Milutin Milankovitch


Early studies by geologists and glaciologists attempted to find a climate mechanism to explain the evidence of massive ice sheets during the recent Ice Age (Pleistocene). Louis Agassiz identified the existence and extent of the ice sheets in Europe as early as 1837, but it wasn’t accepted until the 1860s. There were many theories. One that endured was Joseph Adhemar’s proposal that the likely cause was change in the earth’s solar orbit. James Croll was the major early contributor to the orbital variation idea and calculated orbital eccentricity for different latitudes over 3 million years, and published in 1867. Milankovitch spent years combining changes in the sun/earth relationships including changes in orbit, tilt, and date of an equinox.

He wanted to find the mechanism primarily responsible for climate change. Before computers, he calculated variations in solar energy received at every five degrees of latitude over 650,000 years. He published them in 1920 as Mathematical Theory of Heat Phenomenon Produced by Solar Radiation. His work caught the attention of Köppen, who sent him a postcard. It said he was working with his son-in-law Alfred Wegener on a book about past climates and they were very excited by the mechanism Milankovitch proposed. In 1924, he published, with Köppen, the now classic graph of variation in summer radiation for 65°N that identified the correlation with Ice Ages (Figure 2).



Figure 2: Intensity of radiation curve for 65°N. Ice Ages names are for Europe.

Source: Imbrie and Imbrie, 1979, Ice Ages: Solving the Mystery.

Milankovitch acknowledged that without Koppen’s input from his extensive understanding of global climate patterns he would not have identified that summer temperatures at 65°N were the critical issue.

Milankovitch’s theory was initially accepted as a plausible answer to the major fluctuations and triggered research in the 1950s. Then it was pushed aside because it showed glaciers in a part of Alaska that the new technique of radiocarbon dating indicated were forested. It turned out the radiocarbon was wrong because it assumed constant solar energy. A reference to Milankovitch was immediately challenged. It wasn’t until the 1980s that a reference to his work went unchallenged. The amazing thing is, most people still don’t know that Earth’s orbit changes significantly because of the gravitational pull of the planets, especially Jupiter. They still don’t know the earth tilt swings between 21.8° and 24.4°, or that the date on which critical events, Equinox and Solstices, are constantly changing.

Figure 3 shows a plot of variations in the amount of solar radiation at 65°N for a period of 1 million years.


Figure 2: Variations in the amount of insolation (incoming solar radiation) at 65°N

Source: BERGER, A. 1978. Long-term variations of daily insolation and quaternary climatic changes. J. Atmos. Sci. 35: 2362–2367.

The range of variation is approximately 100 watts per square meter, which far exceeds the 2 watts per square meter the IPCC attributes to humans.


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