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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.
Webbed Reprint Collection
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
73°N    95°W     10,400m ASL
                        Somerset Island, NW Territories
North Poles aren’t what they used to be


If this were 1831, the year that the North Magnetic Pole was first located on the Boothia Peninsula, it would be off to the left of the plane.  But it has moved since then, and is now off to the right, about a thousand kilometers to the northwest of where it was in 1831.

     The North Magnetic Pole is the point toward which all those decelerating charged particles from the solar wind converge to cause the aurora borealis.  From space, the aurora looks like a fountain, spewing light – making the Magnetic Pole look considerably more exciting than the Geographical North Pole.

     No northern lights for us today.  They’re there 24 hours a day, but it’s still noon and we can’t see them for all the summer sunlight that selectively scatters off the thin air to produce a blue sky.  Now, in the winter, when it’s most dark up here, you stand a better chance of seeing the northern lights converging on the magnetic pole.

     And the Geographic North Pole isn’t what it used to be, either.


Back about fifty million years ago, there was no sea ice at the North Pole, just open water like all the rest of the deep oceans.  But more recently, Arctic explorers have always been able to plant a flagpole at the North Pole.

     In the first summer of the new millennium, however, there was no place to stand at the North Pole – unless you stood up in a boat at 90°N.  (These days, one holds aloft for the camera that flag of high-tech - a little GPS unit reading 89.99999°N and with the longitude wildly varying, each time the boat rocks.)

     Radar ice-thickness estimates of the Arctic Sea ice showed that it had been thinning for years, just as they had also shown that the northern coastal glaciers of Greenland were thinning.

  But few guessed that you’d be able to take a Russian icebreaker all the way to 90°N and find open water in the summer, with no ice to stand on.

     That it can happen at all is, of course, thanks to the Norwegian Current carrying water from the North Atlantic Current up to the major downwelling whirlpools at 76°N.  But that it is so much warmer at 90°N in recent decades is likely due to things warming up more generally, bringing greenhouse predictions uncomfortably to mind.

     Some scientists argue that the instrumental record (weather balloons and such) is equivocal on warming in the twentieth century.  Given all of the ice reduction data from recent decades, this just tells you that we’ve been measuring temperature in the wrong places, or weighting the data wrong.  Nature, obviously, is sensitive to some other set of temperatures than the ones we’ve been measuring – and Nature is, generally, what counts.  To cite the twentieth-century instrumental record, without mentioning all the melting – as even the occasional scientist may do before general audiences – is often suspected of being special pleading, trying to confuse nonscientists with carefully selected facts, what lawyers with a losing case sometimes attempt to do with juries.  The radar measurements in the Arctic tell the warming story in a way that will be hard for the global-warming skeptics to refute.

     There’s no place left to stand at the North Pole.  Of course, the open-ocean gap, a fissure in the sea ice called a polynya, will move around, temporarily restoring some footing at exactly 90°N, but the point remains:  Beware of thin ice, and its implications for the world to the south of 90°N.

  [The following paragraph has been corrected from the first printing.]

You can see a mode of operation in the Sahara as the monsoons penetrate farther and farther into arid areas.  Just within decades, less dust blows offshore because the Sahara got major amounts of grass:  small addit­ional changes create major consequences.  This started about 14,800 years ago and soon there were lakes and archaeological sites all around the Sahara .  Then the arid conditions abruptly reappeared about 5,500 years ago as the particularly hot Sahara summers moderated.

     As with the self-perpetuating drought cycle (page 159 ), establishing (and disestablishing) vegetation is thought to have a lot to do with creating regional modes of climate.  The big problem is in figuring out what can cause worldwide modes.  There are some ideas, such as Gulf Stream failure, for how we might flip suddenly from warm-and-wet into cool-and-dry in a matter of decades.  But sudden warming is the real puzzle; no one yet has a good idea for how things can flip back, and even more quickly.

Climate in the past has been wildly variable, with larger, faster changes than anything industrial or agricultural humans have ever faced . . . . Climate can be rather stable if nothing is causing it to change, but when the climate is “pushed” or forced to change, it often jumps suddenly to very different conditions, rather than changing gradually. 
— Richard B. Alley, 2000

     Even the tropics cool down by about 5°C during an abrupt cooling, and it is hard to imagine what in the past could have disturbed the whole earth’s climate on this scale.  We must look at arriving sunlight and departing light and heat, not merely regional shifts on earth, to account for changes in the temperature balance.  Increasing amounts of sea ice and clouds could reflect more sunlight back into space, but Wally Broecker suggests that a major greenhouse gas is disturbed by the far-north failure of the salt conveyor, and that this affects the amount of heat retained.

     In Broecker’s view, failures of salt flushing cause a worldwide rearrangement of ocean currents, resulting in – and this is the speculative part – less evaporation from the tropics.  That, in turn, makes the air drier.  Because water vapor is the most plentiful greenhouse gas, this decrease in average humidity would cool things globally.  As Broecker has said, “If you wanted to cool the planet by 5°C and could magically alter the water vapor content of the atmosphere, a 30 percent decrease would do the job.”

     Just as an El Niño produces a hotter Equator in the Pacific Ocean and generates more atmospheric convection, so there might be a subnormal mode that decreases heat, convection, and evaporation.  In reconfiguring three cells per hemisphere into some other mode of general circulation, it might incidentally reduce the amount of tropical evaporation and thus shift us into a subnormal amount of greenhouse warming.  (Be careful what you wish for!)


To see how ocean circulation might affect greenhouse gases we must try to account quantitatively for important nonlinearities, ones that allow little nudges to provoke great responses - like the typical on-off light switch.  Our usual gradualist extrapolations of the present state of affairs are more like dimmer switches.  All metaphors break down somewhere, and the gradualist scenarios seem particularly likely to fail.  Let me try some nonlinear metaphors to better approximate the climate mechanisms.

     The modern world is full of objects and systems that exhibit bistable modes, with thresholds for flipping.  Door latches suddenly give way.  A gentle pull on a trigger may be ineffective, but there comes a pressure that will suddenly fire the gun.  Household thermostats tend to activate heating or cooling mechanisms abruptly – also an example of a system that pushes back.

     We must be careful not to think of an abrupt cooling in response to global warming as just another self-regulatory device, a control system for cooling things down when it gets too hot.  The scale of the response will be far beyond the bounds of regulation – more like when excess warming triggers fire extinguishers in the ceiling, ruining the contents of the room while cooling them down.  Though combating global warming is obviously on the agenda for preventing a cold flip, we could easily be blindsided by stability problems if we allow global warming per se to remain the main focus of our climate-change efforts.

     Multiple consequences of a single cause are something we can think about, if reminded.  (“You can’t do just one thing.”)  What’s far harder for us is to think about multiple causes at the same time.  We can think about one cause and its most obvious consequences, but factoring in a few more simultaneously-acting causes usually requires much effort.  Even experienced scientists and historians find it challenging.  The politicians and press who read only “executive summaries” of climate change reports routinely oversimplify what they read, and the public gets even less of whatever wisdom was originally there.  And since nothing expensive gets done unless the politicians feel they have the people behind them, the world’s largest democracies may fail to act in time.


We are in a raft, gliding down a river, toward a waterfall.  We have a map but are uncertain of our location and hence are unsure of the distance to the waterfall.  Some of us are getting nervous and wish to land immediately; others insist that we can continue safely for several more hours.  A few are enjoying the ride so much that they deny that there is any imminent danger although the map clearly shows a waterfall.  A debate ensues but even though the accelerating currents make it increasingly difficult to land safely, we fail to agree on an appropriate time to leave the river.  How do we avoid a disaster?

         To decide on appropriate action we have to address two questions:  How far is the waterfall, and when should we get out of the water?  The first is a scientific question; the second is not.  The first question, in principle, has a definite, unambiguous answer.  The second, which in effect is a political question, requires compromises.

- George Philander, Is the Temperature Rising?, 1998 (Princeton University Press, 1998), p.3.



Notes and References
(this chapter
corresponds to 
pages 269 to 274 of the printed book)

Copyright ©2002 by
William H. Calvin

The nonvirtual book is
available from
or direct from
 University of Chicago Press

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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