Earth’s Orbit Influenced Human Migration in Africa in Glacial Periods. 

Models by John Kutzbach suggest this. 

Earth’s Orbit Influenced Human Migration in Africa in Glacial Periods. 
Earth’s Orbit Influenced Human Migration
Earth’s Orbit Influenced Human Migration in Africa in Glacial Periods. 

Earth’s Orbit Influenced Human Migration in Africa in Glacial Periods. 


John Kutzbach was a college graduate in 1961. After that, he joined the U.S. Air Force as an aviation weather forecaster and started working in a station in France. He explored many places there. Some places like Dordogne’s storied caves and prehistoric painted caves from Lascoux attracted him. 

He wondered how these caves became the choice of the ancient people and the animals in primitive times for warmth and shelter. To find the answer, Kutzbach became interested in glaciology. Now, he is an emeritus but he served as a professor of atmospheric and oceanic sciences at the University of Wisconsin-Madison and the Nelson Institute for Environmental Studies before retirement.  He says, "It was interesting to me, as a weather person, that people would live so close to an ice sheet."

His very interest paved him the way to choose a career studying how Earth’s movements affect its climate and how climate change then affects human migration especially out of Africa. He emphasized on Earth’s orbit, its axis’ tilts as movements from the space. Other important factors are- ice cover, greenhouse gases, etc. He revealed Ice Age cave art many years ago. Now, he wants to understand how climate changes affect the migration rate out of Africa. 

Kutzbach and his team have been working on human dispersal 140,000 years ago. For this, they took Africa, Arabia, and the Mediterranean to find out changes in climate and vegetation. The National Academy of Sciences published the team’s report recently. 

In this study, they highlight a model on climate and vegetation. This model is quite a dynamic one explaining plant composition changes throughout  Africa, areas of the Middle East, and the Mediterranean. The places were drier and wetter making the composition tandem and finally opening migration corridors at different times throughout history. 

Kutzbach says, "We don't really know why people move, but if the presence of more vegetation is helpful, these are the times that would have been advantageous to them."

The new model relates all the above-mentioned factors from climate, orbits, the concentration of greenhouse gas, and its ice sheets. 

For understanding, we have to go back 125,000 years ago. This model shows an instance of the summer monsoon rainfall increased and reached northward in northern Africa and the Arabian Peninsula. This rainfall increased grassland in the Saharan and Arabian deserts. The increased amount of winter storm-related rainfall was also evident in the Mediterranean and the Levant.

The positioning of the Sun and the Earth plays a vital role has impacts on these changes. At that time, the Northern Hemisphere was the closest to the sun during summer. So, it is to farthest during the winter. For this reason, summer wae hotter and winter was colder.   

Kutzbach says, "It's like two hands meeting. There were stronger summer rains in the Sahara and stronger winter rains in the Mediterranean."

The given movements of the Earth’s orbitals are called Milankovitch cycles and they appear in the same way every 21,000 years. It takes around 10,000 years every time for the Northern Hemisphere to stay to the farthest position from the sun in summer and closest in winter.  

The model also shows fluctuation levels in rainfall and vegetation in many time limits. The instances of increasing are evident in respectively 125,000, at 105,000, and at 83,000 years ago. While the incidents of decrease became notable in 115,000, at 95,000 and at 73,000 years ago.   

From 70,000 and 15,000 years ago, the Earth had a glacial period. The model clarifies us of the presence of ice sheets then. The greenhouse reduced at that time and caused winter Mediterranean storms with an increased level. At the same time, it limited the summer monsoon to retreat. As the greenhouse gas was available in a decreased amount, many equator areas cooled. This damaged plant’s life cycle and many forests lost their past glory. 

The changing patterns of the regions due to climate and vegetation had an impact on humans living in Africa. For a better life, they migrated one place to another in search of water and plant life.  

Besides Kutzbach, many other researchers joined the team to complete the study. Ian Orland and Feng He are his colleagues from UW-Madison. The team also had experts from Peking University and the University of Arizona. The National Center for Atmospheric Research has developed a method named Community Climate System Model version 3. The research team used this. They ran simulations to see the nature of the orbitals. Related orbital and greenhouse gas changes and found out their influence on each other. Finally, they focused on the overall influence of all of these on ice sheets.  

In the time span of the 1970s and 1980s, Kutzbach was the first to prove that Earth’s orbit strengthens summer monsoons. It is directly related to how much heat the Earth gets from the sun and which part gets the most heat. 

Kutzbach confirmed the world of periodic strong monsoons in Africa forty years ago. The specific part at that time was Africa with the warmest summer and thus with the strongest monsoons. He says, "My early work prepared me to think about this.”


This present model by Kutzbach mostly agrees with the earlier ones. The earlier studies had enough data combining old lake beds, cave features, pollen records, and marine sediments. 

A recent study led by Orland used cave records in the Levant to show that summer monsoons reached into the region around 125,000 years ago.

Kutzbach says, "We get some things wrong (in the model)." For this reason, the team continued working to improve the model. As an example, we can see that southern Europe very cold during the glacial period. But, the model fails to highlight this.

Kutzbach says, "This is by no means the last word. The results should be looked at again with an even higher-resolution model.”