One hundred years of Milanković cycles

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524
correspondence
One hundred years of Milanković cycles
To the Editor — This year marks one
hundred years since the publication of
the book that outlined one of the most
influential scientific theories in climate
science: Milutin Milanković’s Mathematical
Theory of Heat Phenomena Produced by
Solar Radiation1 (Fig. 1). This work was the
first complete mathematical explanation
of how small changes in the rotation and
paths of planets around the Sun influence
the amount of solar radiation that planets
receive, more simply referred to as
insolation. In doing so, it provided the
foundation for understanding Earth’s
climate history and the causes of its great
ice ages2. Although to this day the theory
continues to be applied throughout the
climate sciences, the story of how it
developed despite the hardships of war and
imprisonment is not widely known.
As scientists around the world cope
with the array of challenges caused by
the COVID-19 pandemic, Milanković’s
achievements remind us that great
Earth science can sometimes arise in
the face of adversity.
Milanković’s personal memoirs3
provide unique insights into the scientific
and personal events surrounding the
development of his theory. They describe
his childhood in the Serbian enclave of
Dalj in Austro-Hungary, his student days at
the Vienna University of Technology where
he also completed his doctorate, and his
abandonment of a successful engineering
career in Vienna for a professorship at the
University of Belgrade. They also recount
the turbulent times at the beginning of the
twentieth century and their impact on his
scientific progress.
Upon moving to Belgrade in 1909,
Milanković sought a quiet research area
that avoided the sorts of research questions
that might lead to fierce competition
between scientists. He found this at the
intersection of applied mathematics,
astronomy and climatology, where he
resolved to bring a mathematical perspective
to what had been a rather descriptive
field of climate science. He began to
grapple with a solution to the outstanding
problem of explaining how changes in
insolation occur and affect the global
climate. However, only three years into
his professorship he was drafted into the
first Balkan War (1912–1913). Assigned,
due to his profession, to foreign
correspondence duty instead of being sent
to the frontline, Milanković managed to
publish some of his first works during
this time4,5.
Unfortunately, the end of the Balkan
War in Serbia was only a temporary reprieve
from conflict. Milanković was on the
first leg of his honeymoon in his hometown
in Austro-Hungary when World War I
broke out. No longer an Austro-Hungarian
citizen, he was subject to a new decree
calling for the imprisonment of Serbian
citizens. In the summer of 1914, he spent
six weeks under house arrest, but was
eventually imprisoned and later sent to
a prisoner-of-war camp. His diary entry
from his first night in prison reads
“The guard received me very kindly, like
a hotel receptionist receives an esteemed
guest: ‘I have,’ he said, ‘reserved for you
a separate room.’ […] I thanked him for
his reception […] Sat on the bed, I looked
around and started synchronizing with
my new social position: that lonely room,
away from the noise of people, felt made
for scientific work […] In the suitcase
I had my printed works and my notes
on the cosmic problem, there was clean
paper too and I started writing. It was far
past midnight when I stopped. I looked
around the room, wondering where I was.
It felt like I was in a roadhouse on my trip
through the Universe.”3
At the prisoner-of-war camp in today’s
Austria, research was not possible and
life became more difficult. The monotony
and uncertainty, as well as the threat of
disease, took its toll. Milanković’s freedom
was achieved thanks to the efforts of his
wife, Kristina, who contacted many of his
Austro-Hungarian friends and colleagues
asking for help. Finally, the authorities were
persuaded that Milanković was an esteemed
scientist and he was released and allowed to
live in Budapest under police surveillance.
Despite the adversity faced during his
house arrest and imprisonment, Milanković
continued to think about his scientific
pursuits and, whenever possible, continued
his calculations. Milanković’s manuscript on
a new mathematical theory of astronomical
forcing1 was finished in 1917, while he was
still under police surveillance. Its printing
was initially planned for 1918, but did not
Fig. 1 | Milanković and his masterpiece. A photo of Milutin Milanković upon joining the Serbian
Academy of Sciences in 1924 (left) and the front page of the Theorie Mathematique des Phenomenes
Thermiques Produits par la Radiation Solaire, printed in Paris in 1920 (right). Credit: Image adapted with
permission from Milutin Milanković Society Belgrade
NATURE GEOSCIENCE | VOL 13 | AUGUST 2020 | 524–525 | www.nature.com/naturegeoscience

525
correspondence
come to fruition when the publisher ran out
of paper due to the war. The manuscript was
finally published in 1920.
After the end of World War I, Milanković
was finally allowed to leave Budapest and
returned to Belgrade with his wife and
three-year-old son. In his typically dry way,
he writes “After a comfortable 3-day cruise
down the river Danube, I arrived to Belgrade
on March 15th 1919, thus completing my
exciting five-year wedding trip.”3
While the idea of astronomical drivers
of climate had been proposed before,
Milanković’s work was the first to explain
how the full set of cyclic variations in
Earth’s orbit — eccentricity, obliquity
and precession — caused distinct
variations in incoming solar radiation
at different latitudes and changed on
multi-thousand-year timescales. By finding
a general solution, which was also
applicable to other planets, he laid the
groundwork for understanding the pacing of
cold and warm episodes in Earth’s
climate history.
To ensure that his scientific discoveries
were available to future generations,
he compiled his work on the astronomical
forcing of climate into the famous Canon
of Insolation and the Ice-age Problem6.
During his lifetime, Milanković’s
theory of astronomical forcing gained
substantial attention and support from
scientists like Wladimir Köppen and
Alfred Wegener, but lacked corroborating
geological evidence. The long-awaited
confirmation arrived with James Hays
and collaborators7 in 1976, who finally
demonstrated that climate fingerprints in
ocean sediment records contain the
same temporal cycles as Milanković’s
theory predicted.
From a modern perspective, it may
seem that the importance of Milanković’s
contribution lay solely in his mathematical
achievements. But his memoirs and
life also reveal a remarkable resilience
in the course of developing one of the
most beautiful theories in the Earth
sciences. That Milanković persevered
and prospered scientifically under such
difficult circumstances should give hope
and inspiration to scientists navigating the
uncertain times that lie ahead. ❐
Ivana Cvijanovic 1 ✉ , Jelena Lukovic2 and
James D. Begg3
1Earth Sciences Department, Barcelona
Supercomputing Center, Barcelona, Spain. 2Faculty of
Geography, University of Belgrade, Belgrade, Serbia.
3Amphos 21, Barcelona, Spain.
✉e-mail: ivana.cvijanovic@bsc.es
Published online: 31 July 2020
https://doi.org/10.1038/s41561-020-0621-2
References
1. Milankovitch, M. Mathematical eory of Heat Phenomena
Produced by Solar Radiation (Gauthier-Villars, 1920).
2. Maslin, M. Nature 540, 208–209 (2016).
3. Milanković, M. Special publication by Serbian Academy of Sciences
Vol. 195 (ed. Djaja, I.) (Naucna Knjiga, 1952).
4. Milanković, M. Glas SKA 87, 136–160 (1912).
5. Milanković, M. Rad JAZU 22, 109–131 (1913).
6. Milankovitch, M. Canon of Insolation and the Ice-age Problem
(Königlich Serbische Akademie, 1941).
7. Hays, J. D., Imbrie, J. & Shackleton, N. J. Science 194,
1121–1132 (1976).
Acknowledgements
We thank R. Seager and N. Brandwein. M. Milanković
and D. Milanković helped source the photographs.
I.C. was supported by Generalitat de Catalunya (Secretaria
d’Universitats i Recerca del Departament d’Empresa i
Coneixement) through the Beatriu de Pinós programme
and J.L. by the Serbian Ministry of Education and Science
(grant no. III 43007). All translations are the authors’ own.
NATURE GEOSCIENCE | VOL 13 | AUGUST 2020 | 524–525 | www.nature.com/naturegeoscience