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Thread: Milankovitch, Eccentricity, and Insolation

  1. #1 Milankovitch, Eccentricity, and Insolation 
    Forum Professor Wild Cobra's Avatar
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    I've been having a real hard time finding the information I seek. Anyone familiar with the Milankovitch Cycle?

    Anyway, I'm seeking to quantify something I have never seen reference too within the theory. Milankovich's theory is tied to the ice ages, and the usage of 65 degrees insolation is used. I believe that is an inappropriate measure as the oceans are also warmed by these changes.

    I am looking for the annual average energy received by the earth over long periods of time. First of all, the elliptical orbit changes from a near circular orbit to a small eccentricity of 0.058. I have never seen reference to the total radiation received, only at 65 degrees. The simple answer is that as the earth is in a more circular orbit, global temperature rises. As the earth is in a more elliptical orbit, the aphelion is not only is farther away, but the earth spends even more time at the longer distances, hence, the annual average temperature decreases.

    This cannot be immediately assumed. The other factor critical in this determination is the speed and distance of the earth. The problem cannot be solved without knowing more data. For example. We are in a current orbit of 365.256366 days. If this remained constant for all eccentricity values, then it can be calculated. What if the value changes a bit? What if 100,000 years ago it was 370 days? That would mean a greater average distance from the sun and less solar radiation. What if it was 360 days? Then we would be orbiting close and have more radiation.

    Anyone know where I can find more detailed information of the earth's orbit over time?


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  3. #2  
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    I don't know offhand of any single source which has the information you want.

    I can only suggest narrow searches for each component -
    eg - searching for changes to the length of day or number of days in the year
    might also require the geological period or era (Silurian, Precambrian, etc) ...
    you'll find that it's generally understood that the lengths of days were shorter, and the number of days per year higher in the deep past (hundreds of millions of years ago) ...

    insolation in general is widely explained, as watts per square metre, or as kilowatt hours per square metre per day, and vary according to latitude -
    eg - I think you'll find the "65 degrees" refers to latitude, not temperature ...
    changes to insolation over deep time (ie, hundreds of millions of years) are mostly linked to the long term trend in the Sun (it is ~10% more luminous now than it was a billion years ago) ... other factors include atmospheric scattering (cloud cover, column density, etc) ... to convert into a temperature, you need to factor in albedo and surface absorption - both of which are affected by the type of surface (rock, water, ice, etc) ...


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  4. #3  
    Forum Professor Wild Cobra's Avatar
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    Quote Originally Posted by Cran
    I don't know offhand of any single source which has the information you want.
    That's why I'm asking. As much as I have looked, I cannot find what I seek.
    Quote Originally Posted by Cran
    I can only suggest narrow searches for each component -
    eg - searching for changes to the length of day or number of days in the year
    might also require the geological period or era (Silurian, Precambrian, etc) ...
    I haven't tried yet. Thanx for the suggestion.
    Quote Originally Posted by Cran
    you'll find that it's generally understood that the lengths of days were shorter, and the number of days per year higher in the deep past (hundreds of millions of years ago) ...
    That's an automatic assumption because the earths spin does slow over vast time periods. How many days in a year are only a concern to me if I also know how long a day is. What I seek is to find out how the apogee and perigee have changed over the course of time we have ice core data for. Not just the eccentricity, but how the actual distances have changed to the sun. I'm primarily concerned with either the relative difference of a year that coincides with what time in history, or the apogee and perigee at a given time. Eccentricity alone does not explain what I seek. Changes in distance to the sun changes the heat received by the earth.
    Quote Originally Posted by Cran
    insolation in general is widely explained, as watts per square metre, or as kilowatt hours per square metre per day, and vary according to latitude -
    eg -
    Yes, but insolation is for a region.
    Quote Originally Posted by Cran
    I think you'll find the "65 degrees" refers to latitude, not temperature ...
    Yes, I know that. What I am looking for is how the watts change over the entire surface. Not a region. This changes with distance.
    Quote Originally Posted by Cran
    changes to insolation over deep time (ie, hundreds of millions of years) are mostly linked to the long term trend in the Sun (it is ~10% more luminous now than it was a billion years ago) ... other factors include atmospheric scattering (cloud cover, column density, etc) ... to convert into a temperature, you need to factor in albedo and surface absorption - both of which are affected by the type of surface (rock, water, ice, etc) ...
    I do understand these things.

    The Milankovitch Cycle only deals with 65 degree insolation, and the theory fails to properly explain warming trends and ice ages. I am a firm believe that the eccentricity and orbital distance changes are the primary driver. Not obliquity or precession. I am looking for something to support or disprove my theory.
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  5. #4  
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    Quote Originally Posted by Wild Cobra
    Quote Originally Posted by Cran
    you'll find that it's generally understood that the lengths of days were shorter, and the number of days per year higher in the deep past (hundreds of millions of years ago) ...
    That's an automatic assumption because the earths spin does slow over vast time periods.
    It's an assumption supported by studies of ancient tidal varves and stromatolite layers ...

    Quote Originally Posted by Wild Cobra
    How many days in a year are only a concern to me if I also know how long a day is. What I seek is to find out how the apogee and perigee have changed over the course of time we have ice core data for. Not just the eccentricity, but how the actual distances have changed to the sun. I'm primarily concerned with either the relative difference of a year that coincides with what time in history, or the apogee and perigee at a given time. Eccentricity alone does not explain what I seek. Changes in distance to the sun changes the heat received by the earth.
    If you're limiting your time reference to that of ice cores, then the changes over time in length of day and length of year are very small ... a good planetarium should have that information ...

    Quote Originally Posted by Wild Cobra
    The Milankovitch Cycle only deals with 65 degree insolation, and the theory fails to properly explain warming trends and ice ages. I am a firm believe that the eccentricity and orbital distance changes are the primary driver. Not obliquity or precession. I am looking for something to support or disprove my theory.
    I would think that obliquity and oblation (something Milankovitch didn't consider at all) have a greater effect on insolation (incoming solar radiation - regardless of surface area measured; the reason the calculations are done for specific latitudes is because of the surface curvature ... I'd be very surprised if someone hasn't done an integration for the whole planet - the modern estimate would be the "global average" that is used all over the place in greenhouse discussions ...) than interannual variations to orbital distance - which is a tiny fraction of the intra-annual (seasonal) variation ...

    obliquity determines the number of sunlight hours, and the range of insolation angles ... oblation (the degree of sphericity of the surface) modifies these by a small amount ...

    the reason for the focus on 65 deg N latitude is the very high landcean ratio - because land is much more responsive to changes, it is the key and the starting point for large-scale glaciations and deglaciations ...
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  6. #5  
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    Cran, thanks, but you are explaining the obvious. Obliquity and precession have no place in what I'm looking for, neither does insolation. I was hoping someone knew where such information existed as a link. I understand everything people are explaining already. I do not need explanations, I need data.
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