Paleoclimatology 101 - Milankovitch Ice Age Theory

Figure One - Ice Age Man. Courtesy of 
Manhattan's Museum of Natural History.  

During the Pleistocene, massive sheets of ice flowed southward across Canada into the northern United States about seventeen times. These ice events lasted for approximately 1.65 million years. 

Imagine what Canada and other northern hemisphere countries were like with a mile or so of ice on top of the land for thousands of years. The ice sheets were heavy enough to push the Earth's crust down approximately 1000 feet. Where did all the water for the ice come from. Answer: the oceans! Sea levels dropped substantially! 

Do we know what caused these Ice Ages? 

Let me present one plausible theory.  

Figure Two - An estimate of the depth of ice in meters
 from the Wisconsin glaciation around 21,000 years ago. 

During the last Ice Age, ice sheets spread and shrank roughly on a 100,000 year-long-cycle. Glaciers dominated the land from 60,000 to 90,000 years during the cold phase of the cycle, and then mostly disappeared for 10,000 to 40,000 years during the warm phase of the cycle (Bonnicksen 2000; p. 5). 

The Wisconsin glaciation started about 100,000 years ago in North America and ice sheets reached maximum thickness around 18,000 years ago. The warming trend began around 17,000 years ago and the ice sheets started to melt, and there was a lot of ice to melt! 

Figure Two represents an estimate of how thick the ice sheets were in meters in four future North American cities. It took 11,000 years for Canada to completely thaw out, right around 6,000 years ago. Since then, Earth has had an interglacial climate. 

 

Figure Three - CLICK to BUY the ENTIRE SERIES

Several factors influence climate; the sun’s energy output, carbon dioxide levels, and ocean currents. All three factors are important, but the single most important factor in driving our climate is called insolation! Insolation is the amount of solar energy that reaches the Earth from the Sun. If you don’t believe that the sun has much influence on the climate, try living in Alaska in the middle of January. 

Researchers have found that the sun’s output varies and that the amount of sunlight that reaches certain parts of the globe is affected by how the Earth orbits the sun. A Serbian astronomer-

Figure Four - Milutin
Milankovitch 

mathematician by the name of Milutin Milankovitch (1879-1958) hypothesized that past glacial cycles correlated to cyclical changes in insolation and that the Earth’s circumnavigation around the sun was the main cause of the Earth's cyclical changes in insolation. Milankovitch and others claimed that the Earth’s orbital path had a huge impact on past global cooling and warming cycles. Milankovitch tested his theory against temperature data from the paleoclimate records and proposed a 100,000 year-cycle between ice sheets. He claimed that the ice sheets were not created by dramatic changes in the amount of insolation reaching Earth, but how the solar energy was distributed on Earth. He identified three circumnavigation cycles that were responsible for the Ice Ages: 1). eccentricity, 2). axial tilt or obliquity, and 3). wobble or precession.

Eccentricity. Milankovitch defined eccentricity as the shape of the Earth’s orbit around the sun. I always assumed that the Earth rotates around the sun in a circular orbit, but due to the gravitational pull from other planets, the Earth does not orbit the sun in a perfect circle. The Earth has an elliptical orbit around the sun that varies from five percent to zero percent ellipticity over a 100,000 year-long-cycle (Figure Five). The elliptical orbit of the Earth reduces or increases solar radiation during the various seasons. When the Earth is in its most elliptical orbit, it receives twenty to thirty percent more solar energy at its perihelion (when Earth is closest to the sun) than at its aphelion (when Earth is farthest from the sun). Currently, Earth is in its interglacial cycle and its eccentricity is at a minimum.

Figure Five - Eccentricity. The Earth moves in a slightly elliptical
path during its annual revolution around the Sun.  

Axial tilt or Obliquity. The second circumnavigational cycle proposed by Milankovitch was axial tilt or the inclination of the Earth’s spinning axis in relation to its orbital plane around the sun. 

When I first read that definition, I begged the author to "speak English, please." It is a hard concept to visualize, but it makes sense once it is grasped.  

Earth orbits the sun at a different angle than the angle the Earth rotates around on its own axis (Figure Six). Earth’s rotational axis is currently at an angle of 23.4 degrees from its orbital plane around the sun. Milankovitch calculated that the Earth’s rotational axis and its orbital plane around the sun vary from 21.5 to 24.5 degrees over a 41,000-year-long-cycle. Axial tilt or obliquity is what creates our seasons. When there is less of an axial tilt, the Sun’s solar radiation is better distributed between summer and winter with increased differences in radiation between equatorial and polar regions. Milankovitch's hypothesis claimed that a smaller axial tilt angle promotes the growth of ice sheets because warmer winters hold more moisture and produce more snowfall while cooler summer temperatures cause less ice melt. Under these conditions, ice sheets can grow from year to year to year, etc.!    

 

Figure Six - Axial Tilt or Obliquity. The Earth is tilted on its rotational axis
23.4 degrees from a plane perpendicular to the surface over
which moves during its revolution around the Sun.  

Precession or Wobble is the third circumnavigational cycle. Some years ago, scientists proposed that the Earth’s axis wobbled due to lunisolar forces, changing the orientation of the rotational axis of the Earth. This wobble as Earth spins on its axis is very slow and is on an approximately 23,000-year-long cycle (Figure Seven). The Earth wobbles enough to change from pointing at the North Star (Polaris) to pointing at another star called Vega over time.

Figure Seven - Precession or wobble. The effect of the wobble is to
systematically change the timing of the solstices and equinoxes
relative to the extreme positions the Earth occupies on
its elliptical path around the Sun.  

When the Earth’s axis points at Vega, the Northern Hemisphere winter and summer solstices coincide with aphelion (when Earth is farthest from the sun) and perihelion (when Earth is nearest to the sun), respectively. Winter occurs when Earth is farthest from the sun and summer occurs when Earth is nearest the sun, leading to the greatest seasonal contrast. In the Northern Hemisphere, winter will end up in July and August, and summer in January and February during the 23,000-year-long cycle. This happens because axial tilt or obliquity still accounts for the seasons; summer when that hemisphere leans toward the sun and winter when that hemisphere leans away from the sun. 

Does your head hurt as much as mine did when I first studied this theory? 

You might be asked what is the bottom line? See below.  

Milutin Milankovitch suggested that the right combination at the right time of these three circumnavigational cycles is conducive to glaciation. The first condition is 1). minimal axial tilt or obliquity. Changes in axial tilt have very little effect from solar radiation at lower latitudes but increase the effect toward the poles. As axial tilt increases, summer radiation increases significantly. Therefore, minimal axial tilt is conducive to ice sheet buildup. The second condition is 2). high eccentricity. Eccentricity variations affect the intensity of the seasons because it alters the distance the Earth is from the sun. The third condition is the 3). Northern Hemisphere summer should coincide with an aphelion (when Earth is farthest from the sun) which creates cooler summers which translate to less melting of existing ice sheets. 

When all three conditions converge, we have what is often referred to as a "cold orbit" and the chances are high that ice sheets will expand! Notice, I never once mentioned, "man-made climate change". 

       

2000    Bonnicksen, Thomas M. America’s Ancient Forests from the Ice Age to the Age of Discovery. John Wiley and Sons. New York.

2015    Bradley, Raymond S.  Paleoclimatology - Reconstructing Climates of the Quaternary. Third Edition. Elsevier Publishing. New York.   

The historical fiction novels written by John Bradford Branney are known for their impeccable research and biting realism. In his latest blockbuster novel BEYOND the CAMPFIRE, Branney catapults his readers back to the Late Pleistocene where they reunite with some familiar faces from Branney’s best-selling prehistoric adventure series the SHADOWS on the TRAIL Pentalogy. BEYOND the CAMPFIRE is the eleventh published book by Branney.

Author Branney earned a geology degree from the University of Wyoming and an MBA from the University of Colorado. He lives in the Colorado Mountains with his family.