Milankovitch Cycles Part 1: Orbital Cycles and Earth’s Ice Ages


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In theory, there’s no difference between theory and practice. In practice, there is. — Yogi Berra

This is first in a series by RCA Member David Horne.

This is the story of the imprint of Earth’s orbital cycles on the climate of our planet. Scientists now believe that certain features of Earth’s orbital motions act as a trigger for Earth’s long-term climate cycles, including the cyclical planet-wide glacier cover, popularly called the “Ice Ages” — and termed by science glacial epochs. Scientists first discovered the glacial cycles in the middle of the 19th Century. But the theory linking the glacial epochs to variations in solar radiation caused by cyclical changes in certain of Earth’s orbital motions, had to wait until the first years of the 20th Century.

At the beginning of the 20th century, a Serbian scientist, Milutin Milankovich, developed the mathematics enabling him to accurately calculate a planet’s long term, cyclical variations in solar radiation, and resulting temperature cycles, at cloud top levels, for any latitude. (His calculations for Mars were so accurate they agree with current information from the various Mars satellites and probes.) Over the next decades of the 20th Century, scientists began looking for evidence of these cycles in our planet’s geology and biosphere. Technological advances which began in the last half of the century accelerated the search. What scientists have discovered has generally confirmed Milankovich’s theory: orbital cycles do act as a trigger for the Ice Age glacial cycles. But the advances in technology have also revealed that the mechanisms by which the orbital features trigger these cycles are fiendishly complex, involving multiple systems and feedback loops. The devil, as they say, is in the details. Science has only recently begun to unravel these mysteries in all their complexity. 

Taking my cue from the great American philosopher, New York Yankees’ catcher and manager, Yogi Berra, this article is divided in to two sections. The first section introduces the subject, provides background and reviews the Milankovich Theory, its history and formulation. In the second section, I have selected several topics to try to convey some sense of the complexities and scope of the search to unravel the relationship between Earth’s orbital mechanics, its climate and the life of our planet.

In Theory: Ancient Climates

Simplified chart showing when the five major ice ages occurred in the past 2.4 billion years of Earth’s history. Modified from several sources including Dynamical Paleoclimatology: Generalized Theory of Global Climate Change, 2002, by Barry Saltzman.

Scientists believe that for most of Earth’s history its climate has oscillated, between cooler and warmer periods, lasting millions or tens of millions of years. Some have suggested that during one such extended cold period, around 450 million years ago, the entire Earth was covered in ice and glaciers, the so called “Snow Ball Earth,” think the ice planet Hoth in Star Wars. There are many reasons for these swings early in Earth’s remote past: extensive large scale volcanism, tectonic plate movement, amount and location of land masses and oceans, and long term variations in solar radiation. Within the long cooler Ice Age periods (millions of years) are even colder glacial epochs, when temperatures have plummeted to the point that substantial portions of the Earth were covered in glaciers. Below is a chart providing an overview of what scientists now believe to be Earth’s general, ancient climate history. Note that for most of Earth’s history, the average temperatures have been much higher than today. There are multiple factors for this, many related to tectonics, planet-wide volcanism, and amount of CO2 these processes put in the atmosphere.

This began to change around 55-50 million years ago, during the period science terms the Eocene — at the height of what science calls the “Eocene Optimum.” This condensed chart tracks temperature over time with the temperatures during period 1960 to1990 as the average benchmark.

 

As our planet “aged” it began to “quiet down” relatively (don’t we all). Beginning around 55 million years ago, the amount of CO2 present in the atmosphere began decreasing as the levels of planet-wide volcanism in various forms moderated. Average global temperature went down and stayed down.

As this happened, the average planet-wide temperature also moderated, downward. For thirty million years, periods of glaciation were followed by warming. Antarctica acquired its first snow cap during this period. The “Benthic 18O” in the chart, refers to a particular type or isotope of Oxygen in sea water which is temperature sensitive — more on this later — and so can be used as a type of thermometer.

 

Then, at about 3.5 million years ago, at the beginning of the what science calls Pleistocene Epoch of the Quaternary Age, with Earth’s surface close to its current configuration, average temperatures nose-dived. Earth entered a long cold period, Ice Age, punctuated by cycles of even greater temperature drops and wide spread glaciations, glacial epochs. These epochs were followed by melting and slightly warmer interglacial periods within the Ice Age epoch, followed by another glacial period. Scientists generally recognize eight such glacial epochs during the Pleistocene Ice Age, the last ending approximately 11,000 years ago. We are currently 10,000 years into an Interglacial Period of this same Ice Age on our way to the next glacial epoch. The Pleistocene, the last three and a half millions years, has been described as a series of glacial epochs interrupted by short, interglacial, warming periods. The “Vostok” at the left of the chart refers to ice cores taken at Vostok Research Station in Antarctica.

And when scientists looked more closely at the record they discovered that the glacial/interglacial periods of the Pleistocene cycles have been generally shorter in length of time than previous cycles, exhibiting a degree of regularity and time scale not seen before.

For our purposes, what is important in the graphs is not the temperature values themselves. What is important is that they demonstrate the presence of a recurring cycle in temperature fluctuations. It was towards the end of the 19th Century that scientists first discovered and began searching for some natural cyclical phenomena that could be driving these climate and temperature swings. They began focusing on the cyclic changes in the amount of solar energy Earth receives as a cause of the climate swings, changes linked to certain features of Earth’s orbital mechanics. The Pleistocene cycles are of special interest to us because the Pleistocene saw the development, rise and spread of a small bipedal creature that became homo sapiens. Scientists began seeing the patterns in the middle decades of the 19th Century and that is where the search for a cause begins.

Next: The Beginning of the Search