Climatic resonance

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.^[7][8] 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 feasible—continental drift and sea floor spreading rate change have been postulated as possible causes of such a change.^[9] Free oscillations of components of the Earth system have been considered as a cause,^[10] but too few Earth systems have thermal inertia on a thousand-year timescale for any long-term changes to accumulate. The most common hypothesis looks to the Northern Hemisphere ice sheets, which might expand through a few shorter cycles until large enough to undergo a sudden collapse.^[11] The 100,000-year problem has been scrutinized by José A. Rial, Jeseung Oh and Elizabeth Reischmann^[12] who find that master-slave synchronization between the climate systems natural frequencies and the eccentricity forcing started the 100 ky ice ages of the late Pleistocene and explain their large amplitude.

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 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.^[14][15] Others have argued possible effects from the dust entering the atmosphere itself, for example by increasing cloud cover (on 9 July and 9 January, when the Earth passes through the invariable plane, mesospheric cloud increases).^[16] Therefore, the 100 ka eccentricity cycle can act as a "pacemaker" to the system, amplifying the effect of precession and obliquity cycles at key moments, with its perturbation.^[17]

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-resolution ice cores spanning more of the past 1,000,000 years by the ongoing EPICA project may help shed more light on the matter. A new, high-precision dating method developed by the team^[23] allows better correlation of the various factors involved and puts the ice core chronologies on a stronger 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. An international climate modelling exercise (Abe-ouchi et al., Nature, 2013^[24]) demonstrated that climate models can replicate the 100,000-year cyclicity given the orbital forcing and carbon dioxide levels of the late Pleistocene. The isostatic history of ice sheets was implicated in mediating the 100,000-year response to the orbital forcing. Larger ice sheets are lower in elevation because they depress the continental crust upon which they sit, and are therefore more vulnerable to melting.