92 | Great Geologists
working on practical dam, bridge and aqueduct design using
reinforced concrete, he published a number of key theoretical
papers alongside patenting some of his ideas. Such was his
reputation that in 1909 that he was offered the Chair of Applied
Mathematics at the University of Belgrade.
The move to academia was to signify a change in interests for
Milanković with a focus on fundamental research. A topic that
soon captured his attention was that of Earth’s climate and the
controls acting upon it. He noted that much of meteorology
and climatology was “nothing but a collection of innumerable
empirical findings, mainly numerical data, with traces of physics
used to explain some of them”. He decided to employ advanced
mathematics, in a manner not previously attempted.
Milanković’s ‘Contribution to the mathematical theory of climate’,
published in 1912, described the present climate on Earth and
how the Sun’s radiation determines the temperature on Earth’s
surface after passing through the atmosphere. He built upon this
with ‘Distribution of the Sun’s radiation on the Earth’s surface’
published in 1913. He calculated the intensity of insolation and
developed a mathematical theory describing Earth’s climate
zones.
His ultimate aim was a mathematically-based theory which
connected the thermal regime of Earth to variations in its orbital
parameters. The theory could be applied to the geological past,
especially the onset of ice ages. He wrote: “such a theory would
enable us to go beyond the range of direct observations, not only
in space, but also in time... It would allow reconstruction of the
Earth’s climate, and also its predictions, as well as give us the
first reliable data about the climate conditions on other planets.”
A paper published in 1914 (‘About the issue of the astronomical
theory of ice ages’), was a preliminary statement on the topic,
noting that others, including the French mathematician, Joseph
Adhémar, and the Scottish scientist, James Croll, had attempted
to address the problem in the previous century.
Unfortunately for Milanković, his work was disturbed by
political events. On 14th June 1914, Milanković married Kristina
Topuzovich and went on honeymoon for the summer to his
native village of Dalj, which was then in Austro-Hungary. Whist
the newly-wedded Milanković was staying in Dalj, Archduke
Franz Ferdinand, the heir to the Austro-Hungarian throne, was
murdered in Sarajevo by a Serbian extremist. The consequence
was a declaration of war by Austro-Hungary on Serbia, which
in turn led to World War I. Milanković was arrested as a Serb
and interned. Fortunately he was allowed to spend his captivity
in Budapest and work in the library of the Hungarian Academy
of Science, and the Central Meteorological Institute. His work
Milanković and his ideas celebrated on a Serbian stamp from
2004.
during this period mostly focused on calculating the climate on
the inner planets of the solar system and of the Earth’s moon,
although he continued his research into the drivers of ice ages.
In 1919 he returned to the University of Belgrade and focused
on the climate of the Earth both past and present. His main task
was to produce a mathematical theory of insolation, which was
summarised in a book published in 1920 entitled ‘Mathematical
Theory of Heat Phenomena Produced by Solar Radiation’.
Encouraged by the Russian climatologist Wladimir Köppen and
his son-in-law Alfred Wegener (of continental drift fame), he
began calculations of variations in Earth’s insolation over the
last 650,000 years. Milanković spent 100 days without break
completing the calculations using only pen and paper and
prepared a graph of solar radiation changes at geographical
latitudes at 65° north. Milanković believed that this latitude was
most sensitive to a change of thermal balance. At 65° north, ice
sheets develop not because it gets cold in winter but because
it remains cool in summer. In this way major ice sheets can
develop through a positive feedback, whereby the albedo of the
snow- and ice-covered surface reflects away radiation coming
from the Sun.
We know that it gets colder at higher latitudes in winter each
year because the amount of sunlight reaching the Earth’s
surface decreases, due to the tilt of the Earth’s axis. Milanković
expanded this concept to larger timescales and three key cyclic
variations in Earth’s orbit around the Sun creating variations in
insolation.