Samstag, 26. Juni 2010

Geology and Cyclicity: Milankovitch´s idea

"I do not think that's my duty to teach to the ignorant the most basic things, and I have never forced anyone to accept my theory, on so far nobody could expose something."
Milutin Milankovitch in 1950

Milutin Milankovitch (1879 - 1958) was born in a relatively wealthy Serbian family, so it was almost a kind of obligation for him to archive a higher education degree and later take over the family business. So he studied agriculture, but following a passion for natural sciences he went to Vienna, where he in 1904 concluded his studies as an engineer.
Five years later he returned to Belgrad where he found employment as professor for mathematical studies at the University.
Like Croll he was in search of a scientific problem worth his efforts, and in 1911, sharing some presumably good wine with a friend, he decided to develop a mathematical theory to explain climate changes on the planets of the solar system.

He studied the work of Croll, recognized his previous achievements but also noted his insufficient data. Milankovitch also consulted the work of the German mathematician Ludwig Pilgrim, who in 1904 published exact calculations of the orbital eccentricity, earth's obliquity and the rotation of the axis of earth (change of the perihelion). Pilgrim also tried to correlate the eccentricity with the occurrence of ice ages.
Between 1912 and the beginning of World War I Milankovitch published some preliminary abstracts of his developing theory, concluding that all three factors, in contrast to previous authors, are important to explain earth's climate. At the beginning of the War, Milankovitch was arrested as Serbian officer and imprisoned in his hometown Daly, but fortunately he was carrying with him his work, and so even in the first night as prisoner he continued to work. "When after midnight I looked around in the room, I needed some time to realize where I was. The small room seemed to me like an accommodation for one night during my voyage in the Universe."
Soon after he was released and travelled back to Belgrad, where he continued his work during the entire War and published some ideas about the climate of Mars and Venus.

Finally he published his theory in 1920 "Mathematische Theorie der durch Sonneneinstrahlung ausgelösten Wärmephänomene" (Mathematical theory of thermal phenomena caused by solar radiation).

Fig.1. Variations in the Earth's orbital parameters:
1. Eccentricity: the shape of the orbit around the sun.

2. Changes in obliquity: changes in the angle that Earth's axis makes with the plane of Earth's orbit.

3. Precession: the change in the direction of the Earth's axis of rotation, i.e., the axis of rotation behaves like the spin axis of a top that is winding down; hence it traces a circle on the celestial sphere over a period of time.

Together, the periods of these orbital motions have become known as Milankovitch cycles. These parameters influence the amount of solar energy on earth´s surface, especially during summer of the northern hemisphere (55°-65°N).

In his theory he postulated:
- Glaciations are caused by variations of astronomical parameters

- The parameters influence the amount of solar energy on earth´s surface, especially during summer of the northern hemisphere (55°-65°N)

- It is possible to calculate these changes, and so calculate the climate in the past.

The German meteorologists Wladimir Köppen and Alfred Wegener supported the new theory, and noted the apparent coincidence of the calculated curve with the by Penck and Brückner postulated four European glaciations.

Fig. 2. Figure from KÖPPEN & WEGENER 1924, where they correlated the calculated cycles to the know ice ages at that time.

Fig.3. Outcrop of the Trubi-Formation at Capo Spartivento (South-Italy), a succession of Globigerina-marls from the Pliocene-Pleistocene transition. The regular stripes are caused by organic rich layers, thought to be caused by changes in the biological productivity in response of changes of the astronomical parameters - the Milankovitch cycles.


CHORLTON, W. (ed) (1985): Ice Ages (Planet Earth). Time-Life Books: 176
KÖPPEN, W. & WEGENER, A. (1924): Die Klimate der geologischen Vorzeit. Borntraeger, Berlin: 256


NASA Earth Observatory: Milutin Milankovitch (1879 - 1958). Accessed 26.06.2010

Sonntag, 6. Juni 2010

Geology and Cyclicity

1842, 5 years after Agassiz's "Discourse of Neuchatel", the French mathematician Joseph Alphonse Adhémar elaborated a hypothesis to explain a cyclic occurring of ice ages. He calculated the variations of the "direction" and declination of earth axis and the "movements" of earth around the sun during the geological past.
These cyclic factors influence the time and the energy density of solar radiation that reach earth from sun, causing cyclic climatic change.
Adhémar proposed that in a period of 11.000 years the hemisphere that experiences a longer winter, resulting from these three astronomical factors, would develop in an ice age.
But in 1852 Alexander von Humboldt noted that Adhémar didn't consider an important factor in his calculations, even if one hemisphere experience lower radiation, the opposite hemisphere experience an increase, so in the end the total sum remains more or less identical.

Nevertheless the idea of the French mathematician was intriguing, and would influence later researchers.

In 1833, James Croll (1821-1890), son of a poor stonecutter of Perthshire, purchased a copy of the "Penny Magazine", a magazine for children education. He was fascinated and began extensively to read, and some time later acquired he's first books dealing with natural science; "At first I was totally confused, but then the beauty and simplicity of the ideas provided me with delight and surprise, and I began seriously to study the matter."

Croll had no easy living in the next 20 years; he travelled the country, most time working as casual labourer, and in 1850 managed (for a brief time) the only Scottish pub were no alcohol was allowed.
He then found work as a maintenance supervisor of the Andersonian College in Glasgow, where he had access to the library and the hosted scientific works, a knowledge resource that he grateful exploited.

"At that time, the question of what could have triggered the ice age was much discussed among geologists. So in the spring of 1864, I directed my attention to this topic."

From 1864, Croll corresponded with Sir Charles Lyell, on links between ice ages and variations in the Earth's orbit. This led to a position in the Edinburgh office of the Geological Survey of Scotland, as keeper of maps and correspondence, where the director, Sir Archibald Geikie, encouraged his research. He also corresponded with Charles Darwin on erosion by rivers.

Croll, based on observations of the astronom Urbain Jean Josef Leverrier, used in his calculations an important factor that Adhémar did not know, the "movements" of the perihelion and aphelion on earth's ecliptic (precession of the equinoxes). He published his research in the book "Climate and Time, in Their Geological Relations" in 1875.

Fig.1. Glacial and interglacial conditions when eccentricity is at its superior limit, from CROLL1875, frontispiece (from FLEMING 2006).

Fig.2. Variations in the earth's orbit for three million years before 1800 A.D. and one million years after it, from CROLL 1875, following p. 312 (from FLEMING 2006).

Geikie wrote about the work of Croll: "The astronomical theory seems to me the best solution to the present ice age riddle. It bears in it all the decisive factors for the occurrence of alternating cold and warm periods, and accounts for the peculiar character of glacial and interglacial climates."

But there was a problem, even if dating methods at these times were only approximate, geological evidences supported a very young age of glacial deposits, but after Croll´s theory the last glacial period had ended 80.000 years ago. When Croll died, highly respected, geologists considered his theory wrong.
Geikie resumed: "It may well be, that with certain modifications of his views; one day we will solve the secret. But for now we must be continue to work and wait."

The modifications as hoped by Geikie will come only years later, and a glass wine will be the first step to solve the problem of the cyclicity of glacial periods:
Geology and Cyclicity: Milankovitch´s idea.

Fig.3. Orbitally forced cyclic sedimentation in the Trubi Formation of Zanclean age at Scala dei Turchi, in the Rossello composite section (Sicily), that represents the template for the Pliocene Series.

Fig.4. Orbitally forced cyclic sedimentation as expressed in the uppermost part of the Trubi Formation and in the overlying Monte Narbone Formation at Punta Piccola (Sicily), where the Piacenzian GSSP has been defined.


CHORLTON, W. (ed) (1985): Ice Ages (Planet Earth). Time-Life Books: 176

CROLL, J. (1875): Climate and Time, in their Geological Relations. A theory of secular changes of the Earth's Climate. D. Appleton and Company, New York: 630

FLEMING, J.R. (2006): James Croll in Context: The Encounter between Climate Dynamics and Geology in the Second Half of the Nineteenth Century. History of Meterology 3: 43 - 54