This is final article in a six-part series by RCA Member David Horne.
The Milankovitch theory identified three aspects of Earth’s orbital motion as causes of the Ice Age glacial periods and the cyclical long term climate change: (1) the changing shape of Earth’s orbit from more circular to more elliptical; (2) cyclical changes in the “tilt” of Earth’s axis of rotation (axial tilt and precession) which affect the amount of solar radiation received by each hemisphere; and (3) the first two aspects acting together to determine where along Earth’s orbit maximum and minimum levels of solar radiation are received.
These orbital cycles have always affected Earth’s climate by controlling the amount of solar radiation reaching Earth. Their impact at Earth’s surface has been affected by other factors including: the size, amount and location of continents, oceans, and mountain ranges, and the extent and location of volcanism. As the young Earth aged, planet-wide volcanism decreased.
Gradually the continents and oceans assumed their present relationship. As this happened, the role of Earth’s orbital cycles in determining Earth’s long-term climate cycles became more important. The impression of these cycles on our planet can be seen in the world around us.
Oregon’s Painted Hills of the John Day and Clarno Formation
Anyone who has attended a Rose City Astronomers’ star party at Camp Hancock or traveled to the Clarno Section or Painted Hills of the John Day Fossil Beds National Monument will remember marveling at the beautiful colored bands of soil in the hills. When you look at the bands of color, you are looking at the impression of Earth’s orbital cycles on our planet.
Eastern Oregon has seen a cycling of different climates over the last millions of years from dry grasslands through open forest and hardwood forests to wet humid conditions all determined by the Earth’s orbital cycles. However, the clarity of the record preserved in the Clarno and John Day formation is unique. Oregon’s present and past volcanic activity results from the movement of plates in the Earth’s crust.
Off the coast of Oregon, the plates of the Pacific dive beneath the lighter crustal plate of the continent, today this is termed the Cascadia subduction zone, and it produces our earthquakes and volcanic activity. As the ocean crust containing a high water content melts, a line of volcanoes develops inland. Today, the line of volcanoes forms the Cascades. Fifty-five million years ago this zone and the coast line was much further east.
The area of volcanic activity was much further east in the Clarno and John Day areas of east central Oregon. The eruptions of ash and hot mud flows continued in the areas from about 20 million to 6 million years ago covering the plants and animals in layers of ash and mud. The same volcanic processes later resulted in enormous flows of basalt which have covered large areas of the northern half of the state all the way to the present coast, but not the Clarno and John Day areas. Their layers remain preserved and uncovered to us.
The type of plants and animals living when covered by the ash reflected the climate at the time, including the amount of moisture present and the minerals in the plants and soil. Scientists use a variety of techniques to analyze the fossil remains of the plants and animals and the ancient soils or paleosols. The black layers of soil we now see are termed “lignite” and formed out of plants that grew along a flood plain. The grey layers are mudstone, shale, and siltstone formed in water. The red layers, rich in iron, are laterite soils formed from the vegetation floor when the area was warm and humid.
The scientists’ analysis found that the paleosol of the John Day formation recorded the Milankovitch climate cycles. In the layering, they detected the dominant 41,000-year tilt cycle of Earth’s axis and the combined 100,000-year obliquity cycle. They also found some variations that are believed to reflect local conditions such as volcanism, mountain building and local geography. However, the underlying signals from Earth’s orbital cycles are clear. When you look at the photo, you are looking at Earth’s orbital cycles.
The Human Pulse
Scientists now believe the development and spread of the human species was not gradual over time but “pulsed” in cycles that are linked to the climate cycles of Earth’s orbital features, the Milankovitch Cycles. The development is referred to as the Orbital Monsoon Hypothesis, and the concept of a “pulsed” dispersion of humankind is popularly called the Green Africa Theory.
The human species arose in a rift valley in northern east Africa around 4 million years ago. The valley represents the splitting of two crustal plates, the Somali Plate and the Nubian plate separating at a rate of 6 to 7 mm annually. Eventually, it will split just like the Arabian Plate directly north of it. Over the last 10 million years the changes turned the area from a relatively flat forested land into a long valley with bordering hills and low mountains, heavy vegetation, and a string of lakes that repeatedly dried up and reappeared timed to the Milankovitch Climate cycles. Humankind arose in these lake valleys.
A little over 4 million years ago, in the Rift’s wet and forested valleys, a small creature between 4 and 5 feet tall developed. The ancestors of this creature, a hominid, had split from the last common ape/human ancestor to its own line perhaps 3 million years earlier, according to recent genetic studies. These creatures stood erect and walked upright as we do. Mary Leakey, the well-known anthropologist, discovered the footprints of these small bipedal creatures, hominids, fossilized in the mudstone at Laetoli in Tanzania, east of Lake Turkana. The area of the foot prints now looks much like the high scrub desert of eastern Oregon. But it was wet grassland and forest at the time the prints were made in the mud. The prints were dated to 3.7 million years and are believed to have been made by a group of hominids that included a child.
Scientists believe that the Milankovitch Orbital Cycles caused alternating wet and dry conditions that filled and then dried the lakes in the valley during the first 2 million years of human development. They believe that these cyclical climate and environment stresses presented developmental challenges to these small creatures. Scientists now theorize that these cyclical stresses, caused by the Milankovitch cycles, drove the physical and mental development of these creatures, hominids, ending with modern humans, Homo sapiens.
The climate shifts from wet to dry were initially rapid and extreme fluctuating in 20,000-year cycles later moving to 41,000 year cycles. During the wet periods, the lakes filled. During the dry periods, the early hominids were forced to move to wetter climates. Scientists believe that these climate-caused challenges presented the need for these creatures to develop the physical and mental skills and tool kit necessary to survive the changing conditions. The changes caused by these cycles would eventually permit the creatures to leave their mountain valleys and lakes and spread first in Africa, then Europe, Asia and the Middle East and finally the rest of the world.
Measurements of the amount of vegetation present in the soil or “soil carb” show that, during the period from 4 ½ million to 3 million years ago, the 21,000-year Milankovitch cycle dominated. From 3 million to 1 million years ago, the shorter 41,000-year cycle was predominant. For the last 1 million years the longer 100,000-year cycle has been dominant. Most development occurred when the shorter cycles were dominant. With a larger tool kit and better physical and mental abilities, the creature was ready to move out of the lakes of the Rift Valley, provided there were opportunities for food and shelter. Again, the Milankovitch Cycles provided the opportunities.
The climate of northern Africa is controlled by the location of the moisture from the monsoon cycles. Its location is determined by the Milankovitch Orbital Cycles. So, their migration out of their forested valleys was not steady, but instead pulsed in time to the Milankovitch Orbital Cycles. This climate pulse was responsible for the wet/dry periods of the valley lakes. It was also responsible for the spread of our human ancestors out of east Africa to the rest of Africa and then Europe, Asia and the Middle East. This climate pulse is referred to as the Orbital Monsoon Hypothesis.
The African “monsoon” is a seasonal temperature and moisture circulation pattern. In the summer cooler, moist area flows from the ocean to the warmer land dropping its moisture. The flow in winter is the reverse. The flow occurs because land heats faster than water (summer), but water retains its heat longer than land (winter). The first chart illustrates the flow patterns. Here is a chart of the modern weather and wind flow patterns for ‘northern Africa.
When Saharan desertification occurred the last time, around 5,000 B.C.E, it happened very rapidly in perhaps a generation. But for long periods, the area was green. The belt of moisture and vegetation has cycled north and south in response to the changes in the Monsoon caused by the Milankovitch Cycles. During some periods of the cycles, the belt of moisture is forced so far north that the Sahara Desert becomes open plane and grassland filled with vegetation, game and rivers. During other periods, the line moves south and the area returns to impassible desert.
The African climate record has been reconstructed from the sediment layers off the African coast and these records show a succession of wet-dry cycles with a long-term shift towards a dry climate. The cycles show the characteristic period and frequency of the various Milankovitch Cycles: prior to 2.8 million years ago, the 21,000-year cycle dominated, then the longer period 41,000-year cycle until 1.7 million years ago when the cycle shifted to the longer and larger 100,000-year cycle. The shifts matched the onset of the spread of the polar ice sheets and the cooling of the subpolar oceans. The result was an alternating wet-dry cycle in Africa. This caused a cyclic change or pulsing in the food sources available to the developing specie. As the climate and food supplies available in new areas pulsed, so too did the migration of our developing human ancestors. Within the last 20 years, new satellite imaging technology has revealed beds of enormous river systems below the sands of the Sahara. These systems flowed during the wet periods and some may have been flowing as recently as 5000 B.C.E. . These rivers, flowing hundreds of miles across northern and northwestern Africa to the Mediterranean and Atlantic, would have provided lifelines for dispersion of early man.
By 1.75 million years ago the hominids that would later become us had dispersed throughout Africa. There are a few isolated examples of hominid fossils found in Asia over the next 500,000 years, but no evidence of permanent presence. It was not until 1.2 million years ago that early versions of modern humans (Homo erectus) began showing up in Asia and parts of Europe. By 250,000 years ago their presence had become permanent. But in Africa, in the rift valley, the species continued to change and develop.
Modern humans began appearing in Africa probably around 200,000 years ago. The numbers are not known, but it is believed that they were few at first and lived in small, isolated bands. The last interglacial began around 128,000 years ago followed by a glacial maximum 70,000 years ago which also ended a mega drought in Africa. By that time, of the other early versions of humans to have made it out of Africa, only Homo neanderthalensis remained. But with the ending of the drought and the start of the next African Humid Period, Homo sapiens moved out of Africa in to Europe, the Middle East and Asia and by 30,000 years ago, only Homo sapiens remained.
The pulses of humans and their ancestors out of Africa were timed to the pulses of the Milankovitch Cycles and the Green Africa periods the cycles made possible. By the time Homo sapiens, us, had populated Europe they had the tool kit and the physical and mental agility and ability to allow them to survive the onset of the next Glacial Period and its glacial maximum 21,000 years ago. In fact, they did more than survive it, they were able to use the new opportunities it presented to complete the migration to another continent. None of this would have happened if Earth’s orbital cycles had not imprinted themselves on a small creature and its ancestors several million years earlier.
Conclusion: The Search for ET
The amount of research and material focusing on the Milankovitch Cycles and their imprint on the planet and humans is amazing. But even to an amateur like myself, the take away is obvious: We are the product of a particular planet, at a particular time in its history, circling a particular star, in a particular solar system. And this has important implications in our search for extraterrestrial life.
In their Astrophysical Journal article, "Generalized Milankovitch Cycles and Long-Term Climatic Habitability," David S. Spiegel, et al., the authors note:
The long-term habitability of a planet depends on the properties of its star, on the properties of its own orbit, spin, and geochemistry, and also on the arrangement of other companion planets. When, in the coming years, we find Earth-sized planets at orbital separations that could be consistent with habitable climates, it will be important to study the architectures of those systems to determine what kinds of Milankovitch-like cycles might govern the long-term climatic habitability of such worlds.
It won’t matter what exact form that life takes. Whatever form it takes, it will have developed and exist under a certain range of conditions, a range of conditions set in significant part by the orbital characteristics of its planet.
A very large thank you to Kathy Kornei for her time and patience in editing these articles and Paul Edison-Lahm for editing and his geology expertise. I hope you have enjoyed the effort.
Clear nights and steady skies!
David Horne, Rose City Astronomers
Suggested Further Readings and Note on Articles
The subject of the Milankovitch Orbital cycles and their impact on the planet, is immense. Below are some sources and suggestions for further reading.
Brian Fagan: Fagan is a New York Times Best Selling Author. I strongly recommend 4 volumes: Cro-Magnon: how the ice age gave birth to modern humans; The Great Warming, The Long Summer; The Little Ice Age.
Doug MacDougall: Professor at the Scripps Institute of Oceanography at U.C. San Diego. His book Frozen Earth: The once and future story of the Ice Ages, is an excellent read.
John Imbrie and Katherine Palmer Imbrie: Ice Ages, reviews the history of the Milankovitch Cycles from the beginning to present. John Imbrie is a world acknowledged expert in the Milankovitch Cycles with much research in the area. The book is very readable.
William F. Ruddman: Earth’s Climate Past and Future is a lower division college text introduction to meteorology. It is non-technical and covers many subjects related to the Milankovitch Cycles and the African climate.
JSTOR: This is a resource available on line with a Multnomah County Library card available to residents of Multnomah and Washington Counties. JSTOR is a data base providing access to professional and scholarly journals and publications. It can be searched much as a Google search
Among the technical articles the following were among the most interesting and “readable”, readable being a relative term.
Late Pleistocene climate drivers of early human migration; Timmermann and Fredrich, Nature, September 2016
Human migration: Climate and the peopling of the world; Demenocal and Stringer, Nature, September 2016
Variations in the Earth’s Orbit: Pacemaker of the Ice Ages; Imbrie and Shackleton; Science, 10 December 1976, vol 194, Number 4270
African climate change and faunal evolution during the Pliocene-Pleistocene; Peter B. deMenocal; Earth and Planetary Science Letters 220 (2004) 3-24
Early Human Speciation, Brain Expansion and Dispersal Influenced by African Climate Pulses; Schultz and Maslin; October 16, 2013 https://doi.org/10.1371/journal.pone.0076750
Note on Articles: Subjects Not Included
Two subjects were not included and are current topics of extensive investigation.
Lag: “Lag” refers to the issue of how quickly the cycles change the climate from one extreme to the other. It is a complex issue and a subject of much investigation. However, there seems to be agreement that it requires a less time for the Earth to heat up than cool down. I understood this to be related in part to the CO2 feedback loop.
Carbon Dioxide and the atmosphere: All palaeoclimatological studies find that with the Milankovitch temperature cycles there is also a cyclical CO2 increase and decrease. The oceans are vast reservoirs of CO2. Heat them up a few degrees and they release some in to the atmosphere – warm soda vs. cold soda. A few degrees is not much, but there is an awful lot of ocean. This process is not casually related to the man-made global warming political dispute. But it is part of the feedback loop that warms the planet. The warmer the oceans, the more CO2 they release, the more they release, the warmer it gets and so on. Recall from the first article that CO2 is transparent to ultraviolet radiation from the sun (the radiation that heats the Earth) but relatively opaque to infrared (the radiation that radiates heat from the Earth back out in to space. Increasing the amount of CO2 atmosphere is like adding a layer of overcast on a hot summer evening, the heat gets trapped. Scientists use many of the same techniques developed to research the Milankovitch Cycles to research the Global Warming issue.