The Sahara is “Greening” Again Temporarily: This Can Have Some Local Benefits Such as Reducing Desertification, but Catastrophic Flooding is Also Occurring

 

     When I was an undergrad, I wrote a paper for a geography class on desertification along the Sahara which is basically the spreading of the desert. The problem has diminished a little in recent years, due in part to better land use management. Human activities such as agriculture and especially livestock grazing, deforestation, natural climate variation, and natural processes like wind erosion are driving the spread. It is often difficult to tell how much each is influencing it. Paleoclimatology reveals that the Sahara Desert region was last green sometime between 5000 and 11,000 years ago after the last ice age subsided. Evidence suggests that the greening of the Sahara occurs in cycles every 23,000 years and it is related to the wobble in Earth’s rotation which changes the angle relative to the sun from 23.5 to 24.1 degrees and back again over that period. This is one of the Milankovitch cycles. It is likely that the other cycles also have an effect since they too influence solar radiation. Thus, it is likely a combined effect.

     Evidence from ocean sediments suggests that the current greening and browning cycle has been occurring for more than 5 million years. The desert formed 7 million years ago when the Tethys Sea was closed off due to plate tectonics. The next greening is expected around 10,000 years from now. What scientists think happens is that the change in angle results in changes in solar radiation which trigger the winds to shift and bring the African monsoons into the desert in a new weather pattern. That is what one would call natural climate change. A huge wildcard in the potential for greening is anthropogenic climate change. Could it also trigger a weather shift that brings the monsoons into the desert? It seems it can at least do it on occasion as is appearing now in 2024. It is likely that this is still just a natural fluctuation within a natural climate cycle and perhaps it is being influenced to some degree by anthropogenic climate change. According to a Live Science article from 2020:

 

“The Green Sahara, also known as the African Humid Period, was caused by the Earth's constantly changing orbital rotation around its axis, a pattern that repeats itself every 23,000 years, according to Kathleen Johnson, an associate professor of Earth systems at the University of California Irvine.

 

“The termination of the Green Sahara took only 200 years, Johnson said. The change in solar radiation was gradual, but the landscape changed suddenly. "It's an example of abrupt climate change on a scale humans would notice," she said.

 

     The Sahara Desert as a desert does provide tangible ecosystem services such as delivering sand by air that aids the fertility of the Amazon region and providing sand for Caribbean beaches. It delivers more wind-blown sand around the world than any other desert. It can also be a powerful air pollutant when concentrated.

     There is still uncertainty about all the influences on the Sahara so other periods of temporary greening are possible. Any changes from anthropogenic climate change would likely be less intense than orbital changes and temporary as well.

     Interestingly, the article suggests a possible action to increase the likelihood of an induced greening by deploying solar panels and wind turbines, clearly acknowledging that deploying them influences local weather patterns and is a possible way to ‘geoengineer’ the local climate. The wind and solar farms would increase the local humidity which could increase rainfall and vegetation growth in a positive feedback loop. The idea has not been tested. A more successful project has been reforestation and soil conservation. 

     The Great Green Wall project was launched in 2007 to make a green corridor across the Sahel region which borders the Sahara on the south. The project has several goals, one of which is to slow desertification. 

Map of the Great Green Wall Project

     Despite the current monsoon shift, desertification remains the dominant issue by far. The map below shows the drying up of Lake Chad from 1972 to 2007.

 

 

 

The Current Greening Event and Associated Catastrophic Flooding

  

     An article in the Washington Post explains the current greening event and the weather pattern that is driving it:

“In North Africa, some of the driest places on Earth have seen five times their average September rainfall. Flooding has affected more than 4 million people in 14 countries, according to the U.N. World Food Program. Heavy rain and floods have killed or displaced thousands and disrupted farming activities in areas where there already isn’t enough food for the population.”

“A northward shift in the region of clouds and rain that circles Earth near the equator is responsible for the flooding and greening. In this area, called the Intertropical Convergence Zone (ITCZ), Southern Hemisphere winds blowing from the southeast converge with Northern Hemisphere winds blowing from the northeast. The combination of converging winds, strong sun and warm ocean water leads to rising, moist air and constant clouds, showers and thunderstorms.”

“The movement of the ITCZ north and south of the equator during the year is primarily driven by the difference in temperature between the Northern and Southern hemispheres. It drifts toward the warmer hemisphere, which means it resides north of the equator during the Northern Hemisphere summer, usually reaching its northernmost point in August or September.”

     While this event is catastrophic in terms of flooding, it will likely be quite temporary. Changes in precipitation patterns and greening comparisons of 2023 and 2024 are shown below.

     The transition of El Nino to La Nina weather patterns in the Pacific is also thought to be an influential factor in this year’s event as well as the influence from anthropogenic climate change. The change in weather patterns in Africa has also made a few areas drier than expected by shifting that rainfall to the north into the southern Sahara.

References:

An unusual shift in the weather has turned the Sahara green. Mary Gilbert, CNN Meteorologist. September 13, 2024. The Sahara Desert is greening as unusual storms hit parts of Africa | CNN

Why parts of the Sahara desert are turning green this month. Dan Stillman and Ian Livingston. The Washington Post. September 19, 2024. Why parts of the Sahara desert are turning green this month (msn.com)

Could the Sahara Ever Be Green Again. Donavyn Coffey. Live Science. UCI School of Physical Sciences, September 27, 2020. Could the Sahara ever be green again? | UCI

Desertification in Africa. Wikipedia. Desertification in Africa - Wikipedia

Is the Sahara Desert Growing? Owen Mulhern. Earth.Org. February 17, 2021. Is the Sahara Desert Growing? | Earth.Org