Where have all the 23,000 year ice ages gone.....

Those who care and have a clue are well satisfied with Milutin Milankovitch's explanation of the causes of ice ages, however, they have a secret. Milankovitch attributed ice ages to variations in the Earth's orbit,

• precession of the Earth's axis of rotation (~23,000 year period)
• obliquity, a change angle between the Earth's axis and the orbital plane (41,000 year period)
• changes in the shape of the Earth's orbit (100,000 year period)

which produce changes in insolation. Naively, precession should dominate, but what is inferred from the paleoclimate record is a dominant 41,000 cycle in the early (3 million years ago) to late Pleistoene (1 million years ago). In the July 28th issue of Science, there is not one, but two explanations for this, and a perspective.

Raymo, Lisieck and Nisancioglu observe that precession will increase insolation in one hemisphere while decreasing it in the other. At least potentially this means that the amount of ice in one hemisphere will decrease while that in the other increases by a roughly equal amout. Changes in atmospheric oxygen isotope ratios from ice cores average ice coverage over both hemispheres. RLN believe that between 3 and 1 million years ago the East Antarctic Ice Sheet (EAIS) was much more active than had been thought before, losing ice by melting and calving of icebergs while it would also accumulate ice through snowfall. Changes in the loss to accumulation ratio with changing precession pretty much cancelled northern hemisphere precession driven changes in ice cover. About 1 million years ago, the EAIS cooled to a point where there would be no summer melt and precession could be. Their conclussions include

By allowing modest variations in Antarctic ice sheet size from 3 to 1 Ma, controlled by local insolation, we show that the dominant 41-ky period in marine d18O records may result from out-of-phase ice sheet growth at each pole. Individual ice volume histories in the Arctic and Antarctic realm were likely dominated by both precession (out of phase between poles) and obliquity (in phase between poles) with ice ablation strongly controlled by summer temperatures. Our hypothesis solves the conundrum of why no strong precession signal is observed in global d18O records from this time despite the well-known importance of summer temperatures on ice sheet and glacier mass balance (49). .......

We further propose that long-term cooling resulted in a transition from a primarily landbased to primarily marine-based EAIS margin about 1.0 Ma, resulting in the mid-Pleistocene transition and the strengthening of 23-ky cycles in the d18O record. Ice sheet volume may have increased at both poles at this time because of the establishment of positive globally synchronous feedbacks (such as albedo and CO2) at the precession frequency (50). Lastly, the strengthening of CO2 and albedo feedbacks by enhanced sea level fall or aridity, in conjunction with long-term global cooling, may have led to the establishment of NH ice sheets large enough to survive summer insolation maxima of low intensity, a necessary prerequisite for the development of the ‘‘100-ky’’ cycle (51).

In the same issue Huybers proposes an alternate, and very simple, explanation for the same problem. It turns out that when periods when the average summer insolation increases are also periods during whight the summer is shortest. Thus, the amount of heat available to melt polar ice is roughly independent of precession

Duration and intensity are, however, anticorrelated. This is the Achilles_heel of precession control of glaciation: just when Earth is closest to the sun during summer, summertime is shortest. When the intensity is integrated over the summertime, precession related changes in duration and intensity nearly balance one another (25), and the obliquity component is dominant.

You don't have to pick one over the other. Paillard, in his perspective hits one right down the middle

Both hypotheses could be part of the solution. Huybers’s idea is based on a sound and simple physical premise and is certainly valid to some extent. The hypothesis of Raymo et al. provides a scenario for an increasing contribution of the 23,000-year cycles under a colder climate, through a transition from a land-based to a marine-based East Antarctic ice sheet around 1 million years ago. Indeed, though not dominant, the precessional cycles are present in the climate record of the past 1 million years (the late Pleistocene). Still, neither hypothesis can account for the beginning of Northern Hemisphere glaciations around 3 million years ago. Furthermore, during the past 1 million years, glacial-interglacial oscillations have largely been dominated by a 100,000-year periodicity, yet there is no notable associated 100,000-year insolation forcing. There is currently no consensus on what drives these late Pleistocene 100,000-year cycles.

Anyone with access should read these papers at Science. Some related papers by Huybers can be found on his web site. Material on the Raymo paper can be found here and here
and here.