Between 6,000 to 9,000 years ago, North Africa went through a humid phase in their climate. This was known as the African Humid period. A German explorer Heinrich Barth discovered paintings from that time depicting a very different African landscape, filled with elephants, antelopes, giraffe, and other wildlife, being pursued by hunters. The incongruence of the paintings with the lifeless landscape intrigued the explorer who noticed the contrast between the lifeless terrain presently in the Sahara to the lively wildlife and terrain depicted in the paintings. Here, below is one painting from the Cave of Swimmers:
Nowadays, scientists have pieced together the fact that these paintings depict climate change over a long period of time. A vegetated terrain filled with countless lakes has been transformed into a nearly arid desert known as the Sahara. The previous vegetated terrain was due to climate changes and periodic variations in the orbit of the Earth around the sun. It occurs roughly every 20,000 to 23,000 years. The last African Humid Period was not the first of its kind, it is just the most recent of hundreds of humid periods that have occurred for the past 9 million years! Previously, the Sahara in the AHP was instead filled with huge lakes, rivers and bodies of water that today do not exist.
Causes of the African Humid Periods
- Orbital Changes
One hypothesis for African Humid Periods are the changes in the Earth’s axial rotation due to the different gravitational interactions of the planet with the moon, sun, and other planets. There are three orbital cycles which are known as the Milankovitch Cycles and they have a huge impact on the climate of the Earth, but more particularly, a huge impact on African climate. They have to do with the obliquity, eccentricity, and precession of the Earth’s orbit.
Of all these three, it is precession which has the greatest impact on African climate. Precession is a wobble in the Earth’s top axis. It greatly influences the climate of North Africa because it controls the strength of the monsoon and how far the monsoonal rainfall will penetrate Northern Africa. When the wobble causes the Earth to pass closest to the sun like it did around six to nine thousand years ago, it increases solar radiation absorbed by the North African landmass by up to 8%. This heats up the land and creates a low-pressure region over the African landmass, which results in a large number of summer monsoonal rainfalls. This increase in rain level nourishes the land and results in more growth of vegetation, and thus greater wildlife.
However, in the winter, this effect would be reversed, thus returning the Sahara back to the arid desert that it was.
- Albedo Feedbacks
Orbital changes alone cannot account for the huge changes in temperature and precipitation during the African Humid Periods. Another reason commonly cited are the Albedo feedbacks. Albedo is the phenomenon of measuring the amount of solar radiation that is reflected by a given landmass. A higher level of reflected light would lead to lower temperatures and a lower level of solar energy reflected would lead to higher temperatures. As the landmass had increased vegetation due to the orbital changes, this would affect the Albedo levels of Northern Africa. Increased vegetations means reduced Albedo feedback as more solar energy is trapped and less reflected back into the atmosphere. Thus, more energy is available for the monsoon season.
This creates a feedback loop. As monsoon levels increase due to reduced Albedo, more vegetation grows and thus albedo levels drop even further. Besides this, dust generation is greatly reduced with a longer monsoon season and more vegetation, which also reduces albedo even further. This feedback loop thus further accentuates the effect of orbital changes.
- Intertropical Convergence Zone Changes
The Intertropical Convergence Zone is the area where the northeast and southeast trade winds converge. This also contains something known as the Monsoon trough which is responsible for the generation of many rainforests around the world. The warmer summers may have resulted in the movement of the ICZ northward into Northern Africa, thus further creating the climate needed for the growth of lush vegetation.
- Change in Precipitation Levels
Another possible reason is the decreased seasonality of the precipitation cycle. This shortens the dry season, and further increases moisture inflow, increasing humidity, and increasing the precipitation over Northern Africa even further, thus further increasing the conditions needed for the African Humid Period to proceed.
Effect on Migration
The rise and end of the African Human Period played a role in the migration patterns of Northern Africa. At the height of the AHP, Northern Africa was filled with nomadic hunter and gatherer communities that practiced pastoralism and raised livestock. However, as the African Humid Period ended, more and more people began to abandon those areas. They did not just disappear, those cultures would live on, as most of them would settle along the banks of the Nile River. This huge exodus was around the same time as we would observe a rise of sedentary life along the Nile River. The dispersal of these populations would also lead to a huge rise in pastoral lifestyle in other parts of Africa.
Was the Sahara Desert impassable before 1500 AD?
Many people believe a myth that the Sahara Desert was impassable before the first white man crossed it. Recent research shows that this may not be the case at all. During the African Humid Period, the Sahara Desert was actually crisscrossed by a dense network of rivers, lakes and also many inland deltas. This was discovered using satellite imagery and digital maps of the landscape. Thus, this large waterway, as it was at the time, was anything but impassable and many humans, and indeed, even many animals would pass through the “impassable” Sahara terrain well before 1500 AD.
Effect on Africans of Climate Change over the last 10,000 years
Climate change over the last 10,000 years has thus had a huge impact on Africa, and indeed on the lives of many Africans. The seasonal changes that occurred during the African Human Periods hugely impacted the ways the hunter gatherers lived during the period, and also impacted the growth of the great Egyptian Civilization, how the people in Northern Africa dispersed, and how people settled across the Nile. It has thus contributed hugely to the history of Africa, the way its people lived and also the growth of one of its great civilizations.
Just like in the past, climate change continues to affect Africa and Africans. Even now, according to the UNFCCC, “Climate change is having a growing impact on the African continent, hitting the most vulnerable hardest, and contributing to food insecurity, population displacement and stress on water resources.” We have seen previously that due to the position of Africa on the globe, changes in the Earth’s atmosphere or its orbit will hugely affect the climate of Africa. Thus, the increasing prospect of climate change is one that needs to be settled as it has the potential to hugely impact Africa.
We also see that Africa has gone through many different, distinct periods due to climate change. The most recent African Humid Period is just one of them, and during that time, the terrain and vegetation of North Africa and particularly the arid Sahara was lush, filled with vegetation and lively.