The end of Africa's golden age
If there was a golden age for humans on Earth—a Garden of Eden that flowed with milk and honey—then it was the high point of the Holocene, the era that followed the end of the last ice age. From around 8,000 to around 5,500 years ago, the world was as warm as it is today, but there appear to have been few strong hurricanes and few disruptive El Ninos; and it was certainly a world in which the regions occupied today by great deserts in Asia, the Americas, and especially Africa were much wetter than they are now. Optimists suggest that such conditions might await us in a greenhouse world. As we shall see, there are celestial reasons why that might not happen. But the Holocene era, and its abrupt end, may still offer important lessons about our future climate in the twenty-first century.
No place on Earth exemplifies the fall from this climatically blessed state better than the Sahara. The world's largest desert was not always so arid. Where seas of sand now shimmer in the sun, there were once vast lakes, swamps, and rivers. Lake Chad, which today covers a paltry few hundred square miles, was then a vast inland sea, dubbed Lake Megachad by scientists. It was the size of France, Spain, Germany, and the UK put together. Today, the lake evaporates in the desert sun; but then, it overflowed its inland basin and, at different times, drained south via Nigeria into the Atlantic Ocean, or east down a vast wadi to the Nile.
The difference is that back then, the Sahara had assured rains. The whole of North Africa was watered by a monsoon system rather like the one that keeps much of Asia wet today. Rain-bearing winds penetrated deep into the interior. From Senegal to the Horn of Africa, and from the shores of the Mediterranean to the threshold of the central African rainforest, vast rivers flowed for thousands of miles. Along their banks were swamps, forests, and verdant bush.
Beneath the Algerian desert, archaeologists have found the sand- choked remains of wadis that once drained some 600 miles from the Ahaggar Mountains into the Mediterranean. And in southern Libya, a region so waterless that even camel trains avoid it, archaeologists are finding the bones of crocodiles and hippos, elephants and antelope. If there was a vestige of true desert at the heart of North Africa, it was very much smaller than the desert is today. And, of course, there were people—shepherds and fishers and hunters—and some of the earliest known fields of grains like sorghum and millet. Archaeologists digging in the sands of northern Chad, currently the dustiest place on Earth, have found human settlements around the shores of the ancient Lake Megachad. Paintings in caves deep in the desert depict the lives of the inhabitants of the verdant Sahara of the Holocene.
There are other remains from this time. Rocks beneath the Sahara contain the largest underground reservoir of freshwater in the world. They were filled mostly by leaking wadis in the early Holocene. Some desert settlements today tap these waters at oases. Colonel Gadhafi has constructed pumps and a huge pipeline network to take this water from beneath southern Libya to his coastal farmers. He calls the network his Great Man-made River, though it is a feeble imitation of the real rivers that once ran here.
The wet Sahara and the era known more generally as the African Humid Period began around 13,000 years ago, as the ice age abated; and, except for the Younger Dryas hiatus, it lasted right through to the end of the golden age. It coincided with a time when Earth's precession ensured that the sun was blazing down on the Sahara with full intensity in summer. The land cooked, and convective air currents were strong. As the warm air rose, wet air was drawn in from over the Atlantic to replace it. The process was the same one that creates today's monsoon-rain system in Asia. Meanwhile, the monsoon rains were recycled by the rich vegetation across North Africa. Rather as in the Amazon today, the rain nurtured lush vegetation that ensured that much of it evaporated back into the air. The continually moistened winds took rain to the heart of the Sahara.
But the African Humid Period came to an end very suddenly. In the space of perhaps a century, the rivers of the Sahara emptied, the swamps dried up, the bush died, and the monsoon rain clouds were replaced by clouds of wind-blown sand. The climate system had crossed a threshold that triggered massive change. What happened? The first answer is that the sun moved. Or, rather, the precession continued its stately progress and gradually took away the extremely favorable conditions for Saharan rains. And as summer solar heating lessened, the warm air rose a little less and the monsoon winds from the ocean penetrated a little less far inland some years. The process was gradual, and went on without any appreciable effect on rainfall in most of the Sahara for more than 3,000 years. The vegetation feedback ensured that, at least in most years, the rain kept falling. If Lake Megachad was retreating, we have no evidence of it.
But at some point, the feedback began to falter. Perhaps there was a chance variation in rainfall that dried out the bush for a year or two. The sun was no longer strong enough to make good and revive the rains. Suddenly, what had been a feedback that kept the Sahara watered became a feedback that dried it out. The system as a whole had passed a threshold, and it never recovered. The green Sahara had become a brown Sahara. The North African monsoon rains had died.
Not everybody agrees that the vegetation feedback was the only trigger for the drying of the Sahara. One of Gerard Bond's solar pulses may have had some influence. But climate models show that in all probability, this flip in the Saharan climate was extremely sudden. Martin Claussen, of the Potsdam Institute for Climate Impact Research, in Germany, has played out this tragedy in detail in his model. He turns time forward and backward, recreates the subtle orbital changes, and fine-tunes the vegetation feedbacks. More or less whatever he does to mimic the conditions of 5,500 years ago, the result is the same. The system flips abruptly, turning bush to desert, and seas of water to seas of sand.
Other researchers have replicated his findings. Peter deMenocal, of Lamont-Doherty, calculates that the system flipped when solar radiation in the Sahara crossed a threshold of 470 watts per 10.8 square feet. Jon Foley, of the University of Wisconsin, found that a reduction in Holocene summer sun sufficient to reduce temperatures by just 0.72°F would have cut rainfall across the Sahara by a quarter, and by much more in the farthest interior of the continent. He says that once a region like the Sahara becomes dry and brown, it requires exceptional rains to break the feedback and trigger a regreening. Beyond a certain point—such as that reached 5,500 years ago—virtually no amount of extra rain is likely to be enough. The lack of vegetation "acts to lock in and reinforce the drought."
Back then, the people of the Sahara couldn't have known whether the droughts that suddenly afflicted them were permanent or not. But as the desert asserted control across the region, and the lakes and waterways dried up, they had no alternative but to leave. As part of the exodus, lakeside settlements near the Sudanese border in Egypt were all abandoned at about the same time. One was Nabta, famous now as the site of the world's earliest known stone structures with an astronomical purpose. They predate Stonehenge, in England, by about a thousand years. The key stones point to where the sun would have set at the summer solstice 6,000 years ago. Beneath some of the stones are burial sites for the cattle that the people tended. Nobody can be sure what the precise purpose of the structures was, but it is intriguing to suppose that they were used in an attempt to track the celestial changes that were disrupting the rains and turning their pastures to desert.
It may have been from such places that the myths and legends of past golden ages, and of the Garden of Eden, first emerged. The people who departed from the Sahara to set up new homes on the Nile or even farther afield would have taken their memories of a golden past. Researchers who have tried to date events in the Bible calculate mankind's expulsion from the
Garden of Eden at around 6,000 years ago, when kingdoms across the Sahara would have been collapsing. But the Garden of Eden need not have been in the Sahara, because similar stories were played out elsewhere. Arabia dried out at the same time, leaving behind a huge underground reservoir of water not much smaller than that beneath the Sahara. Claussen calculates that the desertification of Arabia could have been caused by the same combination of gradual orbital change and a dramatic vegetation feedback.
The evidence is as yet sketchy, but the dramatic drying of the Sahara and Arabia appears to coincide with other climate changes around the world. In the Pacific Ocean, El Nino appeared to switch into a more active mode at around this time. There were cold periods from the Andes to the European Alps. In both cases, glaciers advanced strongly down their valleys; many of them are only today returning to their former positions. In the Austrian Tyrol, one victim of the advance was the "ice man" named Otzi, whose freeze-dried remains emerged from melting ice in 1991. In Ireland, a 7,000-year temperature record held in tree rings shows a cold era that included the coldest summers in the entire record, at about this time.
All this is particularly intriguing because—unlike during previous great climatic events of the era of the ice ages—there is little evidence that the primary action had much to do with the polar regions. It seems to have been an abrupt climate change formed in the tropics, with its major impacts there, and only ripples beyond. One in the eye for Wally Broecker, some of its investigators have been heard to say—a point to which we will return.
But what does this say about the future of the Sahara? Could warming in the twenty-first century trigger a greener, wetter Sahara? It is an intriguing idea, with plenty of adherents. Reindert Haarsma, a climate modeler at the Royal Netherlands Meteorological Institute, says the Sahara could be destined for a 50 percent increase in rainfall—enough to trigger a return to the golden age, in which crocodiles floated through swamps where today locusts swarm. Claussen, whose model first stimulated the idea, is more skeptical. He points out that the orbital situation now is very different, so summer solar radiation is not great enough to create a revived African monsoon. DeMenocal says solar radiation is currently 4 percent lower in the Sahara than it was when the Holocene flip occurred. But on the other hand, he admits, much higher levels of carbon dioxide in the air might compensate for this by stimulating an earlier recovery of Sahara vegetation.
Optimists point out that on a very modest scale, something of a revival is going on in Saharan rains and vegetation—albeit from the depths of the droughts that afflicted the region in the 1970s and 1980s. It hasn't happened everywhere, and some places have since slipped back. But, according to Chris Reij, of the Free University, in Amsterdam, improved farming methods, such as digging terraces and holding water on the land, may have encouraged a modest greening of parts of the Sahara, and the resulting vegetation feedback could be one reason for the revived rains. But it would be a big step to predict from that a reversion to the "Garden of Eden" days.
While some in the Sahara may conceivably be able to look forward to greener, wetter times, the prognosis for many other arid regions around the world is not so good. The big fear, from the American West to northern China, and from southern Africa to the Mediterranean, is of a twenty-first century dominated by longer and fiercer droughts.
Again, history is the first guide. DeMenocal has been looking at the history of droughts and civilization in the Americas, and finds strong evidence of periods of drought much longer than any known in modern times. "There is good scientific evidence that vast regions of North America witnessed several such periods during the last millennium, with devastating cultural consequences," he says. "These megadroughts can persist for a century or more."
The six-year Dust Bowl of the 1930s, which caused mass migrations westward, was "pale by comparison" with its predecessors. Droughts in the nineteenth century devastated many Native Americans as well as their bison. At the end of the sixteenth century, a twenty-two-year drought destroyed an early English colony at Roanoke, in Virginia. It became known as the Lost Colony after all its inhabitants disappeared between their arrival, in 1587, and the return of a supply ship four years later. Going back earlier, tree rings show there was near permanent drought from 900 to 1300 west of the Mississippi and through Central America, which destroyed the Mayan and Anasazi civilizations. DeMenocal concludes that complex, organized societies can get by in short droughts. They have stocks of food and water, and know how to trade their way out of trouble in the short term. But few of them can deal with megadroughts. If hunger doesn't get them, the strife and turmoil caused by trying to survive does.
And the signs are that worsening droughts are becoming the norm in regions that have suffered megadroughts in the past. In the American West, the biggest river, the Colorado, is a shadow of its former self. Early in the twentieth century, the average flow was 13 million acre-feet a year. From 1999 to 2003, the average sank to 7 million acre-feet—worse even than the
Dust Bowl years. In 2002, it fell to just 3 million acre-feet. In 2005, the drought was continuing. In Central Asia, the Afghan war of 2002 was fought against a backdrop of drought as debilitating as any Taliban tyranny. The Hamoun wetland, which covers 1,500 square miles on the remote border between Afghanistan and Iran, has for millennia been a place of refuge for people from both countries in times of trouble. But that year it dried out and turned to salt flats. The water has not returned. Southern Europe is increasingly beset by forest fires and desiccated crops.
Richard Seager, of Lamont-Doherty, says that there is a long-standing correlation between drought in the western U.S. and drought in South America, parts of Europe, and Central Asia. And that is a pattern we see reasserting itself in the twenty-first century, as the Arizona desert creeps north, southern Europe increasingly resembles North Africa, and Central Asia takes on the appearance of Iraq or the Arabian Peninsula. Kevin Trenberth, of the National Center for Atmospheric Research, reports that the percentage of Earth's land area stricken by serious drought has more than doubled in thirty years. In the 1970s, less than 15 percent of the land was drought-stricken, but by the first years of the twenty-first century, around 30 percent was. "The climate models predict increased drying over most land areas," he says. "Our analyses suggest that this may already have begun."
That seems to be a common view. Mark Cane, a specialist in Pacific weather at Lamont-Doherty, says scarily: "The medieval warm period a thousand years ago was a very small forcing compared to what is going on with global warming now. But it was still strong enough to cause a 300-to 400-year drought in the western U.S. That could be an analogue for what will happen under anthropogenic warming. If the mechanisms we think work hold true, then we'll get big droughts in the West again." The Garden of Eden it is not.
Many believe that El Nino and the pattern of ocean temperatures in the Pacific are heavily implicated in the historical megadroughts, perhaps as part of a global reorganization of climate systems linked to Gerard Bond's pulses. And this should set modern alarm bells ringing, says Ed Cook, a leading tree-ring expert at Lamont-Doherty: "If warming over the tropical Pacific promotes drought over the western U.S. ... any trend toward warmer temperatures could lead to a serious long-term increase in aridity over western North America." Martin Hoerling, of the National Oceanic and Atmospheric Administration, thinks that such a process is already under way. He blames the increasing droughtiness of the tropics on a persistent ocean warming in the Pacific that, he says, is "unsurpassed during the twentieth century." The pattern of dryness is beginning to look less like a local, short-term aberration and more like a long-term trend, he says, and he predicts that global warming "may be a harbinger of future severe and extensive droughts."
It won't happen everywhere, of course. Climate models predict that a warmer world will, on average, have more moisture in the atmosphere, and that, in general, the wet places will get wetter and the dry places will get drier. They predict that areas of uplift, where rising air will trigger storm clouds and abundant rain, will see the uplift become more intense. But areas of sinking air, which are the traditional desert lands of the world, will see more-intense sinking and drying. In many parts of the world, this "hy-perweather" is likely to set competing forces against each other. Stronger storms will blow off the oceans, and monsoon-type rains may begin again in some places. But the rain-bearing winds will often be confronted by intensifying arid zones of descending air in the continental interiors. It is not obvious which force will win, and where.
Will the Sahara Desert expand and intensify, as drought theorists argue? Or will North Africa be reclaimed by a revived African monsoon? Megadrought or Garden of Eden? Nobody can answer that question yet. Perhaps the greatest likelihood is that in many places, from the Sahara to the American West and Arabia, there will be more and longer droughts, interspersed with brief but devastating outbreaks of intense storms and floods.
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