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
Well done! Thanks Clive!
One of the larger problems facing paleoclimatology is what causes an ice age to end and an interglacial begin. These are the ultimate global warmings, with sea level rising ~130 meters since the last glacial maximum. But it is also the speed with which we emerge from an ice age….
“Recent scientific evidence shows that major and widespread climate changes have occurred with startling speed. For example, roughly half the north Atlantic warming since the last ice age was achieved in only a decade, and it was accompanied by significant climatic changes across most of the globe.” states the opening of the Executive Summary from the Committee on Abrupt Climate Change, National Research Council, 2002, ‘Abrupt Climate Change: Inevitable Surprises’, ISBN: 0-309-51284-0.
Whatever causes that packs a big climate punch over a short period of time.
What we should all appreciate is the pursuit of a novel mechanism, super-tides, that Clive has done for us here. This is a really good example (I think) of why we are all here. I had never heard of this concept before, and I really appreciate the expose Clive has provided. The thing is, this is exactly the sort of kick-start mechanism that could work on a very short time frame. If correct, this might solve the single largest climate mystery which is what brings us out of an ice age?
Again, thanks Clive, your work made my day.
Now if we could only figure out what ends interglacials……………
goldminor says:
January 14, 2014 at 6:37 pm
Your observations about changes to Arctic and Antarctic ice at certain lunar phases are interesting and could be important.
Fig 1a is about, if it’s the same resolution, a long slow global cooling and a treshold that gives increased climate instability.
The reason for the global cooling, last 27 million years, could be many things and probably things working together.
Even a long slow gradual loss of Earths atmosphere?
“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.”
It appears to me that Clive’s premise is that starting 900,000 years ago, an ice free Arctic Ocean during a time when high axial tilt produces increased arctic insolation, is needed to begin the melting away of NH continental ice sheets during a climate optimum.
If the premise were so, wouldn’t we have had an ice free Arctic ocean for the last 12,000 or so years? Or is he suggesting the Arctic froze over again as the Earth’s axial tilt lessened?
SR
The view of Maslin and Ridgewell on the “eccentricity myth” should at least be included in a discussion on glacial timing following the mid Pleistocene revolution (change from obliquity to eccentricity phasing).
Maslin, M. A., & Ridgwell, A. J. (2005). Mid-Pleistocene revolution and the ‘eccentricity myth’. Geological Society, London, Special Publications, 247(1), 19-34.
Abstract:
The mid-Pleistocene revolution (MPR) is the term used to describe the
transition between 41 ky and 100 kyr glacial-interglacial cycles which occurred about
one million years ago. Despite eccentricity having by far the weakest influence on
insolation received at the Earth’s surface of any of the orbital parameters; it is often
assumed to be the primary driver of the post MPR 100 kyr climate cycles. The
traditional solution to this is to call for a highly nonlinear response by the global
climate system to eccentricity. This ‘eccentricity myth’ is a simplified view of the
relationship between global climate and orbital forcing and is in part due to an artefact
of spectral analysis. Our aim here is to clarify the often confused role of eccentricity
and review current theories of the MPR. We suggest the post-MPR ‘100 kyr’ glacialinterglacial
cycles are more closely linked to precession, with the saw-toothed climate
cycles being defined by every four or five precessional cycle. Because the control
over the number of precessional cycles involved is determined by eccentricity,
eccentricity at most only paces rather than drives the system. If true, then one must
also question whether the MPR, itself defined by an abrupt change in spectral
characteristics, is not also somewhat misconceived.
Full paper:
http://andy.seao2.info/pubs/manuscript_maslin_and_ridgwell.pdf
@Fred berple You may be right that the main effect of large tides is to change the ocean circulation of heat through the MOC. If this circulation depends to some extent on tidal strength then this would increase heat flow to the arctic.
The effect of eccentricity on insolation is subtle. When averaged over a year the direct change in solar insolation is small. However eccentricity modulates the precession term which changes the timing of seasons within the earth’s orbit. Today the perihelion coincides with southern hemisphere summers. In 12,000 years time it will coincide with northern hemisphere summers. As the earth’s orbit heads towards low eccentricity the precession effect goes to zero. You can see this well in Figure 2. When eccentricity is large then so too is the 23,000 year precession term.
@goldminor Interestingly enough there was a perigean spring tide at the end of December. This was also the main cause of the costal flooding during the big storms in the UK at threat time. It was a bad storm but the very high tides caused the flooding.
@David – sorry for the confusing sentence! I meant the coincidence of the earth being at the shortest distance from both the sun and the moon simultaneously.
phlogiston says:
January 15, 2014 at 12:10 am
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Thanks for the feedback. I have greatly enjoyed the conversation, exercise, and the mystery of this ongoing search for understanding.
Plus, I fully detest the thought that students of all ages, in many places around the world are being fed a false reality under the supposed banner of ‘science’. I have grandchildren being taught the cagw story.
Whilst not directly related to the article, I have always wondered what effect tides have on the break up of the annual ice sheet. Living by the sea I know the effects of Spring tides especially when combined with winds from a certain direction.
Here is a document from 1911 detailing the tide heights in the Arctic;
http://docs.lib.noaa.gov/rescue/IPY/ipy_005_pdf/Gc311h381911.pdf
I don’t know if there is a recent paper whereby modern tide heights can be compared. Perhaps someone might have practical experience of witnessing the effect of tides on ice sheets?
tonyb
Is it assumed that the tide rises uniformly?
I have witnessed tidal waves.
Much like a tsunami.
I suppose a 4Mtr wave may break 4mtr Ice.
Just a thought.
@Berényi Péter,
The MOC is indeed mainly a mechanical effect. Winds circulating round Antarctica pump surface water northwards and deep water up from the abyss through Ekman transport. This deep water in the Atlantic ocean originated as dense cold saline water from the Arctic. Tidal forces would add more mixing to the MOC.
@Ian,
You have done some great work on matching the D-O events to extreme tides, which I only discovered in the last few days. D-O events are the 25 or so rapid climate fluctuations found in the Greenland ice cores during the most recent ice age. Ian shows that recent ones coincide with what I call above Super Perigean Eclipse Tides occurring every 1832 years. If he takes into account the synchronization with seasons then they match up well. I must admit I haven’t yet understood the maths for this yet.
For me the most important parameter to get my hands on is the change in eccentricity of the lunar orbit due to the same (Milankovitch) effects that change the earth-sun eccentricity. I feel this must have been calculated somewhere but I cannot find any reference to it. This will determine just how strong the largest tides were 17,000 years ago.
Very interesting and thought provoking.
I haven’t seen anyone noting that tidal eccentricities affect the other subsurface fluid, Magma, and also the earth’s plastic crust. Perhaps these periodic perturbations drive shifting Continental movements also. Which then interplay with Oceanic currents.
As a layman, I’d be interested in a Climato Geologists take on the question.
@Ferd Berple your user name is an anagram, no?
@gymnosperm writes
“However, it remains to be explained why there is no signal in ice bubbles or benthic isotopes for the 400kyr cycle, which as you mention is the strongest and most stable of all the Milankovitch cycles.”
I think there is a signal of the 400,000y cycle if you look for it. The two glacial cycles with the lowest maximum eccentricity were the last one and the one 5 cycles previously (cross reference them against figure 2). These two cycles have the sharpest saw-tooth shape because the tidal effect was weaker and needed more of a big push to get it going. If on the other hand you look at the cycles with larger eccentricity say -1 and -2 then these have a more similar 41,000y shape and are brought to an end earlier. In addition, very early on (<3 million ya) in the glaciation cycle there is evidence of a direct 400,000y signal
See this graph here
Nice thinking out of the box. it certainly contributes to the mindset required to solve the 100k not-milankovitch cycle.
But it appears that the real variations in tide amplitude about the full range of orbital variation (less than a meter) is too insignificant to explain the collapse of ice sheets.
http://en.wikipedia.org/wiki/Tide#Amplitude_and_cycle_time
Furthermore the moon, the most important contributant to tidal forces is receding, which implies that tides would reduce over geological time scales, and not increase as per the 0.9Ma transition from the 41K to 100K world.
Looking at several major changes in volcanism, tectonics and deep ocean reorganisations, I would explect that the real cause of waxing and waning ice sheet is below our feet.
“But it appears that the real variations in tide amplitude about the full range of orbital variation (less than a meter) is too insignificant to explain the collapse of ice sheets.”
That is the height of the tidal bulge (in this era) – not the transverse flow near coasts which are much higher. 17,000 years ago the distance of closest approach to the moon and the sun were closer (higher eccentricity). This caused a much larger maximum tidal bulge than experienced today. This is the proposed solution to the 100,000 year cycle of interglacials.
this is why I come here:
@Owen in GA says:
January 14, 2014 at 6:57 pm
and
@dp says:
January 14, 2014 at 11:13 pm
One providing information that to a 46 year old Chartered Engineer has taught an old dog a new trick and the latter – one that made him spray Earl Grey all over his monitor.
leftturnandre says:
January 15, 2014 at 2:28 am
Tectonics is not cyclical and periodic and thus is not a factor in the “current” glacial cycles. In the long term (starting at 10s of millions of years) then yes, tectonics explains most or all of global warmer or cooler phases of earth’s history, e.g. isolation of Antarctica 30 million years ago, the circumpolar current and all the cooling since, together with the separation of Atlantic from Pacific. But it cant explain recent cyclical interglacials with 40-100 kyr periodicity.
phlogiston says:
January 15, 2014 at 3:10 am Tectonics is not cyclical and periodic
Any evidence that it isn’t?
There is evidence that it is,
http://www.sciencedirect.com/science/article/pii/S0264817204001771
http://academic.emporia.edu/aberjame/tectonic/eifel/eifel.htm
Notice that the active periods in the Eifel coincide with interglacial transitions
ferdberple says: @ur momisugly January 14, 2014 at 5:41 pm
I suspect the major effect of the tides on climate is to alter the mixing rate of the oceans…
>>>>>>>>>>>>>>>>
Beat me to it.
It is the oceans not the ice that matters.
The primary mixer of air & water is equator-pole insolation gradients:
• http://imageshack.us/a/img850/876/f0z.gif (credit: JRA-25 Atlas)
• Concise overview of heat engines = p.433 [pdf p.10] here:
Sidorenkov, N.S. (2005). Physics of the Earth’s rotation instabilities. Astronomical and Astrophysical Transactions 24(5), 425-439.
• Elaboration on heat engines = section 8.7 (begins on p.175 [pdf p.189]) here:
Sidorenkov, N.S. (2009). The Interaction Between Earth’s Rotation and Geophysical Processes. Wiley.
I’m not convinced. Nothing seems to fit nearly as well as suggested. It seems to me this is good thinking, good information to add to our investigation, but it really sounds like we are saying that we are seeing a cloud in the shape of a bunny. We humans are good at seeing patterns and likenesses, but we are still good at seeing such even when not actually there.
“leftturnandre says:
January 15, 2014 at 2:28 am
Tectonics is not cyclical and periodic and thus is not a factor in the “current” glacial cycles.”
Now, how can you be so certain? Did volcanoes stop? Did earthquakes stop? Did up heaving up of the crust stop?
When we started using pre stressed concrete floors in high raised office building, floor tile was popping off the floors like popcorn. Why? The thinset (type of concrete glue) bonding the tile to the floor could not handle the mass amount of small vibrations caused by a lot of people walking on the pre stressed floor at the same time.
Ian Wilson says…..
>>>>>>>>>>>>
Thanks Ian.
The Dansgaard-Oeschger event timing and the“1478.00 years (= 2 x 739.00 years) to complete the cycle where a New Moon occurs when Perigee points at the Sun at (or near) Perihelion” is quite interesting.
E.M. Smith mentions the travel of the moon’s effect in a north-south direction. Lunar Cycles, more than one… He has some pretty good illustrations that are worth the visit.
In another essay E.M. brings up this paper that has already been mentioned. (for those who have not already seen it.)
Also see E.M.’s : Lunar Resonance and Taurid Storms
leftturnandre says:
January 15, 2014 at 4:14 am
phlogiston says:
January 15, 2014 at 3:10 am Tectonics is not cyclical and periodic
Any evidence that it isn’t?
There is evidence that it is,
http://www.sciencedirect.com/science/article/pii/S0264817204001771
http://academic.emporia.edu/aberjame/tectonic/eifel/eifel.htm
Notice that the active periods in the Eifel coincide with interglacial transitions
So … the Storegga slide which happened 8200 years ago caused the Holocene interglacial which started 12000 years ago?
OK … and the synchrony of several dozen interglacials with the obliquity cycle before the MPR and 8-9 interglacials after the MPR with the eccentricity cycle – all this is just coincidence?
The proposal of big mudslides in response to thick glaciation is in fact in line with the ideas of Maslin and Ridgewell
http://andy.seao2.info/pubs/manuscript_maslin_and_ridgwell.pdf
who propose that following the MPR obliquity and precession were no longer able via insolation alone to terminate a glacial but the build up of ice eventually triggered some event which caused an interglacial. They had more in mind interference with ocean currents but big mudslides might serve equally well. But this picture fails to address the continued synchrony with both eccentricity and precession.
Clive,
Please notice that the maximum eccentricity in the 400k cycle is 0.058. With some mild math I get to calculate a 11% difference in solar-earth gravity (hence squared) between perihelion and aphelion. Now the current tidal gravity force is 54 cm from the moon and 25 cm from the sun according to my wiki link. With 11% variation at maximum milankovitch eccentricity the solar gravity part account for 25 +/- 1,4 cm, if the relationship was to be linear, a variation of roughly 3.5% of the total tidal variation of 79cm.
Even assumining non linearity, It’s hard to imagine how a 3.5% variation in sun-moon gravity forces can account for super tides with the M2 tidal constituent.
http://en.wikipedia.org/wiki/Tide#Phase_and_amplitude
@leftturnandre
“With some mild math I get to calculate a 11% difference in solar-earth gravity (hence squared) between perihelion and aphelion. Now the current tidal gravity force is 54 cm from the moon and 25 cm from the sun according to my wiki link. With 11% variation at maximum milankovitch eccentricity the solar gravity part account for 25 +/- 1,4 cm,”
The tidal force goes as 1/R^3. The effect of eccentricity = 0.058 is plotted in Figure 4 above. For the sun this gives a 20% increase at perihelion and a 25% decrease at apehelion. Spring tides would be just 10% larger if the much larger lunar component were to remain constant. However the lunar component doesn’t remain constant and any change in moon-earth ellipticity has a far larger effect on tides because the moon is much nearer. If the eccentricity increases by the same proportion as that of the sun then spring tides are 60% greater in magnitude than today.
Here I am asking for help. I would like to get hold of any data on “Milanovitch” effects on the lunar orbit. In particular how has the effective eccentricity of the moon-earth orbit varied over the last million years. I can find no reference to this on the internet. If it can be calculated for the earth then it can be calculated for the moon. The whole hypothesis depends on a significant change in lunar eccentricity with the 100,000 year cycle of planetary gravity effects.
Phlogiston,
Notice that Bryn et al mention a 100ky cycle in tectonics, where the only objective was to demonstrate cyclicity in tectonics. Hence the storegga slide is a strawman.
Notice that Mienert et al suggest that events leading up to the instability of the Orman Langen field was indeed initiated around 12Ka
http://www.sciencedirect.com/science/article/pii/S026481720400193X
But maybe have a good look at Lohne et al 2007 about sea levels in Norway to see a wider interconnection.
http://www.sciencedirect.com/science/article/pii/S0277379107001035