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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
55.620°N    12.656°E     2m ASL
                       The plane where it’s always noon
How ice age climate got the shakes


Archaeologists 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 periods.

     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.

The 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.

 Our 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 comparison.

     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 worldwide effects.

     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.

 I 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 instrumentation.  They 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 temperature.

     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 stabilize climate.


The 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 two.

     Though we tend to talk most about the temperature, the ice cores also reveal at lot about ancient dust, carbon dioxide, swamp-gas methane, and 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 crash.

     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.

So 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 climate change.

     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.



Notes and References
(this chapter
corresponds to 
pages 221 to 228 of the printed book)

Copyright ©2002 by
William H. Calvin

The nonvirtual book is
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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