Guest essay by Clive Best
It is proposed that for the last 800,000 years super-tides caused by maxima in orbital eccentricity have been the key factor needed to break up large northern ice sheets to enable the 41,000 year insolation cycle to initiate an interglacial. Insolation alone was sufficient to melt back the ice sheets over the previous 4.4 million years, as observed by the long series of 41,000 year glaciation cycles in the LR04 Do18 stack[1]. The obliquity cycle was broken once an underlying cooling trend had increased glacial ice sheet extent beyond a threshold for “Milankowitch” summer melting.
Since that time huge tidal forces amplified by increased eccentricity, have been required to bring a glacial cycle to an end by carving and shelving the ice sheets. Once initiated a rapid deglaciation proceeds due to enhanced insolation with positive albedo feedback, resulting in a sawtooth shape. The most exceptional tides occur when the perihelion of the sun and the moon coincide and both orbits are at maximum eccentricity. This process can explain both the origin of the 100,000y cycle of ice ages and the transition from earlier 41,000y glaciation cycles which have so far remained a mystery[2].
Fig 1a. 5 million years of benthic foram dO16 data. The blue curve is a fit to Milankovitch harmonic data described inPhenomenology of Ice Ages.
Fig 1b. Correlation of inter-glacials with maximum eccentricity of Earth’s orbit
Fig 1c. Correlation of larger obliquity and warmer temperatures. A calculation of the insolation at the poles that demonstrating the dominance of the 41,000 year cycle is shown in Fig 2.
Fig 2. Maximum and total solar insolation calculated at the poles during last 600,000 years. The total annual insolation and the N-S asymmetry show the underlying effect of the 41,000 obliquity signal.
The basic hypothesis behind this proposal is the following.
- 5 million years ago a gradual cooling of the climate began (Fig 1a). This was most likely due to plate tectonics. First Antarctica moved further south to sit over the South Pole isolating the Southern Ocean. Second the Panama isthmus closed cutting off circulation between the Atlantic and Pacific.
- A regular glacial cycle began driven by the 41,000 year change in obliquity of the earth’s axis. Higher obliquity brings higher insolation to both poles modulated by the precession of equinoxes. The 41,000y signal dominates glaciation cycles from 5 million years ago until 1 million years ago. Meanwhile the intensity of glacial periods was slowly increasing as global cooling due to plate tectonics continued.
- 900,000 years ago this general cooling reached a critical stage because the increase in spread of ice sheets in the Northern hemisphere became too large to fully melt back during the next peak in obliquity. The cycle of 41,000y ice ages was broken.
- Something else was now needed to trigger ice ages and that something was extreme tidal forces caused by maximum orbital eccentricity. When these coincided with peak insolation in the Arctic Circle the breakup of the northern ice sheets could begin and they collapsed rapidly within one precession cycle.
To understand these tidal forces we need to understand what perigee spring tides are. These are exceptional tides that occur when the new moon coincides with the lunar perigee (closest distance of approach to the earth). These tides are typically 20% larger than normal, because tides are tractional forces that depend on 1/R^3. Perigean tides occurs every 411.78 days (spring tide at lunar perigee). However there are a series of even rarer and more extreme perigean tides:
§ Perigean Eclipse Tides (PET) which occur every 2.99847 years which is when a Perigean spring tide coincides with the Earth-Sun-Moon all aligned in the ecplitic plane. The lunar and solar tides then pull directly together on the earth rather than through a cosine(declination) offset.
§ Finally there are Super Perigean Eclipse Tides(SPET) which occurs every 1832 years. This super tide occurs when a Perigean Eclipse Tide coincides with the earth also at perigee in its orbit around the sun so that the solar tide is also at its maximum value possible. These rare events cause tidal forces some 30% above normal. There is also a 5000 year modulation in the strength of SPET.[3]
Now consider what additional effects variations in the “Milankovitch” cycle of eccentricity would have on these Perigean tides.
The minimum distance of approach at perigee depends on the orbital eccentricity both for the moon and the earth. Tides are a tractional force whose greatest effect is felt near the poles. During both the Arctic and Antarctic winters with zero insolation there are clear signals of tidal effects on temperature (4). Furthermore tides have also a direct effect on sea ice. Postlethwaite et al.[5] write
Tidal mixing within the water column and at the base of the sea ice cover can increase the heat flow from deeper water masses towards the surface causing decreased freezing and increased melting of sea ice and possibly the formation of sensible heat polynyas (Morales-Maqueda et al., 2004; Willmott et al., 2007; Lenn et al., 2010). The tidal currents can additionally increase the stress and strain on the sea ice and cause leads to open periodically within the sea ice cover (Kowalik and Proshutinsky, 1994).
Tidal forces therefore act to break up ice sheets and change ocean heat flows. Fortnightly changes of 20% in ice stream flow have also been observed in Antarctica due to spring tides. [6]
The 100,000 and 400,000 year cycles in the ellipticity of the Earth’s orbit are caused by regular gravitational effects of the other planets as they orbit the sun, particularly those of Jupiter and Saturn. Every 100,000 years the orbits of Jupiter and Saturn align themselves so that their net gravity perturbs the Earth’s orbit causing it to elongate and become more elliptical. This cycle reaches a maximum every 400,000 years in regular fashion. The gravitational force of the sun on the moon is more than twice that of the Earth. For an observer in outer space the moon appears to orbit the sun just like any other planet. Its orbit is perturbed by the Earth’s gravity making it slightly concave. It is only from Earth that it appears to us to be in an elliptical orbit around the Earth. The moon’s orbit is therefore also affected by the gravitational pull of the other planets inducing a similar (Milankovitch) variation of eccentricity in its orbit around the sun. However this also causes an increased elliptical orbit of the moon around the earth because they have different mass.
How large can the tides get during 100,000y cycles of maximum eccentricity? Figures 4 and 5 show calculations of the change in tidal forces due to the sun and the moon for various values of orbital eccentricity. These calculations are based on the distance to the earth for different times in the year for the sun, and in the sidereal month for the moon. Tides are tractional forces which depend on 1/R^3 which explains why the moon has a larger tidal pull on the oceans than does the much more massive sun. At spring tides the two tidal forces are superimposed:
Fig 4: Relative strength of the solar lunar tidal force – proportional to 1/R^3
The largest solar tides are up to 20% higher than those we experience today. I have been unable to find any information about Milankovitch calculations of effects of the lunar orbit but I will assume a proportional increase to that of the earth. Given that assumption we can look at the more important lunar tide.
Fig 5: Variation in strength of lunar tides with orbital eccentricity relative to today.
We see that spring lunar tides for a lunar orbit twice the current eccentricity would be about 60% higher than they are today. Lunar tides are about twice the strength of solar tides so overall spring tides would have been at least 50% stronger than they are today, and Super Perigean tides would have been 20% stronger again.
Are these super-tides the catalyst to break up the large northern ice sheets and exit ice ages once every 100,000 years ?
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References
1. Lisiecki, L. E., and M. E. Raymo (2005), A Pliocene-Pleistocene stack of 57 globally distributed benthic d18O records, Paleoceanography, 20, PA1003
2. Maureen Raymo & Peter Huybers, Unlocking the mysteries of the ice ages, Nature Vol 451/17 P. 284, 2008
3. The 1,800-year oceanic tidal cycle: A possible cause of rapid climate change, Charles D. Keeling and Timothy P. Whorf, PNAS (2000) 3814-3819
4. The influence of the lunar nodal cycle on Arctic climate, Harald Yndestad, ICES Journal of Marine Science, 63(3) 401, 2005
5. The effect of tides on dense water formation in Arctic shelf seas, C. F. Postlethwaite, M. A. Morales Maqueda, V. le Fouest,*, G. R. Tattersall1,**, J. Holt, and A. J. Willmott, Ocean Sci., 7, 203–217, 2011
6. Fortnightly variations in the flow velocity of Rutford Ice Stream, West Antarctica, G.H. Gudmundsson, Nature 444, 1063-1064, 2006
7. On the factors behind large Labrador Sea tides during the last glacial cycle and the potential implications for Heinrich events, Brian K. Arbic,1 Jerry X. Mitrovica,2 Douglas R. MacAyeal,3 and Glenn A. Milne, PALEOCEANOGRAPHY, VOL. 23, PA3211
Interesting hypothesis. I was confused by whether or not you meant that systematic supertides cause the interglacials, or the glaciation. Only the former would make sense, right, but you assert several places that they are coincident with glaciation, I think as typos. It also cannot (IMO) just be a supertide that breaks up the ice all at once and causes an interglacial — it might well be that tidal height varies systematically with orbital eccentricity and eventually reaches a state where the average spring tides break up the ice regularly enough to trigger the kind of positive feedback you assert. That suggests that there might be some sort of critical point in average tide height that favors the transition(s) in both directions.
It would also be interesting to see if the occurrence of supertides over the last 20 years was asymmetrically distributed between Northern and Southern hemisphere summer and winter. It was suggested that a rare coincidence of more of these events than usual over the last 20 years is responsible for the decrease in average Arctic sea ice over that interval (or is otherwise correlated with the years where minimum ice occurred), but in the meantime Antarctic ice has actually grown. For the assertion to be consistent, one would need a matching asymmetry in the distribution of the timing of the supertides and the hemisphere, and actually it wouldn’t hurt to specifically correlate years and even dates in which the supertides occurred and specific sea ice anomalies. It isn’t like a supertide in 2003 is going to cause an ice anomaly in 2005.
Finally, tidal forces can be correlated with other things besides ice breakup — e.g. volcanism. Again things I don’t know or don’t even know where to look for, but is there a geological/historical measure of “net global volcanic activity” vs “contemporary eccentricity of the Earth’s orbit”? Volcanoes could easily be the trigger of glacial/interglacial transitions also, as they modulate both GHGs, soot/ash, and aerosols. Looking for correlations here on any time scale whatsoever would be useful, although I’m guessing that there is damn-all data past maybe 200 years (roughly, Tambora). If there was a burst of supertides over the last two decades, were they warming or cooling? Did they increase or decrease volcanism (potentially, given correlation)?
I’m not convinced by your argument — not yet — but it is certainly plausible. However, there are other things associated with the extremes in orbital eccentricity, such as extremes in peak versus minimum annual insolation and just where/how it hits the pattern of continents on the Earth in the two hemispheres, that are alternative explanations that would obviously have similar timing, and of course it could be that there are multiple explanations, any two out of three conditions favor the start of an interglacial, that sort of thing, that might explain why no particular explanation has perfect correlation. The same sort of thing is true for solar state as a correlate of climate. Even if it is an important cause, it is one of many and it may be that sometimes the stars align for it to “work” and other times they are aligned such that it DOESN’T “work” to effect some particular change. Low solar state (Maunder minimum) AND high volcanism AND a deficit of supertides that causes polar icepacks to grow over 20+ years (with effects stretched out to 100+ years as supertide rates return closer to “normal”.
Everybody looks for correlation one dimension at a time, but the climate trajectory is very likely ten or twenty important dimensional as well as being nonlinearly coupled and chaotic in its local dynamics. But still, interesting idea.
rgb
@rgbatduke
Sorry – Yes I am only addressing interglacials. I agree there is another unsolved issue as to why exactly the earth then gradually cools to enter a new glaciation. The evidence would suggest that this cooling phase for the next ice age should begin within the next 1500 years. Perhaps keeping CO2 levels above 400ppm until then might be our salvation !
I agree that it would be a long process whereby average spring tides increase to a level that slowly undermines ice sheets rather than just one super-tide. There has also long been a suspicion that tidal forces acting on the lithosphere might trigger earthquakes and vulcanism. There are also a number of volcanoes in Antarctica.
So far as I know no climate model includes any tidal effects at all. They are assumed to be negligible. Nor do any of the paleoclimate studies.
@Gail, Yes I get the feeling that E.M. Smith and Ian Wilson should get together because both of them have taken lunar cycle effects on climate far deeper than I am capable – for example this:
Likewise The 18.6 year precession changes the tidal latitude extent of the tidal bulges which could effect the jet stream.
The last perigrean spring tide occurred at the end of december. This was the main reason for the coastal flooding in the UK, rather than the storms were not that unusual for the Atlantic.
Joachim says:
January 15, 2014 at 6:54 am
This post will leave Anthony and Willis, the two upright cyclefighters, with gnashing teeths….
….Cylcemania is rising its head again….10 times worse than Hansen-Warmism! Nèst-il pas?
>>>>>>>>>>>>>>>>>>>>
So the sawtooth pattern over a 5.3 million year span is not cycles it is just random chaos HMMmmm? There are no such things as laws of physics either I take it. Everything is chaos with no rhyme or reason.
That 1/R^3 force can be worked out by a high school algebra student. Here’s where it comes from. Let the distance of the sun from earth be R, radius of earth be r.
The force of the sun on the near side of earth is 1/( R – 1/2 r) ^2
The force on the far side of earth is 1 (R + 1/2 r)^2
The net tidal force pulling the two sides of earth apart is
Rr/(R^4 + 2 (Rr) ^2+ a bunch of smaller terms) (Rr)^2 and those smaller terms can be disregarded if the radius of the earth, r, is small compared to R, the distance between the earth and sun. In the case of the moon-earth system, where r is 4000 miles and R is 240,000 miles,
(Rr)^2 is only 1/3600 the first term, R^4 .
clivebest says: @ur momisugly January 15, 2014 at 10:25 am
…4)Yes glaciers on land do experience tidal forces. Of course it depends how strong they are.
>>>>>>>>>>>>
An interesting bit from E.M. Smith:
(Clive you might want to checkout the data E.M. has link
E.M. Also has an interesting diagram of “…while we are used to thinking of the Moon as doing laps around the Earth, the reality is more that we co-orbit the Sun and do a little ‘speed up / slow down’ dance relative to each other and with some ‘inward – outward’ and ‘up – down’ wiggles.” image
He also has another chart
Also do not forget about comets and other space junk that would have periodic appearance (and possible impact) link
@Clive
Thanks for the discussion, good thinking still. It’s Obvious that the earth tidal amplitudes are subject to significant variation. However the question remains if that would be sufficient to explain all the features of the glacial transitions that are NOT discussed this far, apart from the effect of tides on landlocked ice as mentioned herein. For instance, how to explain the massive reorganisation in deep benthic ocean settings, that caused the gigantic CO2 burp (Marchitto et al 2007 http://www.sciencemag.org/content/316/5830/1456 ).
But on the other hand, resonance at a certain point could very well explain the thus far (not really explained) Meltwater Pulse 1A, not meltwater but excessive tides.
http://en.wikipedia.org/wiki/Meltwater_pulse_1A
rgbatduke says: @ur momisugly January 15, 2014 at 1:53 pm
Interesting hypothesis….
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What I found interesting was the North-South tidal component that E.M. Smith unearthed. If this is real, and the different locations of eclipses indicate that it is, you could have a long slow “tide” driving the oceans first away from the equator and then back towards the equator. Given the lopsided arrangement of land masses this could have some very interesting effects on the oceans and currents.
Clive that first link was link It has a listing of all the lunar essays by E.M.
@Arno
“This is just another example where fanciful explanations of Arctic warming abound, all for lack of knowledge that I provided a full explanation in 2011. Arctic warming started at the turn of the twentieth century, after two thousand years of slow, linear cooling. It halted for thirty years in mid-century, then resumed, and is apparently still active. I say apparently because this year’s ice cap was twice the size of last year’s icecap. Remembering the previous mid-century halt in warming, we must keep in mind that what has happened in nature before can happen again. When the warming first started there was no increase of atmospheric carbon dioxide and this rules out the greenhouse effect as its possible cause. The only logical cause left is a rearrangement of North Atlantic currents at the turn of the century that started to carry warm Gulf Stream water into the Arctic Ocean. The warming pause in mid-century would then correspond to a temporary return of the previous flow pattern. You can download my paper from Climate etc. web site.”
Cant find your paper! What do you suggest causes the said rearrangement of the North Atlantic currents?
Ian Wilson says:
“New or Full Moons that re-occur when the Perigee of the lunar orbit is pointing ta the Sun at (or near) the time of Perihelion are whole multiples of 739 years (i.e. 0.0, 739.0, 1478.0 and 2217.0 years).”
For merely the astronomical interest, I suggest you look at 372 years:
http://www.fourmilab.ch/earthview/pacalc.html
That’s the one I corrected Dr Tony Phillips with back here:
http://wattsupwiththat.com/2010/12/17/lunar-eclipse-and-winter-solstice-to-coincide-first-time-since-1378/#comment-552757
Weather and climate have been going through (increasingly?) wild swings for the past few million years. That much we know. In any case, the end of the movie may not be “they lived happily ever after.” We need to get serious about an exit strategy. Space really is the final frontier.
Ulric,
If I thought that science or physics had anything to do with any of your suggestions and posts I would would look them up. You seem to be correcting everyone except yourself.
Try looking at the fifth figure of this post before you start mouthing off.
http://astroclimateconnection.blogspot.com.au/2013/06/are-dansgaard-oeschger-d-o-warm-events.html
Gail Combs says:
January 15, 2014 at 2:16 pm
During both the full and new moons there is higher moisture content in the soil. The seeds absorb the higher water content, which causes germination to occur more rapidly.
——————————————————————————————————-
I have ‘planted by the moon’ for many years now. There is no question in my mind that there is superior results when following the moon cycle.
It seems to me that if tidal strength influences glacial cycles (which is a very interesting idea), then the effect would probably be through the breaking up of shelf ice areas. Much of the Arctic Ocean and North Atlantic is covered by shelf ice during glaciations, and breakup of this would have a destabilizing effect on the surrounding continental ice sheets. Existing shelf ice areas are known to be strongly affected by tidal effects.
There is evidence that the strongest tides are involved in triggering earthquakes and volcanoes. There are also several volcanoes in and around Antarctica.
http://www.agu.org/wps/ChineseJGeo/47/47.04/articles/yxx.pdf
@Steve Reddish
It might very likely be that breaking up the sea ice allows more water to be exposed to the sun and store more energy. Over time, this would have a substantial negative effect on the land-based ice. I’m sure I calculated that if you go from absolute zero, then the oceans store the equivalent of one whole year’s worth of solar energy received by the Earth. A huge amount.
By the way, great thread. Thank you, Clive Best.
For those wondering about land ice, the Milankovitch cycles are intimately connected to both the tidal forces AND hemispheric insolation summer maximums. When they align in the northern hemisphere, the tides can work on sea-ice/ice-shelves and the insolation on land ice at the same time.
Ian Wilson says:
January 15, 2014 at 8:38 pm
“If I thought that science or physics had anything to do with any of your suggestions and posts I would would look them up. You seem to be correcting everyone except yourself.”
and:
“Try looking at the fifth figure of this post before you start mouthing off.”
In fact I was always correcting your astrocomical figures over at the Talkshop. You lost the plot with the maths past the 93yr figure (which should be 98 eclipse years), there is nothing lunar synchronized with Jan 1st at 177 or 739 years. And further to my comment on DO event periodicity:
http://www.geo.uni-bremen.de/geomod/staff/mschulz/reprint/1470y_pacing.pdf
Puts cold water on MILANKOVITCH CYLES AND TIDAL EFFECTS. They are to slow to account for the many sudden temperatures swings, and even more telling temperature swings within a cold interval such as the Younger Dryas.
Ice ages begin catastrophically
By Robert On November 6, 2013 · 26 Comments
… .
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In less than 20 years.
The Younger Dryas, also referred to as the Big Freeze, was a geologically brief period of cold climatic conditions and drought that was in sharp contrast to the preceding period of warming.
The Big Freeze began approximately 12,800 years ago and ended about 11,500 years ago[1] – about 1,300 years, according to Wikipedia.
The Younger Dryas began catastrophically, with the climate in the higher latitudes of the Northern Hemisphere rapidly descending into glacial conditions.
How rapidly?
Scientists think the transition occurred over a period of a decade or so,[2] perhaps even faster.[3]
Ten years. Or even faster.
My own research shows that all – all! – ice ages in the last 250,000 years began in less than 20 years, sometimes in less than three years.
At the end of the Eemian Period, for example, the climate descended from a period of warmth as warm as, or even warmer than today’s, into full-blown glaciation in less than 20 years.
Can you imagine how unprepared our world would be if we descended into an ice age in less than 20 years?
It has happened before, time after time after time. It’s not a question of “if,” only “when.”
Wikipedia description of the Younger Dryas:
http://en.wikipedia.org/wiki/Younger_Dryas#cite_note-8
Thanks to Steven Rowlandson for this link
1.Berger, W. H. (1990). “The Younger Dryas cold spell – a quest for causes”. Global and Planetary Change 3 (3): 219–237. Bibcode:1990GPC…..3..219B. doi:10.1016/0921-8181(90)90018-8.
2.Alley, Richard B. et al. (1993). “Abrupt accumulation increase at the Younger Dryas termination in the GISP2 ice core”. Nature 362 (6420): 527–529.Bibcode:1993Natur.362..527A. doi:10.1038/362527a0.
3.Choi, Charles Q. (2 December 2009). Big Freeze: Earth Could Plunge into Sudden Ice Age.
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Interestingly, the Younger Dryas began at about the same time as the Gothenburg magnetic reversal.
(See “Magnetic Reversals and
clivebest says:
January 15, 2014 at 2:52 pm
Clive, I see that your thinking is that the Moon and Sun are drawing water toward the point of the Earth’s surface which is “under” them at any given moment, producing a moving area of higher sea level which follows after this point. Thus, as a point in the ocean, say the Hawaiian Islands, is carried by the Earth’s rotation across the side of the Earth facing the Sun, the waters surrounding those islands are first drawn away from the Islands toward the point under the Sun at that moment, then drawn toward the Islands as they pass near the point under the Sun, then drawn away again as the the Islands go over the horizon. The current flow back and forth, and the rise and fall of the sea level produced, would vary with the elevation of the noonday sun at the Islands, and with the distance to the Sun. The same effect, and variations in that effect, would apply to the Moon. As this is perfectly straight forward, I totally agree with you on these points. (Please correct me if I misunderstood you.)
My thinking is that since the Arctic Ocean never enters the region of the Earth’s surface that the Sun is drawing water towards, the Arctic Ocean is never on the receiving end of tidal currents. The Sun only draws water away from the Arctic, as it circles the horizon. Yes, the strength of the Sun’s draw upon the Arctic waters would vary with the sun’s elevation and distance, but there would be a corresponding, though opposite in direction, draw toward the tidal bulge on the far side of the Earth. Thus, also, no back and forth tidal currents as the Earth rotates.
This all also applies to the Moon. Most of the time the moon’s draw upon Arctic waters would be out of alignment with the Sun’s, but even when aligned, the draw of the opposite side tidal bulges would also align. Of course, this drawing away effect has its limits, or the Arctic ocean would be dewatered. Once the limit is reached, tide level would be essentially static, with only slight variations in the strength of the draw due to the orbital eccentricity of the Earth and Moon.
Furthermore, the Arctic Ocean is nearly landlocked. Note how being landlocked stifles tides in the Mediterranean Sea, despite its much lower latitude.
It is hard to imagine how minuscule variations in minuscule tidal currents could have anything other than a doubly minuscule effect on sea ice in the Arctic.
However, the bigger question is how a loss of Arctic sea ice, by whatever means, could cause melting of continental ice sheets.
At our current axial tilt, even the summer sun is too low in the sky in the Arctic for insolation to overcome the heat loss due to evaporation. 1.5 more degrees of elevation would not change this.
Evaporation from open water in the Arctic Ocean would increase snowfall upon surrounding continents, contributing to ice sheet mass. Cooling due to this evaporation, combined with radiation and conduction to the atmosphere, would quickly refreeze the Arctic Ocean during the next winter.
The only way to get, and keep for more than a season, an ice free Arctic Ocean is to have a large supply of warm water, but this would result in growth of continental ice sheets.
SR
beng says:
January 16, 2014 at 7:27 am
“For those wondering about land ice, the Milankovitch cycles are intimately connected to both the tidal forces AND hemispheric insolation summer maximums. When they align in the northern hemisphere, the tides can work on sea-ice/ice-shelves and the insolation on land ice at the same time.”
Beng, Clive was noting that increased insolation due to Milankovitch cycles was not enough by itself to melt land ice, as that ice apparently persisted through some periods of highest insolation.
I note that something other than Milankovitch cycles must be the cause of glacial onset/decline.
SR
Clive does wonderful work. His search into the missing components of Milankovitch cycles is always thought provoking. Lunar variations may be one of these elements that give an added push to the eccentricity in a key phase of the ice age cycle.
Consider the role of the very hot Local Bubble just outside the sun’s heliosphere. The elongated elliptical orbit may contribute to the heat and energy of the sun during a maximum eccentricity.
https://www.americanscientist.org/issues/issue.aspx?id=862&y=0&no=&content=true&page=7&css=print
Placeholder.
Interestingly, the Younger Dryas began at about the same time as the Gothenburg magnetic reversal.
And at least one “substantial” asteroid/meteor fall, and when freshwater glacier melt piled up on North America may have broken through ice dams to the sea “all at once” and caused both substantial SLR and interrupted the thermohaline circulation. The Younger Dryas doesn’t lack for possible causes. It merely lacks any conclusive cause, as far as I know. As always, it might well have been multifactorial — this and that and this other thing all happening in just the right sequence — the Hudson Bay meteor and a consequent or independent or even preceding thermohaline diversion:
http://www.universityherald.com/articles/4425/20130903/younger-dryas-meteor-impact-theory-traced-quebec-canada-scientists-still.htm
A number of things could have caused it — including things nobody has even thought of for which the evidence was long ago erased by time and entropy — but AFAIK nobody has a quantitative theory that can explain it or predict things like interglacial triggers, MWPs, LIAs, and so on. Perhaps because theorists want a “smoking gun” cause, a single big cymbal crash that changed things, where nature may work with syncopated chaotic lemmings playing first a hot jazz riff on a dozen different strings, then a cool one.
rgb