Understanding the Milankovitch Cycles and Their Impact on Climate Change

Understanding the Milankovitch Cycles and Their Impact on Climate Change

The Milankovitch cycles describe the collective effects of changes in Earth’s movements on its climate over thousands of years. Named after Serbian geophysicist and astronomer Milutin Milanković, these astronomical cycles have played a fundamental role in shaping our planet’s climate history, triggering ice ages and warm interglacial periods throughout geological time. Understanding these natural climate drivers provides essential context for comprehending both past climate variations and the current changes affecting our planet today.

Who Was Milutin Milanković?

Milutin Milanković was born on May 28, 1879, in Dalj, Austria-Hungary (now in Croatia), and died on December 12, 1958, in Belgrade, Yugoslavia (now in Serbia). He was a Serbian mathematician, astronomer, climatologist, geophysicist, civil engineer, university professor, popularizer of science and academic. His diverse background uniquely positioned him to tackle one of the most challenging scientific questions of his era: what causes ice ages?

After local schooling, he traveled to Vienna at age 17 to study engineering at the Technische Hochschule (College of Technology). After graduation and a short hiatus for military service, he returned to Vienna and earned a doctorate in 1904 for theoretical research on concrete and the design of concrete structures. Despite a successful career as a civil engineer, he accepted a faculty position in applied mathematics at the University of Belgrade in 1909—a position he held for the remainder of his life.

Remarkably, much of Milanković’s groundbreaking work was conducted under extraordinary circumstances. During his honeymoon in late June 1914, 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. The work he completed during his imprisonment, titled the Mathematical Theory of Heat Phenomena Produced by Solar Radiation, 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.

What Are Milankovitch Cycles?

In the 1920s, Milanković provided a more definitive and quantitative analysis than James Croll’s earlier hypothesis that variations in eccentricity, axial tilt, and precession combined to result in cyclical variations in the intra-annual and latitudinal distribution of solar radiation at the Earth’s surface, and that this orbital forcing strongly influenced the Earth’s climatic patterns. These variations occur because the Earth’s rotation around its axis, and revolution around the Sun, evolve over time due to gravitational interactions with other bodies in the Solar System.

Milankovitch cycles consist of three main components that influence Earth’s climate:

• Orbital Eccentricity: The shape of Earth’s orbit around the Sun changes from more circular to more elliptical over cycles of approximately 100,000 years, with additional component cycles of 95,000 and 125,000 years.
• Axial Tilt (Obliquity): The angle of Earth’s axis varies between 22.1 and 24.5 degrees over a cycle of about 41,000 years.
• Precession: The wobble in Earth’s rotation axis changes the orientation of the poles over a combined cycle of about 23,000 years on average, resulting from both axial precession (approximately 25,772 years) and apsidal precession (approximately 112,000 years).

These cyclical orbital movements cause variations of up to 25 percent in the amount of incoming insolation at Earth’s mid-latitudes (the areas of our planet located between about 30 and 60 degrees north and south of the equator).

The Science Behind Milankovitch Cycles

The interplay between these three cycles affects the distribution and intensity of solar energy received by Earth. These cycles affect the amount of sunlight and therefore, energy, that Earth absorbs from the Sun. This variation influences temperature, precipitation patterns, and the extent of ice sheets across geological timescales.

Orbital Eccentricity: The Shape of Earth’s Orbit

The Earth’s orbit varies between nearly circular and mildly elliptical (its eccentricity varies). The eccentricity of Earth’s orbit is currently about 0.0167; its orbit is nearly circular. However, over hundreds of thousands of years, the eccentricity of the Earth’s orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets.

The eccentricity of Earth’s orbit impacts the distance from the Sun at different points in the year, which in turn affects the amount of solar radiation received. When the orbit is at its most eccentric, the amount of solar radiation at perihelion will be about 23% more than at aphelion. Currently, perihelion presently occurs around 3 January, while aphelion is around 4 July.

The total change in global annual insolation due to the eccentricity cycle is very small. Because variations in Earth’s eccentricity are fairly small, they’re a relatively minor factor in annual seasonal climate variations. However, eccentricity plays a crucial role by modulating the effects of precession. Although eccentricity alone produces only a small direct change in global mean insolation, its interaction with precession and axial tilt has been implicated in pacing the timing of glacial–interglacial cycles during the late Quaternary.

Eccentricity is the reason why our seasons are slightly different lengths, with summers in the Northern Hemisphere currently about 4.5 days longer than winters, and springs about three days longer than autumns. This occurs because Kepler’s second law states that a body in orbit traces equal areas over equal times; its orbital velocity is highest around perihelion and lowest around aphelion. The Earth spends less time near perihelion and more time near aphelion.

Axial Tilt (Obliquity): The Angle of Earth’s Axis

Obliquity is why Earth has seasons. Over the last million years, it has varied between 22.1 and 24.5 degrees with respect to Earth’s orbital plane. The current tilt is 23.446°, roughly halfway between its extreme values.

Changes in axial tilt have profound effects on seasonal intensity and the distribution of solar radiation across latitudes. The greater Earth’s axial tilt angle, the more extreme our seasons are, as each hemisphere receives more solar radiation during its summer, when the hemisphere is tilted toward the Sun, and less during winter, when it is tilted away. Larger tilt angles favor periods of deglaciation (the melting and retreat of glaciers and ice sheets).

Conversely, as obliquity decreases, it gradually helps make our seasons milder, resulting in increasingly warmer winters, and cooler summers that gradually, over time, allow snow and ice at high latitudes to build up into large ice sheets. Increased tilt increases the total annual solar radiation at higher latitudes, and decreases the total closer to the equator.

The tilt last reached its maximum in 8,700 BCE, which correlates with the beginning of the Holocene, the current geological epoch. It is now in the decreasing phase of its cycle, and will reach its minimum around the year 11,800 CE. The current trend of decreasing tilt, by itself, will promote milder seasons (warmer winters and colder summers), as well as an overall cooling trend.

Precession: Earth’s Wobble

As Earth rotates, it wobbles slightly upon its rotational axis, like a slightly off-center spinning toy top. This wobble is due to tidal forces caused by the gravitational influences of the Sun and Moon that cause Earth to bulge at the equator, affecting its rotation. The trend in the direction of this wobble relative to the fixed positions of stars is known as axial precession.

Precession affects the timing of the seasons relative to Earth’s position in its orbit. The cycle of axial precession spans about 25,771.5 years. Additionally, Earth’s entire orbital ellipse—that is, the oval-shaped path Earth follows in its orbit around the Sun—also wobbles irregularly, primarily due to its interactions with Jupiter and Saturn. The cycle of apsidal precession spans about 112,000 years. The combined effects of axial and apsidal precession result in an overall precession cycle spanning about 23,000 years on average.

Axial precession makes seasonal contrasts more extreme in one hemisphere and less extreme in the other. Currently perihelion occurs during winter in the Northern Hemisphere and in summer in the Southern Hemisphere. This makes Southern Hemisphere summers hotter and moderates Northern Hemisphere seasonal variations. But in about 13,000 years, axial precession will cause these conditions to flip, with the Northern Hemisphere seeing more extremes in solar radiation and the Southern Hemisphere experiencing more moderate seasonal variations.

It’s important to note that precession does affect seasonal timing relative to Earth’s closest/farthest points around the Sun. However, the modern calendar system ties itself to the seasons, and so, for example, the Northern Hemisphere winter will never occur in July. This is because our calendar is based on the tropical year, which tracks the seasons, rather than the sidereal year, which tracks Earth’s position relative to the stars.

One observable effect of precession is the changing position of the North Star. The positions of the south and north celestial poles appear to move in circles against the space-fixed backdrop of stars, completing one circuit in approximately 26,000 years. Thus, while today the star Polaris lies approximately at the north celestial pole, this will change over time, and other stars will become the “north star”. In approximately 3,200 years, the star Gamma Cephei in the Cepheus constellation will succeed Polaris for this position.

Historical Context and Scientific Validation

Milanković’s ideas were published in a series of papers and eventually brought together in his influential book Kanon der Erdbestrahlung und seine Anwendung auf das Eiszeitenproblem (1941; Canon of Insolation and the Ice-Age Problem). On the basis of his analysis, Milankovitch concluded that Earth’s orbit changes in three cycles of different lengths.

Milankovitch assumed changes in radiation at some latitudes and in some seasons are more important than others to the growth and retreat of ice sheets. In addition, it was his belief that obliquity was the most important of the three cycles for climate, because it affects the amount of insolation in Earth’s northern high-latitude regions during summer. He calculated that Ice Ages occur approximately every 41,000 years.

However, like that of several predecessors, Milankovitch’s work was greeted with considerable excitement, but was then largely dismissed. Ice ages are difficult to date, partly because each erases much of the traces of its predecessor. Milankovitch’s work was challenged during the 1950s, and it soon fell out of favour. Most scientists thought that Milankovitch’s predicted temperature changes were too minor to choreograph the advance and retreat of glaciers.

The turning point came decades after Milanković’s death. The tables were turned by the late 1960s. Technical advances made it possible for geologists to study deep-sea sediment cores that contain a climate record going back millions of years. This climate record shows remarkably regular variations, which correlate with the mathematician’s figures and which are now known as Milankovitch cycles.

In 1976, a study in the journal Science by Hays et al. using deep-sea sediment cores found that Milankovitch cycles correspond with periods of major climate change over the past 450,000 years, with Ice Ages occurring when Earth was undergoing different stages of orbital variation. Several other projects and studies have also upheld the validity of Milankovitch’s work, including research using data from ice cores in Greenland and Antarctica that has provided strong evidence of Milankovitch cycles going back many hundreds of thousands of years.

Materials taken from the Earth have been studied to infer the cycles of past climate. Antarctic ice cores contain trapped air bubbles whose ratios of different oxygen isotopes are a reliable proxy for global temperatures around the time the ice was formed. These paleoclimate records have been instrumental in validating Milanković’s theoretical predictions.

The 100,000-Year Problem

One of the most intriguing aspects of Milankovitch cycles is what scientists call the “100,000-year problem.” Subsequent research confirms that ice ages did occur at 41,000-year intervals between one and three million years ago. But about 800,000 years ago, the cycle of Ice Ages lengthened to 100,000 years, matching Earth’s eccentricity cycle. While various theories have been proposed to explain this transition, scientists do not yet have a clear answer.

This is particularly puzzling because ice age cycles of the Quaternary glaciation over the last million years have been at a period of 100,000 years, which matches the eccentricity cycle, yet eccentricity has the weakest direct effect on solar radiation. Eccentricity has component cycles of 95,000 and 125,000 years, adding further complexity to understanding this dominant climate cycle.

Impact on Climate Change Through Geological Time

Understanding Milankovitch cycles is essential for grasping how natural factors contribute to long-term climate change. They provide a strong framework for understanding long-term changes in Earth’s climate, including the beginning and end of Ice Ages throughout Earth’s history.

Triggering Ice Ages and Interglacials

Milankovitch cycles have been linked to the timing of ice ages and interglacial periods throughout Earth’s history. When these cycles cause the northern latitudes to get less sun in the summer, it allows ice sheets to begin to expand. The northern latitudes matter much more than the southern latitudes—at least over the past few million years—as it contains more land area (which can more easily become ice-covered than the oceans) and because the Antarctic has remained covered in ice.

The mechanism by which relatively small changes in solar radiation can trigger massive ice ages involves several feedback processes. These ice sheets in turn reflect more incoming sunlight back to space, resulting in a “positive feedback” that drives additional regional cooling. This is known as the ice-albedo feedback.

The Role of Feedback Mechanisms

It is also clear that astronomical factors alone cannot cause the large changes that the Earth experienced. Other factors must also influence climate but scientists still do not know how. As temperatures change due to Milankovitch cycles, feedback mechanisms such as ice albedo and greenhouse gas concentrations can amplify or mitigate these effects.

Even for Ice Age cycles, changes in the extent of ice sheets and atmospheric carbon dioxide have played important roles in driving the degree of temperature fluctuations over the last several million years. During past glacial cycles, the concentration of carbon dioxide in our atmosphere fluctuated from about 180 parts per million (ppm) to 280 ppm as part of Milankovitch cycle-driven changes to Earth’s climate. These fluctuations provided an important feedback to the total change in Earth’s climate that took place during those cycles.

The relationship between orbital forcing and carbon dioxide is complex. Changes in orbital cycles do not immediately cause rises or falls in atmospheric CO2. Rather, initial increases in ice cover in high-latitude areas trigger feedbacks that cause atmospheric CO2 to fall at the start of ice ages. As ice sheets grow, sea levels change dramatically, falling around 120 meters compared to today’s levels and exposing large areas of land currently underwater and allowing growing vegetation to take up more CO2. Colder ocean water dissolves more CO2, absorbing more from the atmosphere, though this is somewhat offset by the effect of higher salinity on ocean CO2 absorption—as fresh water from snow freezes into ice sheets. In addition, ice-age glaciers grind up rocks into dust that provides nutrients to ocean life, helping boost the amount of carbon in the deep ocean as plants get eaten and sink into the ocean.

Natural Climate Variability

Milankovitch cycles demonstrate that Earth’s climate is influenced by natural forces operating over vast timescales. Small cyclical variations in the shape of Earth’s orbit, its wobble and the angle its axis is tilted play key roles in influencing Earth’s climate over timespans of tens of thousands to hundreds of thousands of years. This natural variability is crucial for understanding current climate trends and distinguishing them from anthropogenic effects.

The small changes set in motion by Milankovitch cycles operate separately and together to influence Earth’s climate over very long timespans, leading to larger changes in our climate over tens of thousands to hundreds of thousands of years. Milankovitch combined the cycles to create a comprehensive mathematical model for calculating differences in solar radiation at various Earth latitudes along with corresponding surface temperatures. The model is sort of like a climate time machine: it can be run backward and forward to examine past and future climate conditions.

Current Relevance and Future Climate Predictions

Today, the study of Milankovitch cycles remains highly relevant as scientists seek to understand both past and future climate change. By comparing current trends to historical data, researchers can better predict future climate scenarios and distinguish natural climate variability from human-induced changes.

Why Milankovitch Cycles Cannot Explain Current Warming

While Milankovitch cycles have been instrumental in driving past climate changes, they cannot account for the rapid warming Earth is currently experiencing. Milankovitch cycles can’t explain all climate change that’s occurred over the past 2.5 million years or so. And more importantly, they cannot account for the current period of rapid warming Earth has experienced since the pre-Industrial period (the period between 1850 and 1900).

There are several key reasons for this:

Timescale Mismatch: Milankovitch cycles operate on long time scales, ranging from tens of thousands to hundreds of thousands of years. In contrast, Earth’s current warming has taken place over time scales of decades to centuries.

Recent Orbital Changes Are Minimal: Over the last 150 years, Milankovitch cycles have not changed the amount of solar energy absorbed by Earth very much. In fact, NASA satellite observations show that over the last 40 years, solar radiation has actually decreased somewhat.

Current Orbital Position Predicts Cooling: Earth is currently in an interglacial period (a period of milder climate between Ice Ages). If there were no human influences on climate, scientists say Earth’s current orbital positions within the Milankovitch cycles predict our planet should be cooling, not warming, continuing a long-term cooling trend that began 6,000 years ago.

An often-cited 1980 orbital model by Imbrie predicted “the long-term cooling trend that began some 6,000 years ago will continue for the next 23,000 years”. Earth’s orbit will become less eccentric for about the next 100,000 years, so changes in this insolation will be dominated by changes in obliquity, and should not decline enough to permit a new glacial period in the next 50,000 years.

Human Impact on Climate

The current warming trend is driven by human activities, not orbital cycles. Since the beginning of the Industrial Age, the concentration of carbon dioxide in Earth’s atmosphere has increased 50 percent, from about 280 ppm to 412 ppm (update: 421 ppm in 2023). Scientists know with a high degree of certainty this carbon dioxide is primarily due to human activities because carbon produced by burning fossil fuels leaves a distinct “fingerprint” that instruments can measure.

Since 1850, Earth’s global average temperature has increased by over 1 degree Celsius (1.8 degrees Fahrenheit). Furthermore, recent scientific assessments show that Earth is expected to warm another half a degree Celsius (almost a degree Fahrenheit) as soon as 2030. This relatively rapid warming of our climate due to human activities is happening in addition to the very slow changes to climate caused by Milankovitch cycles.

Applications in Climate Modeling

Incorporating Milankovitch cycles into climate models helps improve their accuracy and provides valuable context for understanding Earth’s climate system. Since orbital variations are predictable, any model that relates orbital variations to climate can be run forward to predict future climate, with two caveats: the mechanism by which orbital forcing influences climate is not definitive; and non-orbital effects can be important (for example, the human impact on the environment principally increases greenhouse gases resulting in a warmer climate).

These models allow scientists to simulate past climates and project future conditions based on various scenarios. By understanding how Earth’s climate responded to orbital forcing in the past, researchers can better calibrate models and improve predictions about future climate behavior under different greenhouse gas emission scenarios.

Milankovitch Cycles Beyond Earth

Milanković’s work extended beyond Earth’s climate. Milanković gave two fundamental contributions to global science. The first contribution is the “Canon of the Earth’s Insolation”, which characterizes the climates of all the planets of the Solar System. The second contribution is the explanation of Earth’s long-term climate changes caused by changes in the position of the Earth in comparison to the Sun, now known as Milankovitch cycles.

Mars, in particular, experiences Milankovitch cycles that are far more extreme than Earth’s. Mars undergoes Milankovitch cycles that are much more extreme than those of Earth. Its obliquity changes by much larger amounts than Earth’s and its eccentricity changes by larger amounts than Earth’s. These dramatic orbital variations have significant implications for understanding Mars’ climate history and the potential for past habitability.

The study of Milankovitch cycles on other planets provides valuable comparative planetology insights. To understand the Milankovitch cycles of an exoplanet, we need to have really good measurements of the orbital parameters of the planet that we’re looking at, but also all the orbital parameters of all the other planets in the system. Milankovitch cycles are a complex thing that is caused by perturbations from all the other planets in the system and perturbations from the host star and any moons that the planet might have.

Educational Importance and Public Understanding

Understanding Milankovitch cycles is essential for educators, students, and the general public. It provides a crucial framework for discussing climate change and the factors that influence it over geological timescales. This knowledge helps distinguish between natural climate variability and anthropogenic climate change, enabling more informed discussions about current environmental challenges.

The story of Milutin Milanković himself serves as an inspiring example of scientific perseverance and interdisciplinary thinking. 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”.

His legacy extends far beyond climate science. Milanković has been honored with a crater on the Moon, a crater on Mars, and asteroid 1605 Milanković. In 1993, the European Geophysical Society established a medal in his name, recognizing his fundamental contributions to understanding Earth’s climate system.

Recent Advances in Understanding Orbital Forcing

Recent research has revealed new insights into how orbital parameters influence Earth’s climate. Scientists have discovered that the role of eccentricity in Earth’s seasonal climate may be more significant than previously thought. Orbital eccentricity produces seasonal radiative changes that are comparable in magnitude to transient climate forcings commonly considered in climate studies.

While textbooks traditionally emphasize that Earth’s tilt dominates seasonal variations, Earth’s orbital eccentricity is relatively small (e ~ 0.0167, meaning that the Earth-Sun distance at aphelion is ~1.67% longer than the mean) and the solar flux changes only by ~7% between aphelion and perihelion. However, researchers are finding that the “distance effect” from eccentricity plays a more important role than previously recognized, particularly in tropical regions and ocean circulation patterns.

Climate records contained in a 1,700 ft (520 m) core of rock drilled in Arizona show a pattern synchronized with Earth’s eccentricity, and cores drilled in New England match it, going back 215 million years. This demonstrates that Milankovitch cycles have been influencing Earth’s climate for hundreds of millions of years, far longer than the recent ice age cycles of the past few million years.

Practical Applications and Future Research

The study of Milankovitch cycles has practical applications beyond understanding past climate. These cycles provide a baseline for natural climate variability against which anthropogenic changes can be measured. They also help scientists understand the sensitivity of Earth’s climate system to various forcings and feedback mechanisms.

Future research directions include:

• Better understanding the mechanisms that amplify the relatively weak orbital forcing into major climate changes
• Resolving the 100,000-year problem and understanding why eccentricity dominates recent ice age cycles
• Improving paleoclimate reconstructions to better validate orbital theory
• Investigating how Milankovitch cycles interact with other climate forcings, including tectonic processes and atmospheric composition changes
• Applying knowledge of orbital cycles to understand climate on other planets and potentially habitable exoplanets

For those interested in learning more about Milankovitch cycles and their role in Earth’s climate system, NASA’s Earth Observatory provides excellent resources and visualizations at https://earthobservatory.nasa.gov/features/Milankovitch. The American Museum of Natural History also offers educational materials about Milanković’s life and work at https://www.amnh.org/learn-teach/curriculum-collections/earth-inside-and-out/milutin-milankovitch-seeking-the-cause-of-the-ice-ages.

Conclusion

The Milankovitch cycles represent a fundamental aspect of Earth’s climate system, demonstrating how subtle changes in our planet’s orbital geometry can trigger dramatic climate shifts over thousands of years. Milankovitch dedicated his career to developing a mathematical theory of climate based on the seasonal and latitudinal variations of solar radiation received by the Earth, and his work has stood the test of time.

By studying these cycles, we gain profound insights into the natural processes that have shaped our planet’s climate history over millions of years. This understanding provides essential context for evaluating current climate change, distinguishing between natural variability and human-induced warming, and predicting future climate scenarios.

While Milankovitch cycles cannot explain the rapid warming Earth is experiencing today—which is unequivocally driven by human greenhouse gas emissions—they remain crucial for understanding Earth’s climate system as a whole. They remind us that climate is influenced by multiple factors operating on vastly different timescales, from the daily rotation of Earth to orbital cycles spanning hundreds of thousands of years.

The legacy of Milutin Milanković endures not only in the scientific principles that bear his name but also in the interdisciplinary approach he pioneered—combining mathematics, astronomy, physics, and geology to solve one of nature’s greatest mysteries. His work exemplifies how fundamental scientific research, even when initially dismissed, can ultimately transform our understanding of the world around us and continues to inform climate science more than a century after he began his groundbreaking calculations.