The following is an article from the book Uncle John's Bathroom Reader Plunges Into the Universe.
(Image credit: Flickr user John Spooner)
Or... who ate the cedars of Lebanon?
In the vast Sahara, seas of sand ripple for miles. They rise up in wavelike dunes that can crest at over 1,000 feet. With temperatures that have been known to reach 136°F (58°C), the desolate Sahara covers most of North Africa and over a third of the African continent. It's the world's largest desert, home to over three million square miles of dust, stone, and sand.
(Image credit: Ferdinand Reus)
In the sweltering Sahara, a cool vision of waving grass and shining lakes can seem like a mirage (or the first signs of heat exhaustion), but it's a glimpse into the Earth's astounding past. Hard to believe, but this hostile ocean of sand was once an earthly Eden. Fish swam in ancient lakes; giraffes, elephants, and gazelles roamed grassy savannahs; and hippos wallowed in ponds and mud.
And people had a great time, too. Prehistoric rock painting show ancient Saharans feasting and drinking and -it's true!- swimming. They had steady jobs, first as fishermen and hunters, later as shepherd, cattlemen, and farmers. But that was in the old days, around 5,500 years ago. But 2,000 or so years later (just a wink of the geological eye), the lakes had dried up, the vegetation and animals were gone, and farmers were forced to leave a land where nothing could grow. So what happened?
Over thousands of years, as rainfall lessened and temperatures rose, what little rain there was eroded the soil and dissolved it, leaving grains of silica sand. Wind slammed sand into rocks, wearing those rocks away, which created more sand. Hot days and freezing nights subjected rock outcroppings to extreme temperature changes. This daily expansion and contraction eventually shattered mineral crystals in the rocks so that the cliffs broke into boulders, boulders shattered into pebbles, and pebbles crumbled into sand.
But what caused the climate to change in the first place? Why did drought transform the Sahara from a wet wonderland to a desert badland? Explanations involve everything from the tilt of the Earth to the apetite of the humble goat.
A VENUS AND JUPITER SANDWICH
Up in space and completely unaware of giraffes, hippos, or swimming Saharans, Jupiter's gravity combined with Venus's gravity to influence the tilt of the Earth's axis. Nine thousand years ago, when the Sahara was blooming, Jupiter and Venus kept the Earth's tilt at 24.14 degrees, compared to the 23.45 degrees it is today. The perihelion (when the Earth is closest to the Sun) occurred at the end of July -now it occurs in early January. This means that a very long time ago, the Northern Hemisphere basked in much more summer sunlight, and the summers in Africa were hotter.
Hotter? So what's the problem? Well, ironically, hot summers meant water for North Africa and lots of it. When the African landmass is much hotter than the Atlantic Ocean, the temperature contrast increases the number of summer monsoons that rain on the Sahara. As the Earth incrementally wobbled into its current axis, the summers grew cooler, the African monsoons grew weaker, abd the cool, moist Sahara became hot, dry, and eventually sandy.
NUDE LANDS ARE BAD LANDS
As rainfall decreased, plants died. The death of vegetation in turn increased the drought. Plants hold moisture in the soil and summer sun evaporates the moisture into the atmosphere, where it's converted to rain. Barren land with no plants or trees holds less moisture, so the surrounding atmosphere produces less rain. Plants absorb sunlight, whereas bare land and bright sandy deserts reflect the sun's heat, which leads to fewer clouds and less rain.
The drought cycle picked up momentum until finally the desert took over fertile ground. And remember the rock paintings showing the farmers, shepherds, and cattle drivers? It may be that these folks helped the drought cycle along, turning their heavenly little Eden into the hot hell of the Sahara.
WHO'S TO BLAME?
Some scientists believe that ancient man was the victim of the climate change, but others think he was responsible for it and that poor farming practices, as well as overgrazing and deforestation, contributed to drought, barren land, and the eventual creation of deserts. In fact, a famous scientific study suggests that deforestation along the southern coast of West Africa could cause a complete collapse of the Sahara's dwindling monsoon system.
BETTER GET YOUR GOAT
Even in ancient times the loss of forests was a problem. A famous example involved the great cedars of Labanon, which were nearly decimated not just by lumbermen but also by heards of goats that chomped on the bark and devoured young saplings. And guess what? Even today, widespread exploitation of those same cedars -goats or not- has contributed to increasing deforestation, which leads to soil erosion, which leads to sand ...and lots of it.
SAND MARCHES ON
Today, there's controversy over whether human activity is causing the Sahara's size to increase. Critics of the idea argue that satellite photos show the desert fluctuating in size, but not steadily growing larger. Nevertheless, people living on the edge of the great desert, in places like Niger, Chad, and Mali, complain that the Sahara has steadily encroached on their precious farmland.
THE GREENING OF THE SAHARA
What about turning back the clock? Can more rain and rivers and even lakes return to the Sahara? Can we help nature re-create an Eden in North Africa? Scientists believe that replanting trees and vegetation, as well as conserving groundwater and runoff, could reverse the desert's drought cycle. Some theorists even propose that global warming (if it increases the heat of the African land in summer) will eventually increase the amount of monsoons. With the help of humans, the desert just might bloom again, but since a reversed drought cycle could take a couple of centuries to work, don't hop your camel just yet.
The article above was reprinted with permission from Uncle John's Bathroom Reader Plunges Into the Universe.
Since 1988, the Bathroom Reader Institute had published a series of popular books containing irresistible bits of trivia and obscure yet fascinating facts.
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