This World War I Prisoner of War Solved the Mystery of the Ice Ages

Milutin Milanković, a 35-year-old Serbian scientist, was honeymooning in his family’s hometown village of Dalj when the world changed overnight. At the time, late June 1914, the town, today part of Croatia, was a Serbian enclave nestled in the Austro-Hungarian Empire. That same week, Gavrilo Princip, a Serbian nationalist, assassinated the Austro-Hungarian heir-apparent, Archduke Franz Ferdinand. Austro-Hungary declared war on the kingdom of Serbia, and suddenly Milanković was caught in the middle of a global conflict.

Milanković was taken prisoner before he and his new wife could make it back to their home and work in Belgrade, Serbia. All he carried with him was his briefcase containing a few scientific papers and some of his notes.

For the first six months, he was held in the Nezsider internment camp under rough conditions. Eventually, though, through the efforts of his wife, Kristina, Milanković secured supervised release in Budapest. There, he was required to check in with the authorities once a week but was otherwise allowed to work among the vast library of the Hungarian Academy of Sciences. Still, despite his improved circumstances, he longed for home and freedom, and sought refuge in his work.

A decade later, reflecting on his time in Budapest and imagining gazing upon the building where he was confined, Milanković wrote, “I was her prisoner of war, and in her my four-year longing for freedom would have drained my core, if I had not found refuge in scientific work. ... There, it is the window where I sat for days and, looking at the blue Danube and proud Buda, wrote my first scientific work.”

The work he refers to is a book written in French, titled the Mathematical Theory of Heat Phenomena Produced by Solar Radiation. It contains the core calculations of what would become Milanković’s life’s work: solving the math behind how Earth’s orbit slowly changes over time to influence the amount of sunlight received by climatically important locales. Today, his name remains on scientists’ lips, as so-called Milanković cycles are widely accepted as the cause of the periodic ice ages of the past 2.5 million years. Geologists have even found evidence of their rhythmic impacts on the Earth system deeper back in time.

But in the earliest years of the 20th century, before the Austro-Hungarians captured him, Milanković wasn’t known as a scientist. Rather, he made his living as a civil engineer. As a teenager in 1896, he moved to Vienna for school, completing a doctoral thesis on reinforced concrete construction in 1904. He found good-paying work for a prominent firm in Austria, designing large factories and military facilities.

He felt something was missing, though. “He knew that his main inclination was science,” says Fedor Mesinger, a meteorologist at the Serbian Academy of Sciences and Arts

In 1909, the University of Belgrade reached out to Milanković and offered him a position teaching applied mathematics. He accepted the professorship in Belgrade despite “a much smaller salary,” says Mesinger. In early October 1909, Milanković got to work brushing up on the latest discoveries in the fields he would be teaching, including theoretical physics and celestial mechanics. But he hadn’t yet settled on what questions to pursue in his own research.

One story holds that, a few years later, around 1911, Milanković was drinking wine at a café with a poet. By then, Milanković was settled into teaching his courses and beginning to think about what scientific issues to pursue in his research. Meanwhile, the poet was feeling good, having just sold his first book. He proclaimed he would abstain from short poems, and instead write about his “entire society, our country, and our soul.” Milanković, swept up in the moment, replied, “I want to do more than you. I want to grasp the entire universe and spread light into its farthest corners.” Milanković was “on the lookout for a cosmic problem.”

He soon found a scientific problem worthy of his cosmic ambitions. In the early 20th century, the cause and timing of the ice ages was being hotly debated in European and American science. Geologic evidence had piled up that mysterious landforms were caused by vast ice sheets that had descended out of the Arctic onto more temperate continents, disproving the lingering idea that the deposits came from icebergs suspended in the biblical flood.

But many more questions remained. When did these ice sheets advance and retreat? How many times? Why did this keep happening?

Ideas abounded. Some thought that the Earth’s surface moved up and down­—when the altitude increased, it got cold enough for ice sheets to form. Others thought that changes in the strength of light emitted from the sun could be large enough to account for major climatic changes. Still more suggested space dust or giant volcanic eruptions like Krakatoa in 1883 could be responsible.

Meanwhile, a small group favored a theory that came from the halls of astronomy. In the 1860s and ’70s, in a remarkable story, a janitor named James Croll got access to the libraries at the Andersonian College and Museum in Glasgow and discovered writings that showed how Earth’s orbit changed over time. He thought that variations in Earth’s distance from the sun might influence the relative coldness or warmth of winter and summer.

Croll’s ideas failed to gain mainstream traction, in part because they were wrong in their specifics. Croll presumed that orbitally caused cold winters would allow vast ice sheets to build up on the continents to the point that they were large enough to last through a warm summer’s melting. It turns out, however, that it’s more important for the weather to stay cold in the summer so that accumulated snow was less susceptible to melting. Warmer winters are also important because they can lead to more moisture in the air and thus more snowfall to build up glaciers.

Milanković’s key insight was combining this idea of the importance of mild seasons with three different variables in Earth’s orbit. First, is the cycle of eccentricity—a measure of how circular the Earth’s large loop around the sun is. When the orbit is less circular, like right now, the Earth spends part of the year far away from the sun and the other part closer. When that happens, the Northern Hemisphere summer season is about 4.5 days longer than winter. When the orbit is more circular, that seasonal length difference disappears. This cycle of eccentricity repeats every 100,000 years or so.

The second variable Milanković defined is obliquity, a term that describes the angle of Earth’s rotational axis relative to a flat orbital plane. When the axis is more tilted, polar regions experience colder winters and warmer summers. A more vertical axis lessens the seasonal extremities, and, as Milanković proposed, those warmer winters and cooler summers help encourage the growth of ice sheets. The axis makes a full cycle of tilting and straightening about every 40,000 years.

Finally, cycles of precession are the third variable. Precession is a wobble in the direction of Earth’s rotational axis, kind of like a toy top spinning in wide circles as it slows down. This wobble enhances seasonal extremities in one hemisphere while weakening them in the other. When the Earth’s axis wobbles so that Northern Hemisphere seasons are milder, that helps ice sheets grow. The cycles vary on a time scale of about 23,000 years.

Together, the cycles of eccentricity, obliquity and precession interact to change the total amount of incoming sunlight at different latitudes. When the variables align in just the right way, this can cause a global ice age or cause even the largest glaciers to melt.

So, while confined to Budapest during World War I, Milanković took the mathematics of these cycles and calculated the amount of solar radiation each latitude on Earth would receive at that moment. He was able to accurately predict the average temperature at locations across the globe, proving that the fundamentals of his method were sound. He then ran his calculations back 600,000 years in time, predicting multiple episodes of widespread glaciation that seemed to match the geological evidence available at the time.

But scientists of the day didn’t readily accept his ideas, and despite becoming respected throughout Europe, his work wouldn’t be proved true until well after his death in 1958. Meanwhile, climate scientists even treated him with some disdain.

“There was a geologist who, publicly, one year before Milanković died, said that the theory of Milanković was rubbish,” says André Berger, a climatologist at the University of Louvain in Belgium.

Even as late as the 1960s, scientists dismissed his ideas out of hand. Berger recalls learning about Milanković and his theory at a conference around that time. “Both meteorologists and geologists were highly criticizing a person that I did not know, whose name was Milutin Milanković. They were finding that still he was totally stupid.” Berger decided to look into Milanković’s work for himself and defend him if possible. Since then, he has worked on refining and expanding on Milanković’s ideas.

The negative reactions didn’t bother Milanković during his lifetime, though. He was rightly confident that his ideas would stand the test of time. Writing in his memoir, he said “As many scientific discoveries, far greater than mine, had remained unrecognized for years, I knew that, if my work was to become a real contribution to science, it would find its way without any help, recommendation or praise.”

By the 1970s, Milanković’s work had truly found its way, but it did need some assistance. Berger’s calculations refining Milanković’s original work helped, and a groundbreaking paper in 1976 found evidence that the cycles Milanković calculated were influencing the composition of deep-sea sediments going back almost half a million years.

Now that scientists had proved Milanković’s ideas had merit, geologists began to see his cycles everywhere. Milanković cycles are even visible in the stratigraphic layers of rocks found around the globe and have inspired a whole new field of science called cyclostratigraphy. For example, geologists have found that layers of rock deposited in far-flung places like New Jersey, Italy and Australia match up to the cycles described by Milanković—even in rocks that are billions of years old.

Before Milanković’s theory won the contest of potential ice age causes, as early as the mid-19th century, scientists understood the greenhouse effect of carbon dioxide and posited that changes in the gas’s concentration in the atmosphere could cause the ice age cycles. Technically, they were wrong about that at the time. Over the past million years or so, geologists have found that atmospheric carbon concentrations didn’t vary enough to instigate the advance and retreat of ice sheets. Through that period, atmospheric carbon ranged from concentrations of about 150 to 300 parts per million.

Now, though, we’re over 420 parts per million, and no longer within the range of conditions under which Milanković cycles have caused ice ages over the past two million years. “We’re just way off the charts,” says Linda Hinnov, a cyclostratigrapher at George Mason University. In fact, the last time carbon concentrations were as high as they are today was about three million years ago, during a geologic epoch called the Pliocene. “You know, here in Washington, D.C.,” says Hinnov, “we were underwater in the Pliocene.”

The planet’s climatic conditions at such high carbon concentrations could render the world of cyclical ice ages that Milanković described impossible. With so much CO2 in the atmosphere, even changes to how much sunlight hits the Earth won’t be able to kindle an ice age. But while the future of the ice age cycles that Milanković sought to understand might be uncertain in a warming world, his legacy is secure—at least among scientists. His impact on climate science has been vast, and hundreds of researchers rely on the calculations he pioneered to learn about both the past and future of our planet.

Outside of scientific circles, Milanković is not widely known. In Serbia, his name is familiar, and he even appears on stamps and one of the most used pieces of Serbian currency. But, even in Belgrade, few non-scientists know the details of his contributions to our fundamental understanding of how our planet works.

Milanković continued to work on the mathematics of Earth’s climate over the ensuing decades, and in the spring of 1939 was ready to write a canonical book detailing his findings. The Royal Serbian Academy of Sciences and Arts agreed to publish the work, titled the Canon of Insolation and the Ice-Age Problem, once Milanković finished writing it.

By 1941, the Canon was ready, and Milanković sent the text to the publisher. On April 2 of that year, he walked across Belgrade to the printer to review the first copies. After flipping through the loose pages, he headed home with assurances that the book would be folded and bound, ready to distribute, without delay. But, later that week, German and Italian bombs began to fall on Belgrade, and by the end of the next week the city was under German occupation.

As it had been 27 years earlier, Milanković’s life was upended by a global war. No longer could he spend long hours secluded in his university office or chatting with other scientists at a local meeting hall. Instead, he wrote in his memoir, “I dug a rubbish pit in the back garden, collected water in cans from a central tap 500 [meters] away and chopped wood to fuel the long-discarded kitchen stove for cooking. Our civilized existence had disintegrated into a life of hard grind.”

It would be some months before he made his way back to the printer, which had been dug out of rubble flung by an explosive bomb, to find that all but a few pages of his Canon had survived. In the fall of 1941, the first copies were fully printed and bound—ready to be sent to scientists around the world, where they were eventually dusted off decades later by climatologists like Berger.

For his part, Milanković felt the publication of the Canon marked the end of his scientific career. “You know, once you catch the big fish you cannot be bothered with small ones,” he told his son, Vasko. “For almost 30 years I worked on my theory of solar radiation, and now that it has been finished and printed I feel too old to start anything new. Theories of that magnitude do not grow on trees!”

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Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.