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/CPH1.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
Available from amazon.com or University of Chicago Press.
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William H. Calvin
University of Washington
wouldn’t think that the direct route from Nairobi to Seattle goes
over Greenland. But just
try stretching a string over a globe’s surface if you don’t
believe me. The shortest
string goes right over the Sudanese and Libyan deserts, then over
Copenhagen, northern Iceland, and Greenland.
I particularly like the view from the great circle route
connecting Copenhagen to Seattle, so I have managed to get a
“geologist’s seat” (window seat, in front of the wing, on the
shady side of the plane, take your binoculars).
One flight in three, the clouds part and make your efforts
From Copenhagen, the plane goes over Norway and then the
Greenland-Iceland-Norwegian Sea (sometimes called the “Nordic
Seas” but I’ll just call it all the Greenland Sea).
Next it passes over a mountainous minor continent of rock and
ice paradoxically called Greenland (a name devised by Eirik the Red to
promote immigration, an early instance of deceptive advertising).
Then we go over the great stores of methane frozen into the
Canadian tundra (and apt to be released into the atmosphere by
greenhouse warming, making things even warmer still).
Those sites are all key players in climate flip-flop scenarios.
They are intimately connected to our African past and our
talked a lot about abrupt climate jumps without mentioning very
much about how they happen. The
big scientific questions often have three major facets: what, how, and why. And
before I can get much further into the how and why of things, I’d
better flesh out the rest of the What’s, the slow background changes
which made the abrupt coolings such a surprise to the scientific world
in the 1990s.
It seems strange to realize that even in the middle of the 19th
century, hardly anybody knew about the ice ages.
Even scientists still talked in terms of a biblical deluge.
But as early as 1787 in Scotland, scientists realized that
giant boulders were carried long distances and deposited in the midst
of a landscape in which their kind of rock was utterly foreign.
Even peasants in Switzerland subscribed to such an explanation.
Charles Darwin, in his Voyage of the Beagle, wrote in
1839 of boulders carried along in icebergs.
Louis Agassiz spent
a lot of time looking at alpine landscapes.
Those Swiss valleys looked as if they had been scoured, over
and over. From his
position as president of a Swiss scientific society in 1837, Agassiz
elaborated the earlier notions and proposed that massive ice sheets
had pushed their way around Europe, sometime in the not-so-distant
past. Darwin provided
further geological details on the subject in 1842.
The landscape evolved. Not
only did the Alps grow long glaciers but giant ice sheets sat atop
much of Northern Europe, flattening the Baltic landscape.
Some piles of ice didn’t even have mountains centered beneath
them; they were like sand dunes, obstacles that grew as the laden
winds ran into them.
We eventually learned that the warm periods in the ice ages
last only about 10,000 years and that it’s a long time until our
next one is “due” – about 100,000 years.
We learned that 32 percent of the land was covered by ice
sheets in the last ice age, but that a warm period like today’s
reduces the ice to only 10 percent (mostly Greenland and Antarctica,
but you can also see some serious ice sheets in Norway, Iceland,
Alaska, Tibet, New Zealand, and the Andes).
How many ice ages have there been?
Dozens. Up until
about 800,000 years ago, the major meltbacks occurred about every
41,000 years. More
recently, the 100,000 year rhythm has been the more obvious one.
second great insight concerned how sunlight might vary to help
melt off the massive amounts of ice.
These slow changes in ice mass are mostly caused by even slower
changes in the summer sunshine at latitudes like Scandinavia.
Elaborating on the 1842 suggestion of the French mathematician
Joseph A. Adhémar, Milutin Milankovitch proposed during World War I
that the sunlight reaching the higher latitudes controlled the ice
ages (he did his laborious calculations as a prisoner of war), and
that slow changes in the earth’s orbit were important.
First of all, the tilt of the earth’s axis varies over a
41,000 year cycle. Our
axial tilt peaked 9,500 years ago at 24.6° and is now at 23.4° and
heading toward a turnaround at about 22.1°.
Furthermore, our closest approach to the sun in our elliptical
orbit is currently in the first week of January (we’re about 3
percent nearer, and get about 10 percent more heat, than in July) –
but in another 12,000 years or so, the closest approach date will have
drifted around to summer again, the precession helped by the
gravitational pull of the other planets causing our
less-than-spherical planet to slowly wobble.
Our orbit is also more circular at some times (repeating at
about 100,000 and 400,000 years), making the month-of-closest-approach
factor periodically less important.
Tilt and precession account for a lot of the slow back and
forth of the glaciers in between the major meltoffs, which occur every
100,000 years or so. The
best setup for melting ice is when there are colder winters but hotter
summers in the Northern Hemisphere (that is, after all, where most of
the meltable ice is, thanks to the general lack of land between 40°S
(southernmost Australia) and Antarctica.
(Yes, I know, there are the southern Andes and some nice
glaciers in southern New Zealand, but that’s not much when compared
to the land mass of Eurasia and North America north of Madrid and
“Exaggerated seasonality” occurs when the closest-approach
bonus is in summer and there’s also another bonus from
near-maximum tilt. In
such cases, there is as much summer sunlight every day at 65° (say, in
northern Iceland or Fairbanks, Alaska) as there is presently at 49°
(say, in Paris or Victoria). Despite
the accompanying colder winters, getting melting going during those
long hot summers is how we got rid of the ice sheets at high northern
latitudes. At the
opposite extreme, 65° gets about what Thule (78°N) gets today.
These rhythms, and the way they reinforce one another
occasionally, also explain a lot of the back-and-forth between one
major meltoff and the next, the so-called interstadials.
it was long suspected that “orbital factors” weren’t the
whole story, as the southern hemisphere ice sheets melted back at the
same time as the northern ones. Why
should ice sheets in the Andes and New Zealand melt when the
closest-approach bonus was in their wintertime?
They should have been out of phase with the northern one, but
they were in phase, synchronized by something.
The other thing that made scientists suspect that it wasn’t
so simple was a wildflower, white with a yellow center, of the rose
family called Dryas octopetala.
It is cold-adapted and is found on the tundra – not among
shrubs and pine trees. Well,
a century ago, Dryas pollen turned up in cores of a lake bed in
Denmark above a layer of trees, at a depth now dated back to about
12,000 years. A return to
cold made no sense then, according the Milankovitch orbital view.
That was when the two astronomical bonuses were combining to
produce particularly hot summers in Denmark.
Half of the ice sheets covering Europe and Canada had already
melted, and all of the ones in Scotland.
There should have been pine seeds in those cores, not the
pollen of a tundra flower.
This return to ice age temperatures, called
the Younger Dryas, lasted more than a millennium, reestablishing
glaciers in Scotland once again.
Then, things suddenly warmed back up.
Nothing in the orbits could explain such things.
I had visited Copenhagen researchers in the late 1970s, no one
would have agreed on when the ice ages began.
The textbooks, noting the age of old moraines plowed up by the
ice sheet frontiers, would have said the ice ages went back about
800,000 years. No one yet
knew about winter sea-ice reaching the British Isles 2.51 million
years ago, nor about all those antelope speciations in Africa about
the same time.
Most people thought, back then, that the ice buildup and the
ice melting were largely due to those slow changes in various aspects
of the earth’s orbit around the sun.
Since they don’t suddenly jump, most people assumed climate
would change gradually. The
cores coming out of the ocean floor indeed showed slow changes, and so
did the ice core from Antarctica – but few understood then about how
special Antarctica is, how insulated from the rest of the world its
weather can be (that ring of westerlies and its vertical curtain of
rising air at 60°S) and how its low rates of snowfall limit time
parts of Antarctica are now known to give different results.)
But the pros knew about such anomalies as the Younger Dryas and
they were anxious to get better data.
Greenland had more snow each year, thanks to weather systems
sweeping up from the Gulf Stream, and the Greenland cores were
starting to show some puzzling results, as the Danish researcher Willi
Dansgaard and colleagues reported in 1982.
I first heard of the abruptness, per se, in 1984 when the Swiss
geophysicist Hans Oeschger gave a talk in Seattle.
The time calibration on one of his slides prompted me to ask
him afterward, about just how quickly temperature had changed.
Oh, he said, the big drop took just a few years.
The enormity of such a whiplash caused me to assume that we
were having some language difficulties and so I persisted, asking,
“Just a few decades?”
No, no, he replied, merely a few years.
The American geochemist Wallace Broecker also heard Oeschger
give a talk in 1984, and the quick flips gave Broecker his idea for
different modes of circulation via failures of the salt conveyor (more
in a minute). It was Broecker who coined the term “Dansgaard-Oeschger
events” to describe the abrupt coolings and re-warmings. (The D-O’s are what I’ve been calling the flip-flops or
Another important discovery was from the ocean-floor cores of
the North Atlantic. Now
and then, a rain of rock had fallen down to the abyss, dropped off the
bottom of passing icebergs. The
rocks mostly came from Hudson Bay (though some came from Iceland).
These “Heinrich events” are not completely understood. They do tend to occur in the coldest parts of the glacial
cycles, when ice sheets extend out on to continental shelves. Perhaps they represent glacial advances that overrun terminal
moraines and then freeze their rock into the bottom of the ice sheet.
Later, when tides manage to float a terminal glacier and break
it off at a weak “hinge,” an iceberg is created which sets sail
for the Bay of Biscay, held upright by its rocky ballast on the
bottom. As it melts en
route, so a trail of debris is deposited on the ocean floor – and
fresh water is leaked all over the surface of the mid-Atlantic Ocean.
That the events are episodic, lasting for several centuries, is
attributed to the Hudson Bay ice mountain collapsing, perhaps from
melting on the bottom in the manner of subglacial lakes in Antarctica.
The D-O flips are not the same thing as the Heinrich events,
though the issue was confused for awhile.
For example, a D-O cooling occurred in the midst of the last
warm period at about 122,000 years ago, when there were no ice sheets
in Canada to create icebergs. But
in icy times, Heinrich events do tend to be
shortly followed by an abrupt D-O warming.
is drilling season and Greenland is where most of the
Copenhagen researchers currently are.
They try to extract long cylinders of ice, ones that will show
annual layers of snow and ice. Greenland
has ice that goes back to about 250,000 years ago, when it was warm
for long enough that it melted most of the accumulation from prior ice
Still, that quarter-million years seen in the ice cores
contains the last two ice ages. And
it includes the last warm period between 130,000 and 117,000 years
ago, when there was a climate somewhat warmer than today’s for much
of the 13,000 year period. (For
Europe, this warm period is called “The Eemian” – more
generally, “marine isotope stage 5e.”) Sea level was 3-5 meters higher than at present (just imagine
shrinking southern Florida). For
comparison, complete melting of the West Antarctic ice sheet (or, for
that matter, the Greenland ice sheet) would today raise sea levels by
6–7 meters, which is several stories high.
Melt all the ice sheets and it’s more like 60 meters,
covering up 18-story Florida condos, with the coastline somewhere up
The last ice age per se had several dozen D-O cycles, a flip
one way or the other occurring on average about every 750 years.
Our current warm-up, which started about 15,000 years ago,
began abruptly in the Northern Hemisphere, with the temperature rising
sharply while most of the ice was still present.
Several thousand years later, temperature declined abruptly
into the Younger Dryas. After
the sudden rewarming at 11,550 years ago that ended the Younger Dryas,
things gradually warmed into the period of modern sea level and
There was a slow-and-gradual view of things in the good old
days, back before we got the time resolution to see the abrupt stuff
that our ancestors suffered through.
These slow “orbital” causes of ice fluctuation have little
to do with the rapid back-and-forth of a whiplash.
Something else seems to be the more immediate cause of the
chattering seen atop the slower orbital trends of ice.
Because some of the D-O cold-and-dry flips last for many
centuries, there is a semi-slow way of viewing them, and indeed a
group of European researchers are looking at the cluster of flips
between 60,000-25,000 years ago for clues about the demise of the
Neandertals, calculating the expected regional climate changes in
Europe and correlating them with the dating of archaeological sites to
see how the Neandertal populations were moving around in response (see
their database web pages at http://www.esc.cam.ac.uk/oistage3/
But my bust-then-boom aspect is keyed to the initial part of
the abrupt changes, just the first century or two after a
cooling-and-drying (when the drought-to-fire-to-grass-to-herds
opportunity occurs) and the century after the abrupt rewarming that
ends it (when grasslands extend into formerly arid areas, and again
dramatically expand herd sizes).
That first century after a flip should be when unusual
opportunities present themselves for experienced hunters to expand
their territory and populations.
Archaeological time resolution cannot, at present, see such
decade-to-century transitions, but that’s likely where the action
ice sheets shown; they are generally connected by sea ice.
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All of my books are on the web.
The six out-of-print books
are again available via Authors Guild reprint editions,