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/noon.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
have always been interested in ancient climates, given how droughts
tend to contribute to the demise of one civilization after another.
There are many indicators of ancient climate, such as wind-blown
soils piling up during periods of great wind storms.
When they blow from Africa into the Atlantic Ocean, those that
don’t blanket the Cape Verde Islands sink to the ocean floor, where a
core can be drilled to sample millions of years of Sahara dusty
Coring old lake beds and examining the types of pollen trapped
in sediment layers led to the discovery of the Younger Dryas early in
the twentieth century. Pollen
cores are still a primary means of seeing what regional climates were
doing, even though they suffer from poorer resolution than ice cores
(worms churn the sediment, obscuring records of all but the
longest-lasting temperature changes).
When the ice cores demonstrated the abrupt onset of the Younger
Dryas, researchers wanted to know how widespread this event was.
The U.S. Geological Survey took old lake-bed cores out of
storage and re-examined them.
Ancient lakes near the Pacific coast of the United States, it
turned out, show a shift to cold-weather plant species at roughly the
time when the Younger Dryas was changing German pine forests into
scrublands like those of modern Siberia.
Subarctic ocean currents were reaching the southern California
coastline, and Santa Barbara must have been as cold as Juneau is now.
(But the regional record is poorly understood, and I know at
least one reason why. These
days when one goes to hear a talk on ancient climates of North America,
one is likely to learn that the speaker was forced into early
retirement from the U.S. Geological Survey by budget cuts.
Rather than a vigorous program of studying regional climatic
change, we hear the shortsighted, preaching of cheaper government at
any cost. These
penny-wise-pound-foolish factions of the U.S. Congress even tried hard
in 1995 to eliminate the U.S. Geological Survey altogether.
main problem with most paleoclimate indicators is that time has
gotten smeared out, so that events that last less than a thousand years
simply disappear into the noise. On land, people tread paths into the floors of caves,
constantly rearranging the deposits of previous centuries. At the bottoms of lakes, worms churn things.
In the oceans, the flat fish burrow and stir.
Fluctuations, alas, can be very important, as in the story about
the statistician who drowned in a lake whose average depth was only
three feet. (And in the
stock market, even a two-year moving average will hide significant
swings, during which fortunes were made or lost.)
For paleoclimate indicators, few have time resolution better
than one millennium.
Great coolings or warmings could have happened and you’d never
know it from such an averaged record (unless the event lasted more than
a millennium, which several did).
There are a few sites, such as the Santa Barbara Basin, with
high accumulation rates (and anoxic bottoms to discourage the worms)
that have resolutions that occasionally achieve fifty year
discriminations. Ice has
the best time resolution of all, especially at sites (such as southern
Greenland) where a lot of snow falls each year.
Ice cores can be made near the poles and at a few elevated low
latitude locations as well.
Once the ice cores from Greenland were analyzed in the 1980s,
experts such as Willi Dansgaard and Hans Oeschger made us aware that
big warmings or coolings had occurred in less than a decade. Greenland does have some weather patterns that affect
regional temperatures, but the big rapid transients recorded in
Greenland are also seen in the tropics and in many places in the
southern hemisphere, and at about the same time.
If the prior ice ages turn out to be anything like this last one
– and all suggestions so far point that way, with evidence for the
Heinrich events already back to 0.8 and 1.1 million years – then our
hominid ancestors suffered through hundreds of these dramatic events.
present warm period has been free of abrupt changes, at least
since 6,000 B.C.
An abrupt cooling got started 8,200 years ago, but it aborted
within a century or so, gradually warming back up instead of stepping
up. I’ll mention the cause of this aborted cooling when we get
a little closer to the scene of the crime.
It probably corresponds to the drought seen about then in
southeastern Europe and the eastern Mediterranean. Temperature changes since then have been gradual in
Indeed, we’ve had an unprecedented period of climate
stability (nothing much worse than the Little Ice Age – though, of
course, droughts of only a few decades duration were sufficient to ruin
civilizations of the past). It
may seem paradoxical to talk of the relative stability of climate in
the last 8,000 years, and then mention the devastation caused by
decades-long droughts and El Nińos, but the latter are truly small
stuff compared to the abrupt climate flips with their synchronous
Why the relative stability for eight millennia?
No one knows, yet. But
we know it is unusual, and see no reason why it should persist.
missed seeing the original ice cores yesterday, but one
industrial-scale freezer with backup generator is pretty much like
another. Being summer,
almost everyone is out at North GRIP, the new site whose cores might
resolve the conflict between the two Greenland Summit cores about the
cold-and-dry centuries during the last warm period 125,000 years ago.
This new site at 75°N 42°W is about 315 km north-northwest of
the pair of summit sites, picked because radar soundings indicated that
the Eemian layers ought to be well above the distortions caused by
bedrock. They’re using a
big C-130 Hercules to fly in equipment and supplies.
(They even hauled in a big road grader to groom the runway!)
I’d have enjoyed seeing the “cold hard data” but you
can’t actually see much in the ice cores without a lot of
measure the hydrogen and oxygen isotopes to infer air temperatures at
the time the snow fell, and the dust particles give a nice indication
of the dusty periods (much of the dust was kicked up far away, in the
Gobi Desert, rather than from sources closer to Greenland).
And they measure air bubbles trapped inside the ice, giving them
a nice look at carbon dioxide, nitrous oxide, and methane
concentrations back through prehistory, and how they co-vary with
There’s a lot of useful information (“Data that makes a
difference” is one definition of information) in one of those cores,
whose cumulative length is about 3 km
(Rich Alley refers to this as the “Two-Mile Time Machine”).
Then comes the hard job of making organized bodies of knowledge
out of mere information, creating a retrospectroscope.
I spent a week listening to 80 paleoclimatoligists and climate
modelers argue about the interpretation of the data from ice and
sediment cores, how it eliminated some proposed explanations for what
was driving the changes in temperature and rainfall, and how it
suggested other possible explanations.
From solid bodies of knowledge sometimes comes wisdom, so you
realize what civilization should avoid and what we might do to help
whiplash climate changes were easily one of the biggest
scientific shocks of the last decade.
You may not have heard of them, however, even though they were
frequently mentioned in the news columns of the major scientific
journals such as Science and Nature, with catchy titles
such as “How ice age climate got the shakes.”
That’s where the popular press usually picks up the important
new science stories. What
happened to delay the news getting out to general audiences about such
an important story, and for more than ten years? The science reporters for the media were surely reading those
Science and Nature summaries.
But the popular press has a pigeonhole labeled “Greenhouse,”
into which all climate change news is forced – and then either put
down as “old news” or given the tired old “Has it really
started yet?” treatment. Even
worse is to assume the cooling will cancel out the warming, just as you
turn up the cold water if the hot water supply suddenly improves in
mid-shower. They have
repeatedly missed the point that, rather than “averaging out,” a
gradual warming may trigger a dangerous cooling – a backlash.
A climate full of whiplash coolings and warmings isn’t a
prediction but a newly-acquired historical perspective.
What happened many times before is likely to happen again,
unless we figure out a means of stabilizing the climate.
The data are very convincing.
Deep in the ice sheets of Greenland are annual layers that
record what the atmospheric gases and the air temperature were like
over each of the last 250,000 years.
That’s the period of the last two major ice ages.
A given year’s snowfall is compacted into ice during the
ensuing years, trapping air bubbles, and so paleoclimate researchers
have been able to glimpse ancient climates in some detail.
Water falling as snow on Greenland carries an isotopic
“fingerprint” of what the temperature was like en route, thanks to
oxygen-18 being a little heavier than the usual oxygen-16.
Counting those tree-ring-like layers in the ice cores shows that
cooling came on abruptly. In
the first few years the climate could cool as much as it did during the
Little Ice Age, with tenfold greater changes over the next decade or
Though we tend to talk most about the temperature, the ice cores
also reveal at lot about ancient dust, carbon dioxide, swamp-gas
salt spray. In the layers
from recent years, you find such anthropogenic gases as the
chlorofluorocarbons used for aerosol spray cans, refrigerators, and
foam containers. When
winter storms have been violent, usually because of an increased
contrast between the average temperature of land and ocean surfaces,
you find a lot more dust and salt spray (indicates wind over oceans) in
the cores. They
confirm the story told by the oxygen isotopes for temperature:
things change very quickly.
In just several years, every terrestrial mammal, including
humans, was likely in big trouble from the droughts and the ensuing
forest fires. Populations
The onset of a cooling lasting centuries is as quick as a
drought. Indeed it’s the
accompanying drought that we’d probably complain about first, along
with unusually cold winters in Europe, were another climate flip to
begin next year. The following summer, we’d complain about the smoke from
the unrelenting forest fires, and about an even more severe crop
failure. The sun would not
so much set as disappear into a red haze above a murky horizon.
that’s the “what” of the ice ages, the major puzzles for
which we need a “how” mechanism and a “why” long-term
perspective that illuminates the predecessors which evolved into the
present state of things. The
how turns out to mostly be about salt, and how you get rid of buildups.
The longest-term why concerns such things as the perpetual
westerlies at the higher southern latitudes and continental drift
moving Antarctica out of their way (the ring of westerlies probably
started about 12 million years ago).
The shorter-term why has to do with the Old Panama Canal getting
dammed up. Maybe before
the end of this nine-hour flight, I’ll say something about “What
next?” and the prospects for intervening to postpone the next abrupt
The flight from Copenhagen to Seattle is, in contrast, as timeless as heaven is reputed to be. We left about noon and we’ll arrive about noon, too – having crossed nine time zones in nine hours of flying. (This plane’s just the place to listen to a CD of Laurie Anderson singing about the little clock on your VCR, always blinking twelve noon because you never figured out how to get in there and change it.) A window seat in the stratosphere certainly provides a better place from which to contemplate the world than most philosophers ever had.
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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,