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Author Topic: Milankovitch Cycles doubts  (Read 2319 times)

ra3000

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Milankovitch Cycles doubts
« on: January 31, 2017, 08:00:35 PM »
We are currently in an Interglaciar Period within the Holocene and in a global warming trend lead by the higher latitudes of the Northern Hemisphere.
Aside from geographic changes, enhanced by the techtonic plates, ocean currents or sudden events such as asteroid impacts, massive vulcanism activity or variation in atmospheric composition & Others, the main driver, the one that seems to dictate when we have Interglaciar either Glaciar periods are Milankovitch Cycles.
My understanding of the Milankovitch Cycles is rather scarce. I know that eccentricity, axial tilt and precession are involved.
The questions are:

1. There must be a high or less degree of eccentricity to develop an Ice Age?
2. Do glaciers grow at both Hemispheres simultaneously? By what I have read, tilt oscillates between 21-24°. When it is closer to 21°, even if this variation does not seem extreme, higher latitudes receive far less sunlight.
But, in a "high eccentric" orbit and a 24° axial tilt let's say, would not it create optimal conditions to start building up Ice caps, glaciers and sea ice in just one of the two Hemispheres?
I still have a lot of doubts about how this works and fits, so I would appreciate someone to help me out in order to improve my knowledge in what I consider to be the main natural driver of the Climate.
I would also request for pages with data of axial tilt&precession&eccentricity, whether I can find a webpage with constantly updated data of these there.

Thanks in advance
« Last Edit: February 01, 2017, 01:36:11 AM by ra3000 »

sidd

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Re: Milankovich Cycle doubts
« Reply #1 on: January 31, 2017, 08:24:08 PM »
Look at Hansen(2007, doi:10.1098/rsta.2007.2052 ) open access, for the link to orbital variation of spring insolation at 65N.

Tor Bejnar

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Re: Milankovich Cycle doubts
« Reply #2 on: January 31, 2017, 08:29:40 PM »
The Cycles are typically called Milankovitch Cycles.  (You can correct the thread's title by editing the first post - not the final "s" but the "t".)

« Last Edit: January 31, 2017, 08:35:01 PM by Tor Bejnar »
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Neven

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Re: Milankovich Cycle doubts
« Reply #3 on: January 31, 2017, 09:16:50 PM »
Wrong category (and could have been posted in another thread as well, but okay). Moved now to the Science Category.
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FishOutofWater

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Re: Milankovich Cycle doubts
« Reply #4 on: January 31, 2017, 09:19:22 PM »
Global temperatures are still less than the peak temperatures of the last interglacial that took place about 115,000 to 125,000 before present.

Martin Gisser

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Re: Milankovitch Cycles doubts
« Reply #5 on: February 01, 2017, 03:53:25 AM »
Almost all I've ever forgotten about Milankovitch cycles I learnt at John Baez' Azimuth Project blog and wiki. Even if you don't like much math you can grasp quite a lot there - just skip the formulas in favor of the images and the plain text.

https://johncarlosbaez.wordpress.com/2013/01/30/milankovich-vs-the-ice-ages/

https://johncarlosbaez.wordpress.com/2012/11/13/mathematics-of-the-environment-part-7/
The Milankovich cycles are periodic changes in how the Earth orbits the Sun. One question is: can these changes can be responsible for the ice ages? On the first sight it seems impossible, because the changes are simply too small. But it turns out that we can find a dynamical system where a small periodic external force is actually strengthened by random ‘noise’ in the system. This phenomenon has been dubbed ‘stochastic resonance’ and has been proposed as an explanation for the ice ages. It also shows up in many other phenomena:

• Roberto Benzi, Stochastic resonance: from climate to biology.

But to understand it, we need to think a little about stochastic differential equations.


More:
https://johncarlosbaez.wordpress.com/2012/11/20/mathematics-of-the-environment-part-8/
https://johncarlosbaez.wordpress.com/2012/12/04/mathematics-of-the-environment-part-10/ (has table of contents of this series at the end)

Slides of a short talk here: http://math.ucr.edu/home/baez/glacial/
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sidd

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Re: Milankovitch Cycles doubts
« Reply #6 on: February 01, 2017, 06:10:13 AM »
Re: "Global temperatures are still less than the peak temperatures of the last interglacial that took place about 115,000 to 125,000 before present. "


Mmm, we are very close if not over.

doi: 10.1126/science.aai8464


" ... results indicate that peak LIG global mean annual SSTs were 0.5 ± 0.3°C warmer than the
climatological mean from 1870 to 1889 and indistinguishable from the 1995 to 2014 mean.
LIG warming in the extratropical latitudes occurred in response to boreal insolation and the
bipolar seesaw, whereas tropical SSTs were slightly cooler than the 1870 to 1889 mean in
response to reduced mean annual insolation."

include fig 1e. note where the zero of the ordinate is with respect to the green curve.

jai mitchell

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Re: Milankovich Cycle doubts
« Reply #7 on: February 02, 2017, 05:29:11 PM »
Global temperatures are still less than the peak temperatures of the last interglacial that took place about 115,000 to 125,000 before present.


a recent paper performed a better analysis of MIS-5 (Eemian peak) sea surface temperatures.  They indicate that the temperatures at that time were not higher than our current global average.  It should also be noted that the period of warming during Milankovich driven cycles is much slower than our current warming and that our current sea surface temperatures won't reach equilibrium to our current warming for at least another thousand years.

In view of this rapid warming driven by atmospheric CO2, the land surface temperatures are likely much warmer today than the Eemian 125,000 years ago.

see:  http://science.sciencemag.org/content/355/6322/276

Abstract

The last interglaciation (LIG, 129 to 116 thousand years ago) was the most recent time in Earth’s history when global mean sea level was substantially higher than it is at present. However, reconstructions of LIG global temperature remain uncertain, with estimates ranging from no significant difference to nearly 2°C warmer than present-day temperatures. Here we use a network of sea-surface temperature (SST) records to reconstruct spatiotemporal variability in regional and global SSTs during the LIG. Our results indicate that peak LIG global mean annual SSTs were 0.5 ± 0.3°C warmer than the climatological mean from 1870 to 1889 and indistinguishable from the 1995 to 2014 mean. LIG warming in the extratropical latitudes occurred in response to boreal insolation and the bipolar seesaw, whereas tropical SSTs were slightly cooler than the 1870 to 1889 mean in response to reduced mean annual insolation.

(or, like sidd said above   ;D  )
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AbruptSLR

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Re: Milankovitch Cycles doubts
« Reply #8 on: February 02, 2017, 06:49:18 PM »
Re: "Global temperatures are still less than the peak temperatures of the last interglacial that took place about 115,000 to 125,000 before present. "


Mmm, we are very close if not over.

Per GISS the 2016 GMSTA was just under 1.25C above the 1870 - 1889 means, so I think that it is safe to say that the world is well above the 0.5 +/-0.3C (w.r.t 1870-1889) for the Eemian peak.
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Laurent

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Re: Milankovitch Cycles doubts
« Reply #9 on: February 03, 2017, 05:16:03 PM »
Hello,
Should I ditch that graph saying 10 meters (or more) of SLR in the Eemian.
(Sorry I store for personal info so no source here)

Laurent

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Re: Milankovitch Cycles doubts
« Reply #10 on: February 03, 2017, 05:18:30 PM »
I have an other one more reasonable considering the temperature that Sidd Graph is showing (0.5°c+-0.3).
I had the rest of the folder content if interested.

Shared Humanity

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Re: Milankovitch Cycles doubts
« Reply #11 on: February 04, 2017, 06:37:56 PM »
Thanks for moving this to the science category, Neven. Also, I appreciate links that help explain Milankovitch cycles. I'll take the advice to skip the math as I am math challenged but would like to understand this idea that is frequently discussed on other blogs.

mati

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Re: Milankovitch Cycles doubts
« Reply #12 on: February 04, 2017, 09:23:10 PM »
and so it goes

jai mitchell

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Re: Milankovitch Cycles doubts
« Reply #13 on: February 05, 2017, 03:51:26 PM »
I have an other one more reasonable considering the temperature that Sidd Graph is showing (0.5°c+-0.3).
I had the rest of the folder content if interested.

Note Laurent:

I was originally very concerned about the spikes in the last graph of methane concentration from the GISP sample.  It turns out that the spikes are actually caused by melt/folding from that period of time.  During MIS-5, as with 2012, there were periods of surface melt at the drill site that caused the reading anomalies, not pulses of methane as they appear to show.
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Tor Bejnar

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Re: Milankovitch Cycles doubts
« Reply #14 on: February 27, 2017, 08:14:33 PM »
Dr. Steve Drury writes on a variety of earth science topics.  Today he blogs on
Odds and ends about Milankovitch and climate change.
He includes:
It is some 40 years since the last explosive development in understanding the way the world works. In 1976 verification of Milutin Milanković’s astronomical theory to explain cyclical climate change as expressed by surface processes has had a similar impact as the underpinning of internal processes by the emergence of plate tectonics in the preceding decade.
...
All manner of explanations have been offered to explain why tiny, regular and predictable changes in Earth’s astronomical behaviour produce profound changes in the highly energetic and chaotic climate system. Much attention has centred on the mathematically based concept of stochastic resonance. That is a phenomenon where weak signals may be induced to show themselves if they are mixed with a random signal – ‘white noise’ spanning a great range of frequencies. The two resonate at the hidden frequencies thereby strengthening the weak, non-random signal. Noise is already present in the climate system because of the random and highly complex nature of the components of climate itself and the surface processes that it induces.
...
Resolving this paradox may lie with three simple, purely terrestrial factors associated with great ice caps: thicker and more extensive ice becomes warmer at its base and more prone to flow; climate above and around large ice caps becomes progressively colder and drier, so reducing their growth rate; the more sea level falls as land ice builds up, the more the vertical structure and flow of ocean water change. The first of these factors leads to periodic destabilisation when ice sheets surge outwards and increase the rate of iceberg calving into the surrounding oceans. Such ‘iceberg armadas’ characterised the last Ice Age to result in sudden irregularly spaced changes in ocean dynamics and global climate to return to metastable ice coverage, as did earlier ones of similar magnitude. The second factor results in dust lingering at the surface of ice caps that reduced the ability of ice to reflect solar radiation back to space, which enhances summer melting. The third and perhaps most profound factor reduces the formation of ocean bottom water into which dissolved carbon dioxide has accumulated from thermohaline sinking of surface water. This leads to more CO2 in the atmosphere and a growing greenhouse effect. Comforting as finding simplicity within huge complexity might seem, that orbital eccentricity’s weak effect on climatic warming – an order of magnitude less than any other astronomical forcing – can tip climate from one extreme to the other should be a grave warning: climate is chaotic and responds unpredictably to small changes …
  (The final ellipsis is his, but the emphasis is mine.)

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Archimid

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Re: Milankovitch Cycles doubts
« Reply #15 on: March 11, 2017, 07:09:08 PM »
Orbits and Ice Ages: The History of Climate: Dr Dan Britt

The best lecture I have seen on global warming. I disagree with his risk assessment for humans, since he only seems to consider SLR, but besides that, the video makes the case for climate change 100% clear. It brings Milankovitch cycles, CO2, Paleo history, early history and the present day into one cohesive vision. 

https://youtu.be/xgNxF2HlN3w

Attached is a screenshot of my favorite graph of his whole lecture, I couldn't find a good version of it so a screenshot will have to do.
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budmantis

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Re: Milankovitch Cycles doubts
« Reply #16 on: March 12, 2017, 06:18:54 AM »
Thanks Archimid for posting this. Dr. Dan Britt's presentation was very informative, and as an added bonus, was pretty funny!

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Re: Milankovitch Cycles doubts
« Reply #17 on: March 12, 2017, 06:41:03 AM »
Posting the link (again) to ScioD for who want extensive reading on the subject. In short, some the latest models DO take post-glacial rebound into account, some are concentrating on the role of WAIS during deglaciations and Dansgaard-Oeschger events, and some are slightly modified Milankovich-style theories.

https://scienceofdoom.com/roadmap/ghosts-of-climates-past/

There are better (most) and poorer (7-8) chapters, but the author gives links on about everything.

All quite interesting. Of course the positions of the planet's glaciers are now different than in the previous de-glaciations, so direct comparisons may not apply.
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