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
goldminor says: @ur momisugly January 15, 2014 at 1:35 am
…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.
>>>>>>>>>>>>>
So challenge them. Nothing like learning young to question authority.
See Jo Nova’s Two high school students take on teacher over climate and win standing ovation
This is quite interesting. The correlations are impressive. Tides could break up sea-ice and even ice-shelves if high enough. Won’t affect purely continental ice, but aligned w/high-latitude solar input, could be the “trigger”. Need time to grok…..
Clive,
In 4. I assume you meant “interglacial” and not “ice age”? Also, how can the the force of sun’s gravity on the moon be 2X larger than that on earth? It’s a function of distance and mass. Since the moon and earth are essentially the same distance from sun and earth is much more massive, I think you need to reword that sentence as well. In the long section near end (2nd to last section).
@Bill_W
1. Sorry – yes it should read “interglacial” not “ice age”
2. What I mean is that the gravitational force of the sun on the moon is twice that of the earth on the moon. The moon is really in a perturbed orbit round the sun when observed far out in space.
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?
clivebest says:
January 15, 2014 at 5:37 am
In particular how has the effective eccentricity of the moon-earth orbit varied over the last million years.
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Consider that the moon and earth both orbit the sun, and the eccentricity in the lunar orbit around the sun will vary in magnitude and timing similarly to the earth’s max eccentricity = 0.058.
So long as these are in phase, then the net earth-moon eccentricity would be close to 0. However, if they were out of phase it seems possible that the net eccentricity could be as much as 0.116 at 180 degrees out of phase, and some sin/cos function in between.
However, that number is calculated from the distance from earth to the sun. 0.116 x 93 million miles = 11 million miles, which would be quite something given the moon is on average only 240,000 miles from earth. So, it would seem likely that the moon’s and earth’s orbits around the sun must also be gravitationally bound to remain in sync with each other, otherwise the moon and earth would eventually collide, or the moon could be thrown out of orbit around the earth.
However, it seems likely that this is not a perfect sync, that the earth’s and moon’s orbits around the sun will alternately lag and lead each other in some sort of complex harmonic oscillation due to orbital mechanics, and could at times become quite large, leading to a large net eccentricity in the moons effective orbit around earth. And it seems likely this net eccentricity could be maximized at times when eccentricity of the earth’s orbit around the sun is maximized.
Something like the motion of a pendulum clock. The amplitude of the pendulum is the eccentricity, and the irregularities in orbital mechanics provides the excitation force. When the excitation force varies in phase with the natural period of the pendulum, the motion can become quite large for even a small excitation force.
ferdberple says:
January 14, 2014 at 6:39 pm
Place an ice cube in a glass of water. Stir gently back and forth with a spoon until the ice is melted. record the time. Repeat the experiment, but this time do not stir.
You forgot the Johnnie Walker.
Ian Wilson says:
January 14, 2014 at 7:24 pm
This makes the long term realignment period for the Perigean spring tides with the seasons = 1478 years, which is very close to the 1470 year periodicity of the DO warming events.
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Likely well within the uncertainty for DO events.
leftturnandre says:
January 15, 2014 at 5:11 am
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.
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this number appears to assume that earth-moon eccentricity remains constant, which is perhaps unlikely.
something else to keep in mind is that the tides on earth do not act in a linear fashion with the tidal forces. they are more like the action of someone pushing a child’s swing. When in-phase the resulting motion is much greater than the exciting force. Thus we routinely see tides much greater than 79cm as would result from gravitational/tidal forces alone.
the tides act somewhat like a shallow bowl of water, where you tip the bowl slightly. If you get the motion of the bowl in phase with the motion of the water, the water will each quite high up the sides of the bowl.
There’s a scaling error in figure 1b. The range of eccentricity is up to 0.05, not 0.5.
something else to keep in mind is that the tides on earth do not act in a linear fashion with the tidal forces.
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for this reason we do not calculate tides using first principles – because it is not accurate. rather, we use astrological methods, which give a much more reliable result.
the problem is that astrology has been misused as a pseudo science, when in fact it has a long history of reliable prediction when used appropriately.
early humans used astrology to predict the seasons for thousands of years before they could do so from first principles. we should keep this in mind when dealing with climate.
“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”
I don’s see any rapid transitions in the Vostok data that would support that, and the last peak in eccentricity was less than the low point around 150K BP:
http://www.jgiesen.de/kepler/img/ecc1Mean.gif
Ulric,
I don’s see any rapid transitions in the Vostok data that would support that, and the last peak in eccentricity was less than the low point around 150K BP:
http://www.jgiesen.de/kepler/img/ecc1Mean.gif
That graph is wrong. The NOAA calculated data for eccentricity is plotted as the top curve in figure 2 and it aligns pretty well with interglacials as shown in Figure 1b.
The Keeling paper on an 1,800-year oceanic tidal cycle sheds light on my discussion with Dr. Svalgaard on the reality of Bond Cycles, the hypothesized extension into interglacials of glacial D-O events. Thanks for posting it.
If you go back to the start of the current interglacial you will find that obliquity was at maximum and NH summer was at perihelion. Obviously those two conditions must be present for an interglacial to begin. Those two conditions had last occurred together prior to this interglacial about 132k years ago, which was the start of the Eemian interglacial. I think you will find that, working back in time, interglacials begin when these two parameters are aligned properly. The amount of eccentricity at the time when the other two align provide the added condition to determine the actual start, length, strength and end of the interglacials.
If you go back to the start of the current interglacial you will find that obliquity was at maximum and NH summer was at perihelion. Obviously those two conditions must be present for an interglacial to begin.
correct – but if only it was that simple.
Those two conditions had last occurred together prior to this interglacial about 132k years ago, which was the start of the Eemian interglacial.
No – another one just as large occurred 80,000 years ago and there are plenty of others that occur within a glaciation but do not trigger an interglacial. Interglacials only occur every 100,000 years. Before 1 million years ago you were right that maxima in obliquity was sufficient. Click on figure 2 for more details. The solid black curve shows the NH summer insolation.
The moon used to be closer to the earth than it is now. I don’t know how much closer 5-million years ago, but perhaps this helped keep glaciation at bay.
@Ian Wilson
Excluding the cluster of Dansgaard-Oeschger events 5 to 7, there is no 1470yr periodicity. It is most likely that these three events arise from intermittent 1542yr (+/-179yr) interval peaks in solar activity.
Clive Best says:
“That graph is wrong. The NOAA calculated data for eccentricity is plotted as the top curve in figure 2 and it aligns pretty well with interglacials as shown in Figure 1b.”
Figure 1b shows exactly the same thing, i.e. the last peak in eccentricity was less than the low point around 150Kyr BP.
I just can’t leave this one alone.
1) In answer to your question, George Darwin calculated that average lunar eccentricity has steadily increased due to tidal braking (along with earth-moon distance). Determination of max perigee and apogee depends on how far back (or forward) you want to go, and the further back you go the less reliable are the calculations: http://en.wikipedia.org/wiki/Talk:Orbit_of_the_Moon#Perigee_distance_should_be_fixed
2) The fact that total global insolation varies little is irrelevant since earth albedo is highly asymmetrical.
3) Of course gravity and insolation are both 4pi functions with corresponding variation. Milankovitch cycles obviously control ice ages by way of insolation, not gravity.
4) Ice caps occur over land, not sea. How are tides supposed to affect them?
–AGF
@agfosterjr
1)Determination of max perigee and apogee depends on how far back (or forward) you want to go, and the further back you go the less reliable are the calculations:
Your link shows that the eccentricity of the moon relative to earth is easily influenced by other planets.
4)Yes glaciers on land do experience tidal forces. Of course it depends how strong they are.
As an old sea dog, I’ve seen lots of tides. I’ve also seen hurricane storm surges, wind-driven currents, and estuary bores. I haven’t seen a major tsunami, but plenty of people have and have been most impressed by its magnitude and consequences. It’s an interesting idea in this article that a periodic 30% increase in tidal activity (caused by astronomical alignment) of a couple feet would trigger dislocations of ice sheets. But here are some order-of-magnitude comparisons with other sea-level fluctuations which are (presumably) non-periodic:
Average open-ocean normal tide level change (low-tide to high-tide level) 3 feet.
Average normal tide-level change in “bottle-neck” coastal land formations (e.g. Gulf of Maine) 15+ feet
Open-ocean extreme tides as described in this article (3 feet x 30%) 5 feet
Hurricane (cat.5) storm surge on open coastline 22 feet
Wind-driven surge recently experienced on North Sea coast 6+ feet
Major tsunami on open coastline (close to quake epicenter) 20 feet
Surge 1,000 miles from major tsunami 5 feet
Claimed historical surges caused by asteroid impact 20 feet
Depending upon the coastline configuration, some of these perturbations also cause major currents in addition to the sea-level impact.
Presumably ice sheets are subject to surges of this type – even attenuated by distance – many times each century. Is it reasonable that a relatively-small fluctuation of 3 feet every so many kiloyears would make much difference?
One of the most interesting discussions ever. It challenges the mind and further reflects the complexity of events that may affect climate. I have wondered about the independently (?) chaotic events and their potential impact should random occurrence coincide. Here’s a what-if for thought. The ENSO/PDO is in warm phase with AMO. The warm water moved to the Arctic thins the ice. A major quake occurs in the Alaskan or Icelandic areas and a tsunami equivalent to that in Japan occurs during late summer. At the same time a major Arctic cyclone develops. The sea ice in the Arctic could have broken up and disappeared very quickly in 2012, and once lost could take a long time to re-establish. If all this coincided with the tidal cycles discussed herein, rapid change might be induced. That’s a lot of mights and what-ifs – but food for thought. As “beng” says – grok time.
Ulric Lyons says:
Figure 1b shows exactly the same thing, i.e. the last peak in eccentricity was less than the low point around 150Kyr BP.
Yes you are right ! But we don’t know how the moon’s eccentricity varies and this is the missing piece of the jigsaw. When the glaciation cycle is at low point in the 400,000 year cycle it seems to be more difficult to trigger an interglacial – the last one and the similar one 5 cycles earlier have strong sawtooth shapes. So for the tide hypothesis to work the lunar component is essential. The solar tide component is smaller.
I cannot find any information about long term (Milankovitch) variations in lunar eccentricity.
One major problem with this hypothesis is that eccentricity also directly impacts total annual insolation. Total annual input energy goes like 1/sqrt(1-e^2). So, the peaks of eccentricity are peaks of total insolation not just tides. You would have to tease apart those effects. Assuming constant sun, the range of eccentricity forcing is from 0 (arbitrary) to about 0.4 watts (surface average).
…and I may as well reference
http://www.robles-thome.talktalk.net/Milank1.pdf
Which describes a similar (and visually quite reasonable) curve fitting exercise from a couple of years back.
Clive, the burning question is what year do we expect to see the beginning of the next interglacial or glacial period? This should be perfectly predictable if this information is correct.
@Ferdberple says …
leftturnandre says:
January 15, 2014 at 5:11 am….
…
I’m not up that early, but I guess it’s a time zone problem.
Anyway you speculate about resonance in the three body problem Sun Moon Earth, which may or may not be the case. I could also imagine that the outcome would be that the moons orbit pertubations counteract the effect of greater eccentricity of the earth orbit. Obviously when the sun is at a closer perihelium, you’d expect the moon also at higher eccentricity in counterphase to balance gravity. But that’s a wild guess. Let’s not forget we’re just at step one of the scientific method here: “Guess”.
Tetragrammaton says:
January 15, 2014 at 9:17 am
Yes, indeed!
Additionally, tides are strongest in those latitudes passing under the Sun and Moon each day. This is why tides are the highest and lowest of the year in the NH at the new and full moons closest to the summer solstice. Having the sun high in the sky matters more than the Earth being at perihelion (which currently occurs during NH winter).
North of the Arctic Circle the sun is never high in the sky, even at maximum axial tilt. Thus the tidal effects in the Arctic are always negligible.
Steve R says:
January 14, 2014 at 9:16 pm
And I join him by asking: How would loss of Arctic Ocean sea ice facilitate melting of continental ice sheets?
SR
“North of the Arctic Circle the sun is never high in the sky, even at maximum axial tilt. Thus the tidal effects in the Arctic are always negligible.
No it is exactly the opposite. The tidal bulge aligned under the moon is not because of the direct gravitational attraction – this is tiny. It is the tractional force piling up water from the edges into a tidal bulge. That is why the strongest tides on earth are at high latitudes. The bulge on the other side is due the centripetal force of the earth’s rotation about the Earth-Moon centre of mass.