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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
75°N     40°W     10,500m ASL
                        Atop Greenland
Why melting can cause cooling


Floods of fresh water might also prevent flushing, long before the average salinity was altered year-round.  In the mid-1990s, we saw floods burst forth from the ice sheets of Iceland.  The mid-Atlantic Ridge goes through Iceland, and there are a number of volcanos and hot springs.  Some are under the ice sheets and, when they heat up, they melt ice near the bottom of the ice sheet.  Eventually, when the pocket of meltwater becomes large enough, its roof may collapse and the steam plume alerts the local pilots.  But even before then, the hot water may find its way downhill, melting the ice in its path, and finally burst forth as a sudden flood, sweeping down the valleys into the sea.

     The fjords of Greenland, because they are occasionally dammed up, offer additional and even more dramatic examples of the possibilities for freshwater floods.  Fjords are long, narrow canyons, little arms of the sea reaching many miles inland; they were scoured by great glaciers when the sea level was lower.  Greenland’s east coast has a profusion of fjords between 70°N and 76°N, including one that is the world’s biggest.  If blocked by ice dams, fjords make perfect reservoirs for meltwater.

     Glaciers pushing out into the ocean usually break off in chunks.  Whole sections of a glacier, lifted up by the tides, may snap off at the “hinge” and become icebergs.  But sometimes a glacial surge will act like an avalanche that blocks a road, as happened when Alaska’s Hubbard glacier surged into the base of the Y-shaped Russell fjord in May of 1986.  Its snout ran into the opposite side, blocking the fjord with an ice dam.  Any meltwater coming in behind the dam stayed there.  At Russell Fjord, it was a serious matter for the seals remaining behind the dam, as they are used to salt water – and all the additional water coming in was unsalted, diluting the original sea water.  A lake formed, rising higher and higher – up to the height of an eight-story building.  That’s why I look for bathtub rings and trimlines on the sides of Greenland fjords, just in case some floods of past centuries went unreported.


Eventually such ice dams break, with spectacular results.  Once the dam is breached, the rushing waters erode an ever wider and deeper path.  Thus the entire lake can empty quickly.  Five months after the ice dam at the Russell fjord formed, it started leaking about midnight and dumped a cubic mile of fresh water in the next twenty hours.  Since the North Pacific Ocean lacks the downwelling so characteristic of the northernmost Atlantic, no great harm was done.  Such an outburst flood from a fjord in Norway, Iceland, or Greenland might be a serious matter.

     Worse, such a flood can break other ice dams in the vicinity.  The Greenland fjords are long, with many side branches.  As water rushes toward the sea, it also has a backwash up other branches, where it can weaken any ice dams there.  So when warming conditions have produced glacial surges damming a number of nearby fjords, there is a domino effect:  all of the meltwater reservoirs can be dumped within a few days.  Were such a freshwater flood to occur from either the Greenland or Iceland coastlines, it might well prevent the flushing that makes room for more tropical water to flow northward.

     Even ordinary flatland lakes sometimes find a new path to the sea, emptying over a century’s time.  The most recent abrupt cooling, a half-sized one about 8,200 years ago, appears to be due to a meltwater lake inland in Labrador about the size of present-day Lake Superior, which found a path into Hudson Bay, its waters thus coming into the Labrador Sea from Hudson Strait.  This cooling was brief, with an exponential recovery over several centuries (unlike the usual sudden warming of the classic flip-flop events), and the flooding perhaps affected only half of the usual flushing sites (the Greenland Sea sites might have continued to flush the Gulf Stream).  There was also a major outburst flood (from the huge meltwater Lake Agassiz) that came down the St. Lawrence, just before the Younger Dryas, 12,900 years ago.

     We’re nearing the northwest coast of Greenland.  Somewhere south of here is that ex-lake in Labrador.  But we’re far north of it now; take a Copenhagen-Chicago flight if you want to see the scene of the most recent major climate crasher.  You wouldn’t think that rain falling in the ocean – nice, clean drinkable water – could cause so much trouble.  Of course, no one thought that water would turn out to be comprised of two gases, either.


Wallace Broecker is worried about the world's health. Not so much about the fever of global warming but about a sudden chill.  For more than a decade, the marine geochemist has been fretting over the possibility that a world warming in a strengthening greenhouse might suffer a heart attack, of sorts: a sudden failure to pump vital heat-carrying fluids to remote corners of Earth.  If greenhouse warming shut down the globe-girdling current that sweeps heat into the northern North Atlantic Ocean, he fears, much of Eurasia could within years be plunged into a deep chill.

- Richard A. Kerr, 1998



Notes and References
(this chapter
corresponds to 
pages 257 to 260 of the printed book)

Copyright ©2002 by
William H. Calvin

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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