while eccentricity's 95 and 125ka periods could inter-react to give a 108ka effect. While it is possible that the less significant, and originally overlooked, inclination variability has a deep effect on climate,[13] the eccentricity only modifies insolation by a small amount: 1–2% of the shift caused by the 21,000-year precession and 41,000-year obliquity cycles. Such a big impact from inclination would therefore be disproportionate in comparison to other cycles.[9] One possible mechanism suggested to account for this was the passage of Earth through regions of cosmic dust. Our eccentric orbit would take us through dusty clouds in space, which would act to occlude some of the incoming radiation, shadowing the Earth.[13]
In such a scenario, the abundance of the isotope
Precession cycles
A similar suggestion holds the 21,636-year[failed verification] precession cycles solely responsible. Ice ages are characterized by the slow buildup of ice volume, followed by relatively swift melting phases. It is possible that ice built up over several precession cycles, only melting after four or five such cycles.[18]
Dust and albedo
It has been suggested that ice-sheet albedo and dust are responsible. The high albedo of northern ice sheets will resist climatic warming from Milankovitch maxima unless they are covered in dust. Dust episodes occur just before each interglacial warming period, and it is claimed that the resulting reduced albedo of northern ice sheets assists in interglacial warming. Dust episodes are said to be caused by low atmospheric CO2 creating CO2-deserts in northern China upland areas, with the resulting dust creating the Loess Plateau and coating the northern ice sheets.[19]
Solar luminosity fluctuation
A mechanism that may account for periodic fluctuations in solar luminosity has also been proposed as an explanation. Diffusion waves occurring within the Sun can be modeled in such a way that they explain the observed climatic shifts on Earth.[20]
Land vs. oceanic photosynthesis
The Dole effect describes trends in δ18O arising from trends in the relative importance of land-dwelling and oceanic photosynthesizers. Such a variation is a plausible cause of the phenomenon.[21][22]
Ongoing research
The recovery of higher-
See also
Notes
- ^ The source for these data is unknown.
References
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All serious students of Earth's climate history have heard of the '100 kyr problem' of Milankovitch orbital theory, namely the lack of an obvious explanation of the dominant ~100 kyr periodicity in climate records of the last 800,000 years.
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