Green Sahara: wait, what?

52 days? That is indeed a long, long way.
Credit: Basil Pao @ www.palintravels.com

Last night I happened to catch my parents enjoying an ageing Python slinking his way across North Africa (a recommended watch), and I was struck by Michael Palin's phrasing:

We're now into what my French guidebook calls desert absolu: absolute desert. The earth stripped clean. As bare as a glacier, as featureless as the sea.

If I asked you to describe what you associate with the word "Sahara", I'd hazard a guess that you'd paint a picture of an incredibly arid environment; maybe endless rocky plains or an expanse of rolling sand dunes only occasionally punctuated by a clutch of palm trees around an oasis. However if I could travel back six thousand years ago to the mid-Holocene and ask the same question to a human living in the region, the response would be very different.

Kakum National Park, Ghana
Credit: Eleftherios Siamopoulos

Water is the key to life, and the above photo shows the vivifying effects of the West African monsoon (WAM). I'm using it here to provide an indication of what a typical Saharan landscape may have looked like to somebody living there during what is known as the African Humid Period (AHP, ~11.5 – 5.5 ka). Though it may seem fanciful, the idea that heavy monsoon rains extended deep into northern Africa, sustaining a lush and verdant terrain criss-crossed by a network of lakes, marshes and wetland ecosystems, is supported by many lines of proxy evidence:

• Marine sediment cores, recording the flux of windblown dust originating from the Sahara, show a marked decrease in the sedimentation rate during the AHP, signalling a reduced desert area.
• An abundance of life in extensive lakes and wetlands is evidenced by thick layers of organic-rich mud in terrestrial sediments, stretching from the southern to the northern Sahara
• Buried in these lacustrine sediments are an assemblage of pollen grains, as well as identifiable pieces of charred wood, which all testify to the presence of the vegetation typical of a  grassland, savannah and forest ecosystems.
• Archaeological findings show that conditions in the Sahara were hospitable to human habitation, as well as for large animals such hippos and elephants, given their depiction in rock art. In addition, massive piles of dung preserved in caves indicate that some areas were repeatedly grazed by herds of domesticated animals towards the end of the AHP.
• Geomorphological evidence for paleo-lake shorelines indicate that the maximum extent of palaeolake Chad (no relation) would make it the largest enclosed body of water on the surface of the Earth today.

Paleomegalakes of the Sahara outlined in white (reproduced from Drake & Bristow (2006))

This mid-Holocene green Sahara was no one-off freak occurrence, either. On the contrary there were repeated northward shifts in the monsoon position over the Quaternary Period, resulting in alternating phases of humidity and aridity in North Africa as inferred from lucustrine and marine sedimentary records.

It's therefore evident that the image we have of the Sahara is one very specific to how we know it to be today. That may seem like a truism, but I believe it's important to stress that the environment around us can change rapidly on a continent-wide scale, and has dramatically done so in the recent past. In the context of ongoing climate change, the motivation to understand how the Sahara is able to change states so dramatically should be clear. If this interests you as much as it does me, then you'll be delighted to find that this is topic of my next post.