The Milankovitch Cycles

Data & Analysis

Glacier Visuals | Ocean Circulation Visuals

Data

In 1992 the group SPECMAP looked at the Milankovitch Model and was able to interpret the cycle’s by dividing it into four primary stages. They are:

  1. Interglacial
  2. Preglacial
  3. Glacial
  4. Post Glacial

From creating these models we are able to understand the relationships between certain conditions that must exist in order for the Milankovitch glacial cycles to occur.

Interglacial

The first stage, the Interglacial period, is often looked upon by scientists to be the period we are living in right now. SPECMAP describes this period as “A state in which, both sea ice and northern hemisphere continental ice caps are at their minimum extents.” (http://www2.ocean.washington.edu/oc540/lec02-22/) The figure above shows the ocean heat pump in the Northern Atlantic Region. The AA represents the Antarctic Ocean and the NOR as the Nordic Ocean. The way the ocean conveyor contributes to the climate on our planet is that it allows weather patterns to be less severe then if the whole system was pushed further down to the mid-Atlantic. It is also believed that if freshwater enters the saltwater system it will alter the path of the conveyor and aid in bringing severe cold weather to North America and might even throw the glacial cycle off balance.

Preglacial

The preglacial state begins when there is a “decrease in northern hemisphere radiation, which leads to freshening of the Nordic Seas” (http://www2.ocean.washington.edu/oc540/lec02-22/) The idea is that the reduction of heat and solar radiation will allow the Antarctic sea ice to grow. The important aspect of this stage is that increase in freshwater. If we remember how the Ocean Conveyor works from what was discussed in the Interglacial Stage, this is an important clue as to how the glacial mechanism works. As SPECMAP continues to describe this stage they mention that “this appears to reduce the rate of overturn of Antarctic waters, which in turn could draw down atmospheric carbon dioxide values.” (http://www2.ocean.washington.edu/oc540/lec02-22/) Further evidence of this can be seen in the discussion of Ice Core samples and the Atmospheric C02 from Siple Dome Antarctica.

Glacial

This stage is pretty straight forward, the idea is that as the ice caps and glaciers continue to grow, there becomes a major change in the wind patterns in the Northern Hemisphere. SPECMAP also mentions that “the loss of water to the ice caps eventually lowers sea level to the point where ice sheets are grounded on large areas of the continental shelf.” (http://www2.ocean.washington.edu/oc540/lec02-22/) If we look at the animation video found here. This gives a very good visual representation of how the ice sheets interact with the continental shelf.

Postglacial

The last stage known as the postglacial or the deglaciation begins as “high latitude northern hemisphere radiation increases, the atmosphere and surface waters warm, evaporation increases, and sea ice retreats from the Nordic Seas.” (http://www2.ocean.washington.edu/oc540/lec02-22/) The transportation of heat to Antarctica acts as a ventilation system and “thereby releasing trapped carbon dioxide and setting the stage for a catastrophic collapse of the continental ice sheets.” (http://www2.ocean.washington.edu/oc540/lec02-22/)

Conceptual Model

The above figure shows the interaction between all four stages and how the responses play a major role in how each stage comes about.

Using Ice Cores to Prove Miilankovitch

One of the most important evidence of the Milankovitch Cycle’s is the use and interpretation of ice core samples. “Ice core from the right site can contain an uninterrupted, detailed climate record extending back hundreds of thousands of years. This record can include temperature, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity and a variety of other climate indicators.” (http://nicl.usgs.gov/why.htm) Looking at this graph of temperature, CH4 and CO2 we can determine each of these levels during each of the glaciation stages discussed earlier.

http://www.sp.uconn.edu/~geo101vc/Lecture24/sld019.htm

We can then take from Milankovitch’s models and compare it to this data and look at the overall trends from the samples. When we do this we get a graph that looks like this:

http://www.sp.uconn.edu/~geo101vc/Lecture24/sld020.htm

The patterns of Milankovitch would appear to line up with the data from the first graph illustrating that there is a correlation between temperature, CH4, and CO2 and the Insolation during the cycles. Several things to note on this graph: there is an error in lining amplitudes from isolation to the CH4. Possible errors could be inaccuracy in data collection or errors in the overall Milankovitch mathematical models. Aside from these errors the overall patterns and relationship is a vital clue in proving Milankovitch.

During the discussion of the preglacial stage we mentioned the importance of atmospheric carbon. This graph made using excel and ice core measurements from http://nsidc.org/data/nsidc-0202.html shows the overall changes from Siple Dome Antarctica of atmospheric carbon by depth since the last ice age. As preglacial stage prepares to transform into the glacial stage, there is a decrease in atmospheric carbon and then later raised during transition of the postglacial stage.

Insolation

The concept of insolation is what gives Milankovitch’s theory basis and stability. Insolation is described as the intensity of Solar Radiation that is received on Earth. The intensity (mathematically speaking is Intensity = SIN (A)) of the solar radiation and its correlation between eccentricity and obliquity help define seasons and play a major role in glacial cycles. Looking at this graph we see the overall changes in isolation during the year for different longitudes.

The 70N is an important key in the Milankovitch Model because he used 65N as a means of comparisons for developing the model because he believed that it was at 65N that glacial formations begin. Looking at the next graph we can see that for a perfectly clear atmosphere the changes in insolation at 65N during the present and 12,800 years ago. The x-axis represents total days from the vernal equinox and the y-axis represents the radiation changes (Wm)

This image (Below) is a graphical represtentation of the Milakovitch Climate Theory over a 15,000 year period. It illustraits changes in insolation due the Milankovitch Cycles. Note the uniform insolation levels at -19000 yr, this would denote the end of a glacial period. Looking at this model we can see a visual representation of our current interglacial period. At -11000 we were experiencing a maxima in solar radiation in the northern hemisphere.

Conclusion: Contradictions and Drawbacks of the Milankovitch Theory

In a article by the European Geophysical Society, author V.A.Bolshakov discusses several issues regarding Milankovitch’s work and argues the following:

  1. “Number of glaciations and their dating disagree, on the whole, with similar glaciations characteristics obtained by Milankovitch”
  2. “Glaciations coincide in time with minima eccentricity values, whereas the Milankovitch theory mostly associates them with eccentricity’s Maxima”
  3. .The main period of climatic cyclicity changed from 41 kyr to 100 kyr at a time about 1 Ma.” (http://www.cosis.net/abstracts/EAE03/00721/EAE03-J-00721.pdf)

Besides Bolshakov, other articles such as one in Nature magazine point out that “we now know that today's conditions are not like those of the last warm period, the Eemian, which was around 125,000 years ago.” And that because these conditions are so different the overall idea of Milankovitch could not have any effect with today’s world. As opinionated as both of these author’s are we can conclude from what we know so far from ice cores and changes in eccentricity, obliquity, and precession that we are currently in the interglacial period in which according to the Milankovitch Glacial Models will result in a new Ice Age. Also we can also gather that the same forces that drive glacial cycles can be manipulated by current warming trends. (See: preglacial models) These occur from changes in the freshwater and saltwater systems illustrated by the ocean conveyor belt.