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William H. Calvin, A Brain for All Seasons:  Human Evolution and Abrupt Climate Change (University of Chicago Press, 2002). See also

copyright ©2002 by William H. Calvin
ISBN 0-226-09201-1 (cloth)    GN21.xxx0     
Available from or University of Chicago Press.
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This 'tree' is really a pyramidal neuron of cerebral cortex.  The axon exiting at bottom goes long distances, eventually splitting up into 10,000 small branchlets to make synapses with other brain cells.
William H. Calvin

University of Washington
Seattle WA 98195-1800 USA

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To:                  Human Evolution E-Seminar
From:             William H. Calvin
22°N   14°E   10,000m ASL
                        The Sahara

Subject:          Why climate can suddenly flip


The Sahara down below gets no rain at all.  It doesn’t even get dew from offshore fog drifting inland, like some deserts near a coast.  It is “hyper arid.”

     Not always, however.  There have been “pluvial” periods when the Sahara got enough rain.  Between about 14,800 and 5,500 years ago (except for the Younger Dryas), it was a verdant landscape covered with grasses and shrubs, with numerous lakes.  There were grazing animals of many kinds, even elephants.

     There have also been periods in the ice ages when the arid area was even larger than at present.  So why is there a Sahara at all?

     Well, we’re into the “horse latitudes,” those bands of fickle winds and dryness that surround the globe near 30° North and 30° South.  Lack of vegetation makes them brighter-looking.  The Sahara is an example (the arid band actually extends east across Asia), and the Southern Hemisphere has the Kalahari and Australian deserts, plus Patagonia.

     If hot air tends to rise, then what goes up at the equator has to come down somewhere else.  All of those tropical rains are because the moisture drops out, once the dew point is reached during the ascent to cooler levels.  By the time the tropical air comes back down from the stratosphere hereabouts, it is dry.  These are examples of what the atmospheric scientists call the Hadley Cell circulation, named after the 1735 analysis by the British lawyer George Hadley (scientists used to make their livings in more diverse ways).

     It is now known that each hemisphere is divided into three cells:  rising air at the equator falls between 20° and 35° North (creating the Hadley Cell).  Air rises again at roughly 55° to 60° North and descends over the North Pole (creating the Polar Cell).  In between the descent at about 30° and the rise at about 60° is the third one (called the Ferrell Cell after a nineteenth-century American meteorologist).  All of this varies with the season.  Ditto for the Southern Hemisphere.  This six-cell general circulation pattern is one of the reasons why the North and South Poles are so dry, as they are being flushed by moisture-free air that descends from on high, just like the Sahara.

     They are, of course, high-pressure areas.  In low-pressure areas, air rises and any moisture may precipitate out when the dew-point temperature is reached.  Thunderheads are vast upwellings and can carry some heavier molecules (like refrigerator coolants) into the upper atmosphere; they’d never diffuse there on their own, but they go with the flow, another one of those package deals like brain size.

     Another consequence are the bands of fickle winds (“the doldrums”), cursed by sailors for centuries, that occur near the equator and at the horse latitudes.  They are because winds tend not to cross between cells, thanks to the vertical curtain of air separating adjacent cells.  At the horse latitudes, sailors also cursed the relentlessly sunny skies (few clouds) and dry air, along with the lack of reliable wind to carry them out of the situation.  Even the ocean surface is more salty in the horse latitudes, because it doesn’t get rained on.  The next time you walk through one of those building entrances with an air curtain rather than a door separating indoors from outdoors, remember the cell boundaries of the earth.


While horizontal winds may not often pass through the vertical curtains, the curtains themselves create the important “trade winds” and “westerlies” on which sailors and weather forecasters rely.  To understand this, recall that even if you are standing still at the equator, you are moving eastward at a speed of about a thousand miles per hour (it takes 24 hours to rotate through the 24,902-mile equatorial circumference).  But halfway to the North Pole at 45°N, your daily path is about 70 percent of the equatorial circumference, and so your eastward speed is 30 percent less (at 60°N, it’s down by half).  Same thing applies to the air, which is dragged along at the same local speeds by the surface features.

     Now consider what happens to a wind blowing north.  It also has a certain eastbound speed which it doesn’t lose (conservation of angular momentum and all that) as it goes north, a speed greater than the local eastbound speed.  So this northbound wind will also move east relative to the ground.  It will seem to turn right.  Try to move north and you really move northeastward.

     Now consider the fate of the air that descends from on high.  Air that descends in that vertical curtain at 30°N and turns north continues to travel east at the velocity characteristic of 30° – even though its surroundings are now traveling eastward more slowly.  This northeast-bound air stream will thus appear as a wind coming out of the southwest to a local observer.  These southwest winds get called “westerlies.”     Air from the 30° descending curtain that turned south will be traveling eastward at the 30° velocity but, in this case, their surroundings will be traveling faster.  So these winds will appear as a wind out of the northeast to a local observer, as if they too had turned right.  They got called “trade winds” not for commerce but because they were so steady, “threading” along at a constant pace.

     This is, you may have guessed by now, the so-called Coriolis effect at work (it’s not a “force” so much as just conservation of momentum).  It appears to turn moving things to the right in the Northern Hemisphere, and to the left down south.  That’s what George Hadley figured out.  Sailing ships heading to North America from Europe went south past the doldrums to pick up the trades, but returned on a more northern path using the westerlies.


Just scanning the globe for deserts, you can see exceptions to the 30° ideal all around 30° North and 30° South.  (Florida would be a desert were it not surrounded on three sides by the Gulf Stream.)  And the Ferrell-to-Polar Cell boundary at 55-60° North is something of a statistical thing, not exactly a vertical curtain, and there are eddies (alias weather systems) that wander around.

     It’s also not clear that things have always been the six-cell way, or that this pattern must continue.  Maybe there are two-cell possibilities or no-cell chaotic arrangements.  Any such reorganization of this cellular circulation would, of course, have profound all-bets-are-off consequences for regional climates and the world’s average temperature.  No more steady trade winds or westerlies.

     This is not like the more familiar droughts, where your rain happens to fall elsewhere for a decade (and so others prosper for awhile).  These reorganizations last a long time and have global consequences.  The best-studied case so far is the abrupt warming in the Northern Hemisphere about 15,000 years ago that started the ice sheets to melting.  Prior to that time, Lake Victoria had dried up for lack of sufficient rainfall; abruptly, it got enough rain once again.  Prior to then, there were big lakes in Nevada and western Utah; abruptly, they dried up and became as arid as today.  Such simultaneous occurrences in distant places are why people think that the atmospheric circulation pops into a new mode of operation.

Whereas the atmosphere is quick and agile and responds nimbly to hints from the ocean, the ocean is ponderous and cumbersome.
George S. Philander,
Is the Temperature Rising?, Princeton University Press, 1998

     Even if you conveniently happened to live in a place where the local climate changes balanced out, reorganization would likely affect global average climate as well and so you’d still suffer from the worldwide flip from warm-and-wet into cool-and-dry.  That’s because reorganization likely wouldn’t maintain the same average amount of water vapor in the atmosphere (it’s the most important “greenhouse gas,” not carbon dioxide or methane).  The whole earth could get warmer and wetter, or cooler and drier, in just the few years that it would take the winds to rearrange themselves.  Oceans are usually thought to take much longer to change (it takes six years for ocean currents to get from Labrador to Bermuda), so those who worry about abrupt climate change tend to look at potential cell reorganizations in the atmosphere with considerable interest.  The other quick effect is albedo, with a brighter atmosphere (from dust storm or salt spray) bouncing more sunlight back into space.

     Apropos causation, remember the difference between proximate causes and what, in turn, causes them.  The coup de grace is likely delivered to the old climate via new winds and an altered greenhouse, but ocean changes may be what sets up the flip to a new mode of operation – and, even more ultimately, it may be continental drift that sets up the modes into which ocean circulation can shift.

     I’ll return to this Rube Goldberg chain of causation later, when I fly home over the Gulf Stream up above 60° North, whose warmth encourages the air to rise and thereby helps to stabilize the present cell pattern.  But, when the Gulf Stream falters and no longer extravagantly warms the air at 60° North, the atmospheric cells may be vulnerable to disruption by such usual decade-scale climate oscillations as El Niño and the North Atlantic Oscillation.


If anyone looks up and sees this plane high over the Sahara of northern Africa, he or she is likely to wonder where it is heading.  Having heard of a moon landing back in 1969, some might think we were heading there (improbable, but it takes a lot more knowledge to distinguish between the possible and the probable).  Others might suppose our destination was their nation’s capital, though the more experienced would realize that our 10,000 meter altitude would make that unlikely.

     Would the observer watch long enough to judge our direction, which is a little east of south?  Would the observer know the map of Africa well enough to realize what lay farther south and east, several countries beyond their immediate neighbors? (A continent with 62 countries, constantly changing their names, would certainly stretch my abilities.)  Or have enough education to know about the shortest-and-fastest great circle routes, and that this particular one led to Johannesburg?

     So too, our knowledge of gross anatomy attained via butchering grazing animals does not necessarily prepare us for understanding the relationships between the pigs and the antelopes, much less their separate evolutionary paths.  Or what evolutionary forces moved things along those paths and not other likely ones.  If there are shortest-and-fastest paths, some evolutionary equivalent to a great circle route, we sure don’t know about them yet.

When we trace food production back to its beginnings, the earliest sites provide another surprise. Far from being modern bread- baskets, they include areas ranking today as somewhat dry or ecologically degraded: Iraq and Iran, Mexico, the Andes, parts of China, and Africa's Sahel zone. Why did food production develop first in these seemingly rather marginal lands, and only later in today's most fertile farm- lands and pastures?
- Jared Diamond, 1997

Seeing the Sahel reminds me that population size always fluctuates.  That has a lot of implications for the usual view of evolution, the one that says that improvements in place are always happening when something proves useful.  However true that may be, it is slow compared to what happens when population size shrinks and expands.  If you want to see how things happen quickly, pay attention to the climate transitions, not the more static periods, and look for things that are amplified by environmental change.

     Take the Sahel down below (the “shores of the Sahara” in one metaphor), that transition zone between the arid Sahara and the tropical rain forests.  A relatively sparse savanna vegetation of grasses and shrubs now covers most of the Sahel, which stretches from Senegal in the west to the Sudan in the east.

     When rainfall improves in the Sahel and the shrunken lakes re-expand, humans move into the newly grassy areas.  You don’t get a uniform expansion of central population (centered, say, on the Congo) into its neighbors and so filling in the space left by those who moved to the vacant territory.  That is simply too slow, compared to the time scale of the climate fluctuations (besides, territories are contested).  What you see instead is an expansion of the subpopulation that just happens to live next door when the opportunities open up.  In short, the people who are already adapted to living in arid regions get opportunities that the central population doesn’t.  They’re the ones who see the new resources, and when no one is yet contesting them, they’re the ones who get more grandchildren as the result.

     Now consider the flip side, the Sahel drought that often occurs a few decades after the expansion.  There was a severe drought in the 1970s and, if the quarter-century cycle of Sahel rainfall and Atlantic hurricanes  holds up, it might again be in trouble.  You get hungry people trying to migrate back into the already filled southern regions of the Sahel.

For the production of man a different apprenticeship [from forests] was needed to sharpen the wits and quicken the higher manifestations of intellect - a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking played a preponderating role in the preservation of the species.
- Raymond Dart, 1925

     Because the frontier people may have survival skills that are somewhat better than those of settled people, they may do better in the competition.  The Huns invading Europe is just a recent episode of an old story.  The other thing that happens, of course, is that species explore new foods during the hard times.  They are forced to innovate, and some of those new skills may allow them to exploit resources not being used where the central population lives, so the central population density can actually increase somewhat when the immigrants arrive.  The result is eventually the same:  peripherally-useful genes are infused into the more central population.

     So far, none of this story is particularly human; the same thing likely happens on a more local scale with baboons, which are monkeys that have adapted to treeless settings but will happily invade woodland.  Even some chimpanzees can survive in semi-arid Senegal environments rather than their usual forests (knowing how to dig for roots and tubers can confer survival, as you can get water along with calories).  Some anthropologists believe that an important part of our ancestors’ story involves learning how to thrive in arid environments.  There are a lot of large antelope species that live in such places, and until you learn how to gather grains and bake bread, humans in arid lands likely thrived by being able to eat meat on the hoof.

     It’s an interesting “pump the periphery” principle that tends to make useful-on-the-frontier genes much more common in the central population than if the population size were static and only frontier peoples needed frontier genes.  Whatever the speed of the apocryphal improvements-in-place, you are far more likely to see substantial changes when climate is fluctuating.  That’s true for short cycles, like the Sahel cycle, for mid-range cycles like the one every 1,500 years that makes West Africa swing between wet and dry within a generation’s time, and for the “glacially slow” ice age changes as well.

     And when it’s so bad that the central population fragments into isolated refugia, even more dramatic things can happen.


Oceans and deserts are powerful engines in human affairs.  The Sahara is another enormous pump, fueled by constant atmospheric changes and global climate shifts.  For tens of thousands of years its arid wastes isolated the very first anatomically modern humans from the rest of the world.  But some 130,000 years ago, the Sahara received more rainfall than today. The desolate landscape supported shallow lakes and semi-arid grasslands.  The desert sucked in human populations from the south, then pushed them out to the north and west. The Saharan pump brought Homo sapiens sapiens into Europe and Asia. And from there they had spread all over the globe . . . . But the pump shut down again. As glacial cold descended on northern latitudes, the desert dried up once more, forming a gigantic barrier between tropical Africa and the Mediterranean world. Fifteen thousand years ago, global warming brought renewed rainfall to the Sahara.  The pump came to life again. Foragers and then cattle herders flourished on the desert’s open plains and by huge shallow lakes, including a greatly enlarged Lake Chad.  Then, as the desert dried up after 6000 B.C., the pump closed again, with its last movements pushing its human populations out to the Sahel, where they live today. Like the North Atlantic Oscillation, the Sahara is a pump with the capacity to change human history.

- Brian Fagan, Floods, Famines, and Emperors: 
   El Niño and the Fate of Civilizations
,  Basic Books, 1999



Notes and References
(this chapter
corresponds to 
pages 64 to 73 of the printed book)

Copyright ©2002 by
William H. Calvin

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All of my books are on the web.
You can also click on a cover for the link to

Conversations with Neil's Brain:  The Neural Nature of Thought and Language (Calvin & Ojemann, 1994)

The Cerebral Code:  Thinking a Thought in the Mosaics of the Mind (1996)

How Brains Think:  Evolving Intelligence, Then and Now (1996)

Lingua ex Machina:  Reconciling Darwin and Chomsky with the Human Brain (Calvin & Bickerton, 2000)

The six out-of-print books are again available via Authors Guild reprint editions,
also available through (click on cover):

Inside the Brain

The Throwing Madonna:  Essays on the Brain

The River That Flows Uphill


The Cerebral Symphony

The Ascent of Mind

How the Shaman Stole the Moon