Wednesday, December 5, 2007

Travails of the Milankovitch hypothesis

The grandiose name “Milankovitch theory” purports to account for the recurrence of the ice ages. In fact, it is more correct to call it the “Milankovitch hypothesis” especially as few astronomers that I know accept it. On that note, I would like to discuss the basis for general non-acceptance of Milankovitch’s theory into conventional astrodynamics and astrometry.. (The former is what has been called “celestial mechanics” in the past, while the latter focuses on methods of position updating for celestial objects)

Start with its contention that the obliquity of the ecliptic (inclination of Earth to its orbital axis) varies from 21 to 24 degrees over a 41,000 period in a process called ‘nutation’. This is certainly a magnitude in excess of a half degree (1800”) on either side of its current 23.5 deg.

Astronomers-astrometrists recognize no such period or differential of axial tilt. The following is from the book, Astronomy- Principles and Practice by A.E. Roy and D. Clarke, 1978, Adam Hilger Books, p. 118:

“Because of the nutational wobble in the Earth’s axis of rotation, the obliquity of the ecliptic (KP in Fig. 10.32) varies about its mean value. The magnitude on either side is about 9.”2.”

For the benefit of non-astronomers, the magnitude cited (9.”2) isn’t even one hundredth of a degree! Indeed it is nearly a factor 4 LESS than a hundredth of a degree! (which translates to 36”- there are 3600” = 1 degree))

Going now to Eichhorn and Mueller’s standard text in astrometry and geodesy- p. 69, “astronomic nutation’:

”The main term of astronomic nutation is produced by the non-coincidence of the Moons’ orbit with the ecliptic in conjunction with the retrograde moton of the lunar nodes. This results in a periodic change in the obliquity of the ecliptic termed nutation in obliquity, denoted by d x

The astronomic nutation, from now on called simply ‘nutation’ is not to be confused with the true nutation appearing as a force-free precession (Eulerian motion) of the Earth’s rotation axis about its principal moment of inertia axis, which is part of the polar motion described in 4.13.

The first six terms of the expression for nutation in obliquity are:

delta eta =

(9.”2100 + 0.”00091t) cos Z - (0.”0904 - 0.”0004t) cos 2Z – (0.”0024 cos (2w _m + Z) + 0.”0002 cos (2w_ s – Z) + 0.”0002cos 2 ( w_ m + Z) + (0.”5522 – 0.”00029) cos 2L_ s

where t denotes the time interval measured from 1900 January 0.5 d ET in Julian centuries (1 JC = 36525 mean solar days), Z is the longitude of the mean ascending node of the lunar orbit on the ecliptic measured from the mean equinox of date, w_m is the ‘argument’ of the point where the Moon is nearest the Earth (i.e. from the lunar perigee), w s is the mean longitude of the solar perigee measured from the mean equinox of date, and L_ s is the geometric mean longitude of the Sun measured from the mean equinox of date. The terms are illustrated in Fig. 4.6

Most interesting in the above – which I merely give for the sake of completeness- is that even jacking up the value of t by 41,000 yrs. (e.g. 410 JC) doesn’t appreciably alter the magnitude from seconds of arc – very small seconds of arc (e.g. about 8.”85 with Z = 160 deg and counting only the first order term)

After giving all this a lot of thought – here is what I think, and why safi and I are at loggerheads:

The Milankovitch theory’s first problem is that it was developed by a Serbian civil engineer (& later meteorologist) who acquired the avocation of astronomy – as a personal abiding interest. This is admirable, but puts him basically in the same class as Immanuel Velikovsky – who was a Russian psychologist – trying to do astronomy. Since astronomers wouldn’t let Velikovksy into the club, it was doubtful they’d let Milankovitch. (Though to be sure, his theory had much more to commend it than Velikovksy’s gobbledegook)

The second problem is that his theory acquired its cachet and bulk of earnest support from outside the astronomical community and inside the geological research community that also spawned Wegner and Croll. The astronomical community cannot claim it as uniquely their own, so like the solar physics community earlier – in regard to space physics incursions into their ‘territory’ in the mid- 20th century – has left it “still born” or a step child. This is perhaps why it appears in no standard astrometric texts or astrodynamic texts. It is the bastard offspring of a non-astronomer and doesn’t follow at all from any standard celestial mechanics principles, equations or theories.

This marginalization can also be rationally justified (within the astronomically- affected community) on the basis that the Milankovitch theory hasn’t yet been adequately tested (e.g. beyond finding geological correlations - which of course is not causation) to prove itself worthy to compare to rigorous astronomical theories – say like the one for the lunar libration. All we have are assorted correlations, which most in the astronomical community can still attribute to flukes or haphazard research.

For example, when conditions are favorable for an ice age in the northern hemisphere, they’re not favorable for one in the southern hemisphere. How could the Milankovitch Cycles cause a global change in climate then? Also, Milankovitch cycles can only account for a temperature difference of 1° to 2°. How is it possible then that sediment records show temperature differences of 7° to 10°? The 100,000 yr cycle is dominant in the record, yet it has the weakest astronomical effect; moreover, in the record, it doesn’t always occur at 100,000 years - ranges from 80,000 to 125,000. How can these variances be explained? Until they are – most astronomers won’t embrace the theory.

Adding to that, in the paper ‘A Causality Problem for Milankovitch”, Daniel B. Karner and Richard A. Muller from the Dept. of Physics, University of California, note an earlier paper by W. Broeker (1992) ‘Upset for Milankovitch Theory’ – in which he discussed a troublesome new measurement. That is, oxygen isotope data from a cave in Nevada called ‘Devils’ Hole’ appeared to show that the timing of the penultimate termination of the ice ages- called ‘Termination II’ – was incompatible with the standard Milankovitch theory (cf. Winograd et al, Science, Vol. 258, p. 255; Ludwig et al, Science, Vol. 258, p. 284)

The data indicated a shift in (delta 16) to interglacial values that was essentially complete by 135 thousand years ago (ka). But at this time, the Northern Hemisphere summer insolation had not yet warmed to the point at which it should have triggered anything extraordinary, let alone a glacial termination. The termination event appeared to precede its cause.

Though the Milankovitchites attempted valiantly to rebut this, as the authors noted, the “causality” problem remained and it was really all an (initially) skeptical community needed to keep the Milankovitch theory from being cemented into standard celestial mechanics. Too many loose ends!

Worse, the Devils Hole data had not been the first to indicate a problem. As far back as 1974,Bloom et al. (Quatr. Research, Vol. 4, p.185) had suggested that sea level had reached a high point, from melting glacial waters, by as early as 142 ka. Their work was based on U-Th ages of coral terraces from the Huon Peninsula in Papua New Guinea. These results were not used when Imbrie et al. (‘Milankovitch and Climate – Part I’, Doredrecht Reidel)) created the SPECMAP template, the most widely used model for explaining how insolation could drive ice age cycles. Instead, Imbrie et al. set the termination at 127 ±6 ka, based on radiometric dates from Barbados corals by Mesolella et al. (J. Geology, Vol. 77, p. 250) and Shackleton and Matthews (‘Nature’, Vol. 6, p. 445)

All of the above provides just enough ‘ammunition’ to those already skeptical – to justify their resistance to Milankovitch theory and to preventing supporters from nudging it into text books (like evolutionists seek to prevent ID’ers)

This is reinforced by the fact that most astronomers’ prevailing skepticism is fuelled by the lack of a precise dynamic time scale, which would make it possible to test the match between the supposed cycles recorded in ocean sediments and the Milankovitch cycles calculated on the basis of the Earth's orbit in standard celestial mechanics. Until this is done, the Milankovitch theory will rightly not be regarded as a part of legitimate celestial mechanics – but rather a marginal or fringe spinoff.

While the cycles with periods near 100,000 years, 41,000 years, and 23,000 years, based on sediment data are intriguing – they don’t get an astronomer’s blood boiling. They merely show circumstantial evidence for the claim. Ultimately, the claim has to be tested and verified in space – to get an astronomer to invest credulity.

Theoretically, at least, there is more than enough Earth orbital data right now to be able to make solid predictions. The trick is to be able to make testable predictions, and then, meld those into a coherent theory of exactly how the orbital forcing occurs and what it does.

A first start would be using sophisticated numerical simulations – piping in the orbital (a, e, i, pi etc.) elements and their perturbations claimed by the Milankovitch crew. Thus, use standard equations of celestial mechanics (e.g. Kepler’s equation, n(t – T) = E – e sin E) and show that the assumed changes actually occur in space. Show that 100,000 yrs. from now Earth will be in such and such predicted position (according to the Milankovitch theory), and ditto for the 41,000 year scale, and so on. Capture these graphics, then publish them.

Interestingly, this was exactly the method used by space physicists to make their case for accepting dynamo processes as applicable to solar flares! Years of work finally paid off, when the most elaborate numerical simulations could no longer be disputed.

When all that happens, the Milankovitch theory may finally be embraced by the key segment of the astronomical community for which it would matter most: astrometry and celestial mechanics. This might be like the space physics community (at least part of it!) was finally embraced by the solar physics community after yrs. of squabbles..

One can, of course, look on these turf battles and territorial defense mechanisms as “immature” and unbefitting science – but scientists are human too. They want to protect what they have, what they have fought for – and not surrender turf without a good fight. If the Milankovitch lot are up for that, their "theory" may finally find its place in standard celestial mechanics text books – maybe in the next fifty yrs. Maybe earlier.

We will see.

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