I think we are all aware of Milankovitch and his cycles as an explanation of the Ice Age Glacials.
It fits pretty well, and I think on the “big lumps” it is most likely correct. There are some odd bits around the edges that don’t quite fit, though. Room to tune, for sure.
So I was reading this (rather well done) site on one of those “issues”:
In particular, this link from that aggregator page:
That, while very readable, goes into painful detail about exactly what we know about timings and exactly where they “have issues”. A sample:
Drysdale et al 2009
Variations in the intensity of high-latitude Northern Hemisphere summer insolation, driven largely by precession of the equinoxes, are widely thought to control the timing of Late Pleistocene glacial terminations. However, recently it has been suggested that changes in Earth’s obliquity may be a more important mechanism. We present a new speleothem-based North Atlantic marine chronology that shows that the penultimate glacial termination (Termination II) commenced 141,000 ± 2,500 years before the present, too early to be explained by Northern Hemisphere summer insolation but consistent with changes in Earth’s obliquity. Our record reveals that Terminations I and II are separated by three obliquity cycles and that they started at near-identical obliquity phases.
Standard stuff by now, for readers who have made it this far.
But the Drysdale paper is interesting on two fronts – their dating method and their “one result in a row” matching a theory with evidence (I extracted more text from the paper in note 5 for interested readers). Let’s look at the dating method first.
Basically what they did was match up the deep ocean cores that record global ice volume (but have no independent dating) with accurately radiometrically-dated speleothems (cave depositions). How did they do the match up? It’s complicated but relies on the match between the δ18O in both records. The approach of providing absolute dating for existing deep ocean cores will give very interesting results if it proves itself.
The correspondence between Corchia δ18O and Iberian-margin sea-surface temperatures (SSTs) through T-II (Fig. 2) is remarkable. Although the mechanisms that force speleothem δ18O variations are complex, we believe that Corchia δ18O is driven largely by variations in rainfall amount in response to changes in regional SSTs. Previous studies from Corchia show that speleothem δ18O is sensitive to past changes in North Atlantic circulation at both orbital and millennial time scales, with δ18O increasing during colder (glacial or stadial) phases and the reverse occurring during warmer (inter- glacial or interstadial) phases.
You get the idea. Pretty much goes through it all. The implication being that the insolation pattern alone is not sufficient and creates inconsistencies and incompatibilities with actual events.
So I took a look, again, at some old familiar friends / graphs / and all. I’ve seen again, something seen a dozen times+ and had a new thought (in the context of the above page). Here’s that standard graph:
OK, look at it for a minute. Temperatures get colder and colder. CO2 levels drop to plant starvation levels below 200 PPM (gee… never hear much about how nature cuts the CO2 down to near nothing in a glacial, do we…) and we are all very comfortable with that story. Gets cold, CO2 gets sucked down. Then “magic happens” and it gets warm enough to melt the ice and let the CO2 free ( 800 years later ) and good times return to the planet.
While waiting, it is fun to take a dividers (“compass”) and set the points the width of the last interglacial at the -2 line. Just about when the “plunge” gets going really fast. Then move those dividers over to “now”. My thought on that is that the Younger Dryas was an impact event that chopped the overshoot peak off of our interglacial. But now we are just about the same width as before from that -2 C point. Not much longer to go… Now back to the main line:
And we all ignore the dust.
At most it gets a bare mention as cold times being dry and dusty times.
I’m going to add what may be a new wrinkle.
So the speculation at SOD was that it was obliquity (axial tilt of the planet) cycles that set the pattern (though using variably 3 or 5 of them, and sometimes needing a ‘kicker’ from the other changes such as eccentricity). I’m going to point at dust. Yes, it rises over time. The world oh so slowly desiccates and dies. It takes time to dehydrate the soil and kill everything. Then grind it to a fine dust.
Eventually we have giant sand and dust storms that must be way bigger than the present ones from the Sahara. Sand and dust from there blows out into the Atlantic and eventually reaches Florida and the Caribbean in significant quantity. Yet during our present regime, we are “low dust” in the icy poles. There had to be much wider desert areas and much more dust in the wind to load the poles with dust.
My hypothesis is simply that “the dust matters”. During low obliquity, there isn’t enough sunshine to matter much. At the start of the process, there isn’t enough desert and blowing dust to move enough dust to the ice sheets. Only after enough life has died and enough soil is sand and dust, and we’ve gotten rid of all that dust trapping plant life between mid-latitudes and poles, only after enough ocean has gone to be icy poles exposing ever more mud flats globally and THEY dry; only then can enough dust be delivered onto those mid latitude ice sheets to make a significant albedo difference. At that point, the very next obliquity event can finally start that ice sheet melting. That, then, starts the positive feed back.
When this happens just before the right Milankovitch configuration, it can run to completion as an interglacial warm (and wet and living) period. When not so timed, it’s a ‘squib’ melt and fails to run to completion (like at about 60 MYBP where on the lower graph there is a spike of dust, but then life takes over and dust drops again, but without enough sun at 65 N to make it stick, and the glaciers return…)
I would also note that the dust peaks are getting larger over time. Artifact? Or is it getting ever harder to break the grip of ice…
So that’s the idea. That it is a ‘3 way’ of obliquity, dust cycles, and Milankovitch as the gating event post initiation. That, then, lets Milankovitch drivers show up at the observed delay, while also helping to explain some of the “squib” events and why sometimes the sun is enough, and sometimes not.
Think of a very brown dry world and how it would respond to an obliquity kick in insolation. Then think about a wet green world. Then think about a very dry white ice world. Ice and cold can withstand the added sun. Green and wet can withstand the added sun (even if only the tropical belt and a bit into the temperate zones). But once “life dies and dries”, it’s a brown dusty land with brown dusty ice. That can’t stand the heat of more sun in the modest lattitudes when obliquity puts the sun right overhead.
So kick it around. See if it has legs, or is stuck in a sand trap ;-)