With Speed and Violence Fred Pearce - Global Commons Institute

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