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
We can measure this hypothesis in how many years?
Keeling’s paper is outstanding. And makes a lot more sense to me. If correct, we are on the verge of another big Cold Spell lasting decades.
Chaco Canyon New Mexico.
Three rounded rocks with circles.
The old ones following the sun and moon cycles.
These old ones lived out in the open, they slept out in the moon light.
They knew some things, possible more than Michael Mann and his tribe.
Necessity the mother of invention.
I thought interglacials were caused by continential drift.
But it’s good to have alternate theories too.
I look forward to the Day when all theories are thrown out the window and we collectively say to ourselves. “Oh.. So that is what causes Humanities extinction.” By Ice or by Fire it matters not.
Cooper says:
January 14, 2014 at 3:49 pm
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Go away.
That the large changes to Earths climate are cyclical like clock work, one can only conclude that the answers lay in the cosmos.
The coinciding alignments of our solar system and our place in the galaxy seem overwhelmingly to be the cause. The mechanism or mechanisms that cause the swings are open to conjecture.
Astrology this is not, but some basic premises of astrology coupled to astronomy may explain why the ancients were so enamored by the concept.
Notwithstanding that the mechanism is not known, this knowledge can give us some predictive skills. Fathoming the mechanism may take some time, as it is not just climate science that has succumbed to consensus.
Some very interesting points made in this paper.
Along the same lines its interesting to note that over the period of 1991-2011 there were 7 ‘supermoon events’ where the moon orbits closer to the earth than is usual, when usually there is only 3 such events in any 20 year period. It so happens that in this time period we have also witnessed the thinning of the Northern Ice Cap, so this paper would explain that behaviour much better than the missing heat scenario.
I think its highly likely that these variations in the orbits of our planetary system is the main factor in all global, and most local geological and atmospheric events.
Alan Robertson says:
January 14, 2014 at 3:59 pm
Why? I’ve been following this issue for the better part of a decade, The staggering amount of information we don’t know, yet pretend we do to a remarkable tenth’s of a degree spanning 100’s of thousands of years. Getting tiring. Like to get it over with sooner or later..
Great post Clive! Thanks!
this certainly seems to be on the right track.it has always puzzled me why there was not more investigation into the HUGE effects caused by the masses of the oceans movement variation depending not only on tidal range,but the changing areas where the greatest gravitational pull is.
this is most likely the main cause of shifting ocean currents,and of their variation in strength in recent times. it would be interesting to see a plot of the various oceanic oscillations over the same timescale if that is possible.
The key to understanding the glaciation cycle is to understand that, while orbital variations don’t cause large changes in the “global average TOA radiative forcing” they cause significant shifts in the seasonal and latitudinal *distribution* of solar insolation. It is likely that this (and things like changing geography) act on the climate system by their effects on dynamics, the atmospheric circulation, and in turn, clouds. This allows such factors to give rise to changes in the mean climate state, without a need to appeal to global radiative forcings like CO2, and without the sensitivity to such being high. This, moreover, is why such changes have typically involved tropical climates not much different than the present, but polar climates *dramatically* different. Changes in the meridional heat transport as induced by such heterogeneous forcing-this would naturally arise, given the above, when combined with a strong negative feedback in the tropics. In that regard it is worth noting that, at least according to CLIMAP, there were even parts of the tropical Pacific ocean that were actually *warmer* than the present during the Last Glacial Maximum.
Of course, changes in ocean circulation could cause a similar effect, and would probably be related.
The idea that ocean tides break up large floating ice-sheets runs afoul of considerations of spatial scale. There is scarcely enough difference in tidal elevation over the typical dimensions of such ice-sheets (tens of kilometers) to exert the forces necessary to fracture them. Only in the case of ice-sheets fastened to shore is such a mechanism plausible.
Sounds good, but…..
“…..that something was extreme tidal forces caused by……” Or maybe an asteroid hit an ocean. Or maybe an mile high avalanche of ice and rock cause a tsunami. Just not enough information yet.
Which reminds me whether global warming kick starts ice sheets growth?
See my last comment. The game plan of Warmists is to have a paper ready for any eventuality so they can point to it as backing their crazy ideas. Whether the IPCC agrees or not is (at that point in time) irrelevant. Same for the endless model runs.This is a scam and people must try and see through it.
Just one point
‘to an end by carving and shelving the ice sheets’ = calving?
I agree with Old Construction worker. What is heartening is that despite the consensus, real science still is happening. And perhaps we will overcome the handicap of the meme.
>> The most exceptional tides occur when the perihelion of the sun and the moon coincide and both orbits are at maximum eccentricity.
…the perihelion of the sun and moon? …both orbits?
If I didn’t already know what you were talking about, I would have no idea what you were talking about with that sentence.
I would quantify the insolation caused by orbital eccentricity. That it does increase insolation is seen with a simple thought experiment: imagine the greatest eccentricity such that the Earth grazes the Sun once a year, with apehelion of twice the average orbital radius — we would all vaporize at perihelion. This is because insolation goes as the inverse square of the Earth-Sun distance, so perihelion is hotter than the apehelion is cool. So quantify this to see how much of the 41,000-yr temperature variation is accounted for. Already done, I suppose, but not seen in this article.
I quote Tim January 14, 2014 at 4:01 pm:
“…. over the period of 1991-2011 there were 7 ‘supermoon events’ where the moon orbits closer to the earth than is usual, when usually there is only 3 such events in any 20 year period. It so happens that in this time period we have also witnessed the thinning of the Northern Ice Cap, so this paper would explain that behaviour much better than the missing heat scenario…”
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.
I suspect the major effect of the tides on climate is to alter the mixing rate of the oceans, bringing more (or less) warm water to the polar surface, as well as to alter the circulation between northern and southern hemisphere.
Any change in the mixing rate is significant, due to the overwhelming amount of thermal energy stored in the oceans as compared to the atmosphere.
The larger tides may not lead to increased ice breakup except in shallow water, or in areas where there are large tidal streams.
Gifford H. Miller* & Anne de Vernal†
* Center for Geochronological Research, INSTAAR, University of Colorado, Boulder, Colorado
Here we examine the recent geological record (130 kyr to present) to obtain an independent assessment of ice-sheet response to climate change. The age and distribution of glacial sediments, coupled with marine and terrestrial proxy records of climate, support arguments that initial ice-sheet growth at the beginning of the last glacial cycle occurred at high northern latitudes (65–80° N) under climate conditions rather similar to present.
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so, 130kyr ago there were cavemen driving around in SUV’s. Or were conditions similar simply due to natural variability? In which case our current climate is completely explainable without any human causes required, and our most likely future is glaciation.
The problem with the insolation theory of glaciation is that an eccentric orbit does not significantly change the total amount of energy the earth receives from the sun. Thus the 100kyr problem in climate science. Radiative changes are insufficient to explain the peak in glaciation at 100kyrs.
However, this problem goes away if one considers that tidal forces also peak on a 100kyr cycle, and the importance of the oceans in determining climate. Celestial mechanics still cause the ice ages, but the mechanism is tidal forces as well as isolation.
Thousands of years ago humans used the position of the stars in the heavens to predict the coming of the seasons, long before they understood the cause. Our brains are no larger than theirs.