Cold Dry Sahara, Hot Wet Savanna
August 10, 2010 by E.M.Smith
Pumping Sahara Sand
There is a peculiar thing that happens to the Sahara. Periodically, it turns into a green Savanna. This happens when it gets more heat input. Yes, more.
With a little more heat (not necessarily just as temperature) there is an influx of rain. More rain falls, and plants start to grow. This growth puts even more water into the air via transpiration. This causes more rain, leading to more growth. This continues until the whole area becomes a lush place to live, complete with elephants, hippos, and crocodiles.
Some long time later, as the earth changes its tilt and orbits shift, the Sahara cools some. The heat no longer pulls in the rain clouds, and the drying begins. As plants die, it becomes even more dry with less rain. This cycle continues until the Sahara is once again a desert.
http://en.wikipedia.org/wiki/Sahara_pump_theory (At least as it stands today. But I’ve saved a copy just in case the AGW Langoliers decide to change history again…)]
An example of the Saharan pump has occurred after the Last Glacial Maximum (LGM). During the Last Glacial Maximum the Sahara desert was more extensive than it is now with the extent of the tropical forests being greatly reduced.
During this period, the lower temperatures reduced the strength of the Hadley Cell whereby rising tropical air of the Inter-Tropical Convergence Zone (ITCZ) brings rain to the tropics, while dry descending air, at about 20 degrees north, flows back to the equator and brings desert conditions to this region. This phase is associated with high rates of wind-blown mineral dust, found in marine cores that come from the north tropical Atlantic.
So lets get this straight. During the glacial maximum the Sahara is bigger and dryer than now? COLD makes it grow and makes life harder?
Around 12,500 BC, the amount of dust in the cores in the Bølling/Allerød phase suddenly plummets and shows a period of much wetter conditions in the Sahara, indicating a Dansgaard-Oeschger (DO) event (a sudden warming followed by a slower cooling of the climate).
The moister Saharan conditions had begun about 12,500 BC, with the extension of the ITCZ northward in the northern hemisphere summer, bringing moist wet conditions and a savanna climate to the Sahara, which (apart from a short dry spell associated with the Younger Dryas) peaked during the Holocene thermal maximum climatic phase at 4000 BC when mid-latitude temperatures seem to have been between 2 and 3 degrees warmer than in the recent past.
Analysis of Nile River deposited sediments in the delta also shows this period had a higher proportion of sediments coming from the Blue Nile, suggesting higher rainfall also in the Ethiopian Highlands. This was caused principally by a stronger monsoonal circulation throughout the sub-tropical regions, affecting India, Arabia and the Sahara. Lake Victoria only recently became the source of the White Nile and dried out almost completely around 15 ka.
The sudden subsequent movement of the ITCZ southwards with a Heinrich event (a sudden cooling followed by a slower warming), linked to changes with the El Niño-Southern Oscillation cycle, led to a rapid drying out of the Saharan and Arabian regions, which quickly became desert. This is linked to a marked decline in the scale of the Nile floods between 2700 and 2100 BC.
So it can be colder, and drier, or significantly warmer (right up there with IPCC predictions) and we can have a lush thriving North Africa.
And it’s not just a theory. [See above link for a picture via radar of the old riverbeds under the blown sand]
Yes, it can take a long time to cycle. Though the transition from a wet to a dry Sahara can be measured as a few hundred years.
So what have we got here. Heat drives a giant heat engine to evaporate ocean water and deposit it as rain, bringing life and plant growth. It was hotter by 2 to 3 degrees then, than it is now.
The collapse into a lower heat level, and much dryer, Sahara is believed to have driven the formation of the Egyptian empire as people fled to the rivers where water could be found.
When we get cooler, the Sahara gets quite large. As recently as 2000 years ago, the Roman Empire used horses to cross the desert for trade. As it has cooled from that Roman Optimum to now, the Sahara has gotten dry enough that horses can no longer cross it. Camels were imported from Asia.
When did this round of cooling and drying start? From the same source:
The Saharan pump has been used to date four waves of human migration from Africa, namely:
Homo erectus (ssp. ergaster) into Southeast and East Asia
Homo heidelbergensis into the Middle East and Western Europe
Homo sapiens sapiens “Out of Africa theory”
The spread of Afro-Asiatic languages (Berber and Egyptian to North Africa and Semitic to the Arabian Peninsula and Middle East).
The formation of the modern Sahara, as a result of the 5.9 kiloyear event is also considered to be a part of the same mechanism in operation.
http://en.wikipedia.org/wiki/5.9_kiloyear_event (Also saved as private copy)]
The 5.9 kiloyear event was one of the most intense aridification events during the Holocene. It ended the Neolithic Subpluvial and probably initiated the desiccation of the Sahara desert. Thus, it also triggered worldwide migration to river valleys, e.g. from central North Africa to the Nile valley, what eventually led to the emergence of the first complex, highly organised, state-level societies in the 4th millennium BC. It is associated with the last round of the Sahara pump theory.
So it was hotter more than 5.9 thousand years ago, started cooling off, and we sprouted a very dry Sahara, had a collapse of ecosystems, and mass migrations of people to river valleys. Oh, and started the era of state empires.
Folks want more cooling, exactly why again?
I also note that the Heinrich Event sure sounds a lot like our ‘Little Ice Age / modern optimum” process (perhaps a small “dig here?”) though on a much larger scale. So with all these cycles inside cycles and DO Events and Heinrich Events and Bond Events; exactly how do we know what’s happening now isn’t one of them? And with it cooler now than in the past (The Sahara is a desert right now, as an existence proof…) exactly why would folks say it’s warmer than ever? And if it DID get warmer and the Sahara turned into a vast Savanna and jungle, this would be bad how? It’s done it before and life was good.
A model by Claussen et al. (1999) suggested rapid desertification associated with vegetation atmosphere interactions following the 5.9 kiloyear cooling event (Bond event 4).
Bond et at. (1997) identified a North Atlantic cooling episode at 5,900 BP from ice-rafted debris, as well as other such now called Bond events that indicate the existence of a quasiperiodic cycle of Atlantic cooling events, which occur approximately every 1500 years. For some reason, all of the earlier of these arid events (including the 8.2 kiloyear event) were followed by recovery, as attested by the wealth of evidence of humid conditions in the Sahara between 10,000 and 6,000 BP.
However, it appears that the 5.9 kiloyear event was followed by a partial recovery at best, with accelerated desiccation in the millennium that followed. For example, Cremaschi (1998) describes evidence of rapid aridification in Tadrart Acacus of southwestern Libya, in the form of increased aeolian erosion, sand incursions and the collapse of the roofs of rock shelters.
OK, so we have a 1500 year cycle that’s slowly getting colder. 4 x 1500 = 6000. 6000 minus 5900 = 100 years. Given that we have about that much error in the estimate (Bond Events are 1470 years +/- a hundred or two) we could have the next one any time now.
And what happens when it gets colder?
In the Middle East the 5.9 kiloyear event led to the abrupt end of the Ubaid period.
The 5.9 kiloyear event was also recorded as a cold event in the Erhai Lake (China) sediments.
Historically the period of the 5.9 kiloyear event is associated with the increased violence noticed in both Egypt and throughout the Middle East, leading eventually to the Early Dynastic Period in both Egypt 1st Dynasty and Sumeria. James Meo and Steve Taylor suggest that this period is associated with the rise of patriarchy, institutionalised warfare, social stratification, abuse of children,
I think I’ll stop there…
And folks want more of that?
It looks to me like the history and the geology and the biology all pretty much say “Warm is good, cold is bad”. Unfortunately, the patterns of the cycles say “it’s cold now, but the last warm didn’t get very warm, and we’re headed for even colder real soon in geologic time scales. Hundreds of years scale.”
There is more, but I think I’ll leave it for folks wanting to know more to look on their own. The “pluvials” are interesting.
The Neolithic Subpluvial — sometimes called the Holocene Wet Phase — was an extended period (from about 7,500-7,000 BC to about 3,500-3,000 BC) of wet and rainy conditions in the climate history of northern Africa. It was both preceded and followed by much drier periods.
The Neolithic Subpluvial was the most recent of a number of periods of “Wet Sahara” or “Green Sahara”, during which the region was much moister and supported a richer biota and human population than the present-day desert.
The Neolithic Subpluvial began during the 7th millennium BC and was strong for about 2000 years; it waned over time and ended in the 4th millennium BC. Then the drier conditions that prevailed prior to the Neolithic Subpluvial returned; desertification advanced, and the Sahara desert formed (or re-formed). Arid conditions have continued through to the present day.
This also means that all the hand wringing about “mining” all the “irreplaceable” groundwater in the Sahara is somewhat silly. Yes, we need to make it last a couple of thousand years, but that’s not forever. (One of the bodies of water is about the size of the Great Lakes…) It gets replaced in the next “pluvial”.
Which also means it was hotter from 7500 BC to 3000 BC than it is today…