Cyclicity of Ice Ages

Milutin Milankovich was a Yugoslavian astronomer, who in the 1920-1930’s proposed a theory for the cyclicity of glacial periods.  His basic theory was derived from the idea that the northern hemisphere summer temperatures were the key to the onset of glaciers, and ultimately the start of an ice age. Cool summer temperatures meant the snow that accumulated during the winter months was unable to melt.  This build up of snow over many years was able to bring about glacial formation.

 

 

 

 

 

Glacial and interglacial events occur in cycles of varying length, and within these there are three smaller cycles, of approximately every 100 thousand years, 43 thousand years and 23 thousand years each.  Peak glaciation occurs when the maximum of these individual cycles occur at the same time.

 

Milankovich proposed that three sources of variation in the orbit of the Earth may explain the cyclicity of glacials seen.

 

1.  Eccentricity¹

The shape of the earth's orbit changes from circular to elliptical over a period of approximately 100 thousand years.  This change in the orbit shape causes only a 0.03% variation in annual insulation, but has a larger effect on the seasons.

· When the earth's orbit is perfectly circular, there is no difference between the total amount of radiation in summer and winter.

· However, when the earth's orbit is elliptical, at its maximum there is a difference in radiation of 30% between the position closest and furthest from the sun.  When this is the case, snow is more likely to survive the summer than when the sun is further away. 

Milankovich stated that the more elliptical the shape of the orbit, the more the seasons in one hemisphere are exaggerated, and this may contribute to the onset of glacials, as seen below.

 

 

 

2. Obliquity¹

The tilt of the earth's orbit varies between 22.1^o and 24.5^o (as seen below) over a period of approximately 43 thousand years.  Milankovich hypothesised that the greater the tilt, the greater the difference between the total amount of radiation in the winter and summer months, and hence more likely snow will be retained all year round.

 

 

3. Precession¹

The rotation of the earth's axis completes a full circle every 24 thousand years.  Again this causes the distance between the earth and the sun to vary between the seasons, and increase the likelihood of a glacial period (as seen below).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Individually these variations in the earth's orbit are unlikely to start glacial periods on their own, but when they occur collectively glaciations may result, as shown

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(http://www.geocomplexity.net/Milankov_theoretical.gif)

 

 

 

 

¹ http://www.emporia.edu/earth'sci/student/howard2/theory.htm

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