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When the Sahara was Green

Posted by | December 13, 2008

As schoolchildren know, dinosaurs lived in a much warmer world.  Back in the era of T Rex and Stegosaurus, dinos flourished all over the globe beside truly balmy seas.

Fantasize with me for a moment about going back in time to those days, when we could swim in the surf all year round and our December heating bills would simply evaporate in the warm breezes.

Earth’s climate, clearly, has changed from what it was in the dinosaur era.

Recently a famous dino hunter came across evidence of much more modern climate change. Paul Sereno of the University of Chicago is a geologist who is expert at finding and unearthing dinosaur bones all around the world. (If you know an 8-year old, ask about Sereno and you’ll learn he’s the cream of the crop in the realm of dinosaur discovery.)

In recent years Sereno has made a specialty of hunting dino fossils in the Sahara. And by chance, his crew stumbled across dozens and dozens of human skeletons in loose sands, lying above the dinosaur-rich solid rock below the dunes.

The serendipitous find led Sereno to unearth the bones of people who once lived in what’s now the barren desert of Africa. The bones tell a story of early Stone Age humans from cultures of different times. They reflect two distinct burial styles and are associated with pots having quite different patterns. In other words, there are twin pieces of human history wrapped up in this one special spot in what’s now an empty desert.

Here’s the climate connection: the human bones are accompanied by bones of animals, including crocodiles, hippos, turtles, clams and various fish. The great supply of water made evident by these bones has led to the nick-name “Green Sahara” for time of the Stone Age peoples.

Both archeological evidence and radiocarbon dating show the age of the younger culture as 6,000 years ago and the older one as 9,000 years ago. Just before that earlier time, Earth’s climate had warmed up from the bitter cold of the Ice Age – the frigid period in which Canada, the upper Great Plains and Midwest, as well as New England, were all under thick glacial ice.

In Africa temperatures didn’t climb much when the Ice Age ended, but weather patterns shifted. The Saraha suddenly got rain each year – lots and lots of rain. The humans Sereno found were buried in what was then a small peninsula, jutting into the local lake in which the crocs, clams and fish lived.  It was, clearly, a good spot, one used by people for literally thousands of years during that wet period.

As a geologist I’m happy to report the people of the Green Sahara knew the value of rocks. They made stone tools out of an unusual volcanic rock found about 100 miles away, either traveling there to bring it back or trading with other people for the valuable material. We know they made stone tools from that rock beside the lake because of abundant flakes of the unusual rock in the loose sands.

Sereno’s work shows the Stone Age dead were buried with care. A woman reaches out her arms to two small children in one gravesite. Other individuals were buried with pots and even with flowers.

But then the whole scene changed. The wet winds shifted, and by 3,500 years ago, the Green Sahara literally dried up. It was a climate upheaval more massive than anyone alive at the time could have understood. In short, the Sahara lurched back to its previous status as a desert. The water-loving creatures and the people of the area were soon replaced by arid sands.

In each chapter of Earth history, Mother Nature makes it obvious she likes a climate story with a truly vigorous plot. That’s not to our liking, but such is the clear record geologists and archeologists have learned to read from all over the world.

And in its own small way, December is once again showing us a tiny glimpse of that fundamental truth.

Good luck with your heating bills.