100,000-year problem

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An average of several samples of δ18O, a proxy for temperature, for the last 600,000 years

The 100,000 year problem is a discrepancy between past temperatures and the amount of incoming solar radiation, or insolation, which grows and shrinks periodically on a 100,000 year (100 ka) timescale.

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 solar heating drive changes in the climate of the Earth, and are recognised as a key factor in the timing of initiation and termination of ice ages. Spectral analysis shows that the most powerful climate response is at 100,000 year period, but the orbital forcing at this period is small.

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[edit] Reconstructing past climate

A δ18O record for the past 120,000 years

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 δ18O. This fractionation is controlled mainly by the amount of water locked up in ice and the absolute temperature of the planet.

[edit] Comparing the records

The δ18O record of air (in the Vostok ice core) and marine sediments has been compared with estimates of solar insolation, which should affect both temperature and ice volume. Nicholas Shackleton orbitally tuned the Antarctic ice core air δ18O (ie he matched the signal to this assumed 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, allowed him 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 concluded: "The effect of orbital eccentricity probably enters the paleoclimatic record through an influence on the concentration of atmospheric CO2".[1] The mechanism causing these cyclic temperature changes remains at the heart of the 100,000-year problem.

[edit] Solutions to the problem

The effect of obliquity variations may, in concert with precession, be amplified by orbital inclination.

As the 100,000-year cyclicity 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[2] and James Zachos' composite isotopic record helps to put the last million years in longer term context.[3] Despite these records, which often show eccentricity components of climate variation, some researchers believe that the 100 ka cycle may be statistically insignificant.[4] Hence there is still no absolute consensus on the mechanism responsible for the 100 ka periodicity — but there are several popular contenders.

The mechanism may be internal to the Earth system. The Earth's climate system may have a natural resonance frequency of 100 ka — that is to say, feedback processes within the climate automatically produce a 100 ka effect, much as a bell naturally rings at a certain pitch.[5][6] Opponents to this claim point out that the resonance would have to have developed 1 million years ago, as a 100 ka periodicity was weak to non-existent for the preceding 2 million years. This is not infeasible — continental drift and sea floor spreading rate change have been postulated as possible causes of such a change.[7] Free oscillations of components of the Earth system have been considered as a cause,[8] but too few Earth systems have a thermal inertia on a thousand-year timescale for any long-term changes to accumulate.

Precessional cycles may produce a 100,000 effect.

Orbital inclination has a 100 ka periodicity, whilst eccentricity's 95 and 125 ka periods combine to give a 100 ka effect. Whilst it is possible that the less significant, and originally overlooked, inclination variability has a profound effect on climate,[9] the eccentricity only modifies insolation by a small amount: 1-2% of the change caused by the 21,000-year precession and 41,000-year obliquity cycles. Such a large impact would therefore be disproportionately in comparison to other cycles.[7] 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.[9] In such a scenario, the abundance of the isotope 3He, produced by solar rays splitting gases in the upper atmosphere, would be expected to decrease — and initial investigations did indeed find such a drop in 3He abundance.[10][11] However, further investigation showed this pattern of negative excursion to be due to a focussing effect in the sedimentary sequence rather than any genuine variation in radiation, invalidating the hypothesis.[12] However, there is still the possibility that the 100 ka eccentricity cycle acts as a "pacemaker" to the system, amplifying the effect of precession and obliquity cycles at key moments, pushing a system usually in an accumulatory state "over the brink" into a rapid melting phase, by providing the smallest of taps.[7][13] A similar suggestion holds the 21,000-year Precession cycles solely responsible. Ice ages are characterised by the slow build up of ice volume, followed by relatively rapid melting phases. It is possible that ice built up over several precession cycles, only melting after four or five such cycles.[14]

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 modelled to in such a way that they explain the observed climatic shifts on earth.[15] However, the He3 signal again appears to contradict this finding.[16]

An algal bloom. The relative importance of land- and sea-based photosynthesis may fluctuate on a 100,000 year timescale.

The Dole effect describes changes in δ18O arising from changes in the relative importance of land-dwelling and oceanic photosynthesisers. Such a variation is a plausible perpetrator of the phenomenon,[17][18] but only really passes the buck: what changed the importances of land- and sea-based photosynthesis?

The recovery of higher resolution ice cores spanning more of the past 1,000,000 years by the ongoing EPICA project may help to shed more light on the matter. A new, high precision dating method developed by the team[19] allows better correlation of the various factors involved and puts the ice core chronologies on a firmer temporal footing, endorsing the traditional Milankovitch hypothesis, that climate variations are controlled by insolation in the northern hemisphere. The new chronology is inconsistent with the "inclination" theory of the 100,000-year cycle. The establishment of leads and lags against different orbital forcing components with this method — which uses the direct insolation control over nitrogen/oxygen ratios in ice core bubbles — is in principle a great improvement in the temporal resolution of these records and another significant validation of the Milankovitch hypothesis.

[edit] See also

[edit] References

  1. ^ Shackleton, N.J. (2000). "The 100,000-Year Ice-Age Cycle Identified and Found to Lag Temperature, Carbon Dioxide, and Orbital Eccentricity". Science 289 (5486): 1897–1902. doi:10.1126/science.289.5486.1897. PMID 10988063. Retrieved on 2007-05-09. 
  2. ^ LR04 Benthic Stack
  3. ^ Zachos, J; Pagani, M, Sloan, L, Thomas, E, Billups, K (2001-04-27). "Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present". Science 292 (5517): 686–693. doi:10.1126/science.1059412. PMID 11326091. Retrieved on 2007-09-14. 
  4. ^ Wunsch, C. (2004). "Quantitative estimate of the Milankovitch-forced contribution to observed Quaternary climate change" ([dead link] – Scholar search). Quaternary Science Reviews 23 (9-10): 1001. doi:10.1016/j.quascirev.2004.02.014. Retrieved on 2007-05-09. 
  5. ^ Ghil, M. (1994). "Cryothermodynamics: the chaotic dynamics of paleoclimate". Physica D 77 (1-3): 130–159. doi:10.1016/0167-2789(94)90131-7. ISSN 0167-2789. 
  6. ^ Gildor, H.; Tziperman, E. (2000). "Sea ice as the glacial cycles' climate switch: Role of seasonal and orbital forcing". Paleoceanography 15 (6): 605–615. doi:10.1029/1999PA000461. 
  7. ^ a b c Ruddiman, W.F. (2006). "Orbital changes and climate". Quaternary Science Reviews 25: 3092–3112. doi:10.1016/j.quascirev.2006.09.001)>> (inactive 2008-06-25). Retrieved on 2007-05-09. 
  8. ^ Saltzman, B.; Hansen, A.R.; Maasch, K.A. (1984). "The late Quaternary glaciations as the response of a three-component feedback system to Earth-orbital forcing". Journal of Atmospheric Sciences 41: 3380–3389. doi:10.1175/1520-0469(1984)041<3380:TLQGAT>2.0.CO;2. 
  9. ^ a b Muller, R.A.; MacDonald, G.J. (1995). "Glacial cycles and orbital inclination". Nature 377 (6545): 107–108. doi:10.1038/377107b0. Retrieved on 2007-05-09. 
  10. ^ Farley, K.A. (1995). "Cenozoic variations in the flux of interplanetary dust recorded by 3He in a deep-sea sediment". Nature 376 (6536): 153–156. doi:10.1038/376153a0. Retrieved on 2007-05-09. 
  11. ^ Kortenkamp, S.J.; Dermott, S.F. (1998-05-08). "A 100,000-Year Periodicity in the Accretion Rate of Interplanetary Dust". Science 280 (5365): 874. doi:10.1126/science.280.5365.874. PMID 9572725. Retrieved on 2007-05-09. 
  12. ^ Higgins, S.M.; Anderson, R.F.; Marcantonio, F.; Schlosser, P.; Stute, M. (2002). "Sediment focusing creates 100-ka cycles in interplanetary dust accumulation on the Ontong Java Plateau" ([dead link] – Scholar search). Earth and Planetary Science Letters 203 (1): 383–397. Retrieved on 2007-05-09. 
  13. ^ Hays, J.D.; Imbrie, J.; Shackleton, N.J. (1976-12-10). "Variations in the Earth's Orbit: Pacemaker of the Ice Ages". Science 194 (4270): 1121. doi:10.1126/science.194.4270.1121. PMID 17790893. Retrieved on 2007-05-09. 
  14. ^ Imbrie, J.; Imbrie, J.Z. (1980-02-29). "Modeling the Climatic Response to Orbital Variations". Science 207 (4434): 943. doi:10.1126/science.207.4434.943. PMID 17830447. Retrieved on 2007-05-09. 
  15. ^ Ehrlich, R. (2007). "Solar resonant diffusion waves as a driver of terrestrial climate change". Journal of Atmospheric and Solar-Terrestrial Physics 69 (7): 759–766. doi:10.1016/j.jastp.2007.01.005. 
  16. ^ Marsh, G.E. (2007). "Climate Change: The Sun’s Role". eprint arXiv: 0706.3621. Retrieved on 2007-10-17. 
  17. ^ Bender, M.; Sowers, T.; Labeyrie, L. (1994). "The Dole effect and its variations during the last 130,000 years as measured in the Vostok ice core". Global Biogeochemical Cycles 8 (3): 363–376. ISSN 0886-6236. Retrieved on 2007-05-09. 
  18. ^ Sowers, T.; Bender, M.; Labeyrie, L.; Martinson, D.; Jouzel, J.; Raynaud, D.; Pichon, J.J.; Korotkevich, Y.S. (1993). "A 135000-year Vostok-SPECMAP common temporal framework". Paleoceanography 8 (6): 737–766. Retrieved on 2007-05-09. 
  19. ^ Kawamura, K.; Parrenin, F.; Lisiecki, L.; Uemura, R.; Vimeux, F.; Severinghaus, J.P.; Hutterli, M.A.; Nakazawa, T.; Aoki, S.; Jouzel, J.; Others, (2007). "Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years". Nature 448 (7156): 912–916. doi:10.1038/nature06015. 
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