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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
71.0°N      8.9°W     10,000m ASL
                        Jan Mayen Island
Flushing the Gulf Stream


Down below is Jan Mayen Island, featuring a volcano called Beeren Berg whose erect cone sweeps upward like the tip of Japan’s Fujiyama.  There’s also a panhandle, a long ridge to the southwest of the volcanic cone.  There’s a Norwegian weather station down there, and not much else.  But it would make a nice base for a dozen ships to monitor those downwellings year-round.  The Greenland Gyre is just to the north, where the warm waters swing away from the ice-free Norwegian coast and head west, sinking here and there.

     Besides the downwellings of the Greenland Sea, there are others in the central Labrador Sea between Canada and the southern tip of Greenland.  Without such far-north downwellings, the world’s climate is utterly different, far colder and drier in many important places around the world, though Europe takes the hardest hit.

     Downwellings are hard to see, as only a few sink the surface waters via obvious whirlpools.  River runners sometimes have to cope with downwellings in turbulent waters.  If you see one side of your boat being sucked under, you shout “Highside!” and everyone throws their weight to the other side, trying to counterbalance.  Waves that appear to flow beneath other waves are a tip-off that you’re close to a downwelling.  But those are little localized downwellings, and their cause is simply turbulence, usually from trying to force a square river into a round hole.  The downwellings of the Greenland Sea and the Labrador Sea, usually in late winter, are likely much larger and even include giant 10-15 km wide whirlpools.

     You know they’ve happened, however hard they are to see, because towing an instrument package on a long cable through the ocean depths will occasionally reveal a great blob of water with lots of dissolved oxygen and other atmospheric gases, quite unlike the surrounding waters in the depths.  Oceanographers can follow a blob around for years.  Remnants of the great 1988 downwelling in the western Labrador Sea were, eight years later, nearing Scotland at intermediate depths – it’s a downwelling that wasn’t salt-heavy enough to sink all of the way to the abyss.

     “Almost all the way down” is what often happens to the northern end of the North Atlantic Current.  The reason for the surface instability isn’t just the cooling, though that helps.  It’s also because of all the evaporation promoted by the tropical heat when it reaches the high latitudes.  The cold dry winds blowing eastward off Canada eagerly evaporate the warm surface water of the North Atlantic Current, and leave it heavy with excess salt.  Eventually, late in the winter, the surface layer sinks in a big way.  The heaviest blobs sink to the abyss and flow south, down near the bottom of the Atlantic.

     What is so significant for our present climate is that such profound salt sinking makes room for warm water to flow much farther north than it might otherwise do.  This “extension of the bus route” produces a 30 percent bonus of heat beyond that provided by direct sunlight to these seas, accounting for the mild winters downwind in Western Europe.

     If it isn’t obvious why salt sinking attracts warm water further north than otherwise, an analogy may help.  A colleague once discovered that the plumbers had inadvertently gotten their pipes crossed in a university building:  the tank for a toilet was being refilled with hot water.  This was not immediately apparent because the tank would ordinarily cool down to room temperature between flushes.  But in periods of heavy use, it was noticed that the toilet tank was acting as a radiator, nicely warming up the room on cold winter days.  We joked about inventing an automatic flushing device, triggered by the water cooling down to room temperature, so as to attract more hot water into the room.

     That’s an imperfect analogy for how the northern loop of the North Atlantic circulation is maintained and why Europe stays anomalously warm – but only so long as there is frequent flushing to replace the cooled Greenland Sea water with warm water attracted in from elsewhere.

     Nothing like this happens in the Pacific Ocean.  And sometimes failures of flushing occur in the North Atlantic.





Notes and References
(this chapter
corresponds to 
pages 243 to 245 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