Look at Africa from space, and you’ll notice a belt of dark green across the continent’s midsection. A friend studying a poster of Earth once asked me what caused that “shadow.” It’s vegetation, of course, and what makes it stand out is the contrast with the continent’s arid regions, most notably the Sahara Desert. Scientists at the University of Kiel, Germany, however, have shown that three times within the past 120,000 years, the Sahara region was covered in grassland, lakes, and ponds (see The Green Sahara, A Desert in Bloom. Those wetter episodes correspond to the changing orientation of the Earth’s rotational axis, which periodically increases the solar radiation falling on the tropical Atlantic, boosting the moisture in the air while also pushing the African monsoon farther north into the Sahara.
The Sahara’s past is one of the keys to understanding how our climate can change rapidly. As a geologist, I love timelines; for me they are the only way I can visualize the order of historical events. The record of the region’s “green” periods is well illustrated at Dienekes’ Anthropology Blog, where a simple chart based on the new research gives you 120,000 years of the Sahara’s rainfall history at a glance. But how did these green periods look on a map, the geologist’s other visualization aid? For that I stumbled across a site on Lake Megachad, a paleolake that reached its maximum extent some 7,000 years ago. Today’s Lake Chad (Africa’s second largest wetland) is all that remains of the body that once exceeded the size of the Caspian Sea, the largest lake (by area or volume) in the world today (note: the Caspian is not a freshwater lake). Some of the ancient rivers and lakes are often easy to pick out in satellite images. An excellent example of how archaeologists are using these long-gone waterways to guide their search for prehistoric settlements can be found at The Geoarchaeology of Western Sahara. Subtitled “Preliminary results of the first Anglo-Italian expedition in the liberated zone,” it reports on a 2002 exploration of Western Sahara, a sparsely populated, contested territory south of Morocco.
To see how changes in Earth’s orbit could add enough heat to the tropical Atlantic to produce the Sahara’s green periods, go to Orbital Fluctuations, a page at the Department of Meteorology at Lyndon State College in Vermont. The Flash animation shows how the three cycles—eccentricity, precession, and obliquity—combine to alter the amount of sunlight falling on any given region. After quite a bit of searching, I found this Graph of Insolation at a site on the ice ages at the Illinois State Museum. It shows the irregular curve produced by adding and subtracting the effects of the three cycles. In this case, it shows the amount of sunlight falling in June at latitude 65 degrees north. For more on this subject, look at Peter Huyber’s page at the Department of Earth and Planetary Sciences at Harvard University. Scroll down to “Downloads” and click on “Insolation plots, movies, and tables.”
The National Climatic Data Center’s End of the African Humid Period explains how the Sahara expanded rapidly 5,500 years ago, when the African monsoons weakened. This page is part of a section on abrupt climate changes that have occurred in distant past. Other examples (which you can access by clicking on “The Data” in the menu on the left) include the collapse of the Classic Maya civilization and the demise of the world’s first empire, the Akkadian, in what is now Iraq and Syria.
Archaeological discoveries in the Sahara (in addition to those mentioned above from Western Sahara) are revealing the lives of the people who thrived during the wetter climate. Paul Sereno and collaborators recently published their work on a Neolithic graveyard in the Sahara. Anthropology.net has a good summary of the find entitled, “The Kiffian & Tenerean Occupation of Gobero, Niger: Perhaps the Largest Collection of Early-Mid Holocene People in Africa”. In 2000, Mike Hettwer, a photographer for National Geographic, stumbled upon the graveyard during an expedition in search of dinosaur and giant crocodile remains. The burial site has some 182 individuals, but the most remarkable discovery is that they represent two different cultures, one inhabiting the area between 7700 and 6200 b.c. and the other between 5200 and 2500 b.c. Paul Sereno and his team, famous for uncovering new dinosaur species, did a superb job of preserving and documenting this important prehistoric burial site. Their detailed report is linked in the article above, or you can access it directly at “Lakeside Cemeteries in the Sahara: 5000 Years of Holocene Population and Environmental Change”. Related finds, made in the late 1990s in Niger, are the beautiful stone engravings of giraffes, some of the largest prehistoric artworks discovered. At the 153 Club (a club for Sahara Desert travelers named for the old Michelin 153 map of northwestern Africa), I found this entry with photos: “New” Giraffe Engravings Found.
Dust: You cannot talk about the Sahara’s influence on global climate without talking about it. Each year, 60 million to 200 million tons of Africa’s mineral dust are carried high into the atmosphere by hot, rising air currents. Propelled westward by powerful trade winds, some of it reaches the Amazon, fertilizing the rainforest with much-needed nutrients. Go to the International Geosphere-Biosphere Programme and read “The Sahara Desert and Amazon Basin—‘Achilles heels’ in Earth’s Armour.” Courtesy of EUMETSAT, a European consortium charged with monitoring the weather and climate from space, you can see an animation of “Dust Wind Tracers over the Central Sahara (5 January 2005)” blowing across northern Chad from the vantage point of a satellite. At the SeaWiFS site there is a NASA animation of a Sandstorm the Size of Spain blowing across the Atlantic A NASA image of a Dust Cloud over the Western Sahara Desert shows a massive dust storm blowing from Algeria to Western Sahara.
But what happens if the desert greens again, perhaps owing to warmer oceans, and the dust diminishes? In 2002, NewScientist magazine ran an article by Fred Pearce, “Africa’s Deserts Are in ‘Spectacular’ Retreat,” which suggests a greening of sorts may be underway. Of further concern is a NASA finding that African dust reflects sunlight into space, which cools the planet; see the article “Sahara Dust Storms & Sea Surface Temperatures: Dust Is Responsible for Lower Atlantic Temperatures.” This implies that less dust from a greener Sahara would lead to further warming.
To end on a positive note: I found that some Europeans are angling to generate much of their electric power from the Sahara winds, some of the most consistent gales on the planet. For a look at the ambitious plans for wind farms and transmission lines to bring green power to Europe go to North Atlantic Trade Winds. Someday, nearly half the Europe’s electricity could be generated along the western edge of the Sahara, cutting greenhouse emissions drastically to help “green” the whole planet.