William H. Calvin, A Brain for All Seasons: Human Evolution and Abrupt Climate Change (University of Chicago Press, 2002). See also http://WilliamCalvin.com/BrainForAllSeasons/Sahara.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
Available from amazon.com or University of Chicago Press.
Webbed Reprint Collection|
William H. Calvin
University of Washington
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
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.
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
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
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
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
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.
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.
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
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.
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.
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:
On to the NEXT CHAPTER
Copyright ©2002 by
Book's Table of Contents
All of my books are on the web.
out-of-print books are again available via Authors Guild reprint