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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
60.0°N    10.7°E     10,000m ASL
                        High above Oslo, Norway
The ocean has a conveyor belt


Glaciers descending from the north hereabouts lead most people to think that the center of the ice cap must have been the North Pole (that is, after all, the way it works at the South Pole).  Wrong.  It took a while after the discovery of the ice ages before anyone realized that glaciers don’t form over open ocean:  the pack ice at the North Pole is a few meters thick.  Terra firma is four thousand meters farther down.  The bottom of the Arctic Ocean is as deep as the Atlantic.  It has features such as Nansen Basin, underwater ridges such as the Nansen Cordillera.  They’re both are named for the Norwegian scientist Fridtjof Nansen, an early Arctic explorer and, incidentally, one of the first neurobiologists.

     As a naval officer sitting next to me on another airplane flight once remarked, “If anyone ever builds a house there, they’ll get a surprise if they dig a basement!” (his submarine had punched through the ice, and he had gone walking on top of the world).  But his trip was a few decades ago when the average sea ice depth was 3.1 meters; now it is down to 1.7 meters and models suggest that it will continue thinning and retreating with our global warming.

     Although it may snow up on top, the floating ice seldom gets more than a few meters thick.  To build up ice to the height of a mountain range, as happens during an ice age, requires a solid foundation such as Greenland.  Down at the South Pole, there is a whole continent (9.3 percent of the Earth’s land surface) to house tall piles of ice.  When an ice age really gets going, then the northern hemisphere has a lot more land on which to house ice sheets than the southern.  Typically, there have been big piles, housed on the continents either side of Greenland.  Greenland itself still has a pile two miles high.  The Canadian one created a 1000-meter-deep ice shelf pushing out into the Arctic on the continental shelf off northern Alaska.

     The sea ice itself is of interest.  It reflects 60 to 90 percent of the arriving sunlight back into space, thus keeping the earth somewhat cooler.  At present, the warm Norwegian Current prevents a lot of sea ice.  In the winter, one now sees sea ice along the eastern coast of Greenland above 70°N, but not along the Norwegian coast.  Without that warm influence in the sub-Arctic, sea ice may come down past Norway to France – and that’s a considerable percentage increase in whiteness, reflecting back summer sunlight that might help re-warm things.  Like the self-perpetuating droughts (page 158 ), there are positive feedbacks for ice that make things even worse.


There are big signs of civilization down below.  Civilizations accumulate knowledge, so we now know a lot about what has been going on, what has made us what we are.  We puzzle over oddities, such as the climate of Europe – which is far warmer than, by rights, it ought to be.

     Oslo is anomalous, when you look around the globe for other major cities so far from the equator.  In the southern hemisphere, latitude 60° is halfway between the tip of South America and Antarctica, down in the Drake Passage where strong westerly winds circle the world, stirring up prodigious wave heights.  Though the long summer twilight of the higher latitudes may be nice, the flip side includes those dim winter days where the sun makes only a brief midday appearance low in the sky, barely edging the thermometer upward.

     The populous parts of the United States and Canada are mostly between the latitudes of 30° and 45°, whereas the populous parts of Europe are ten to fifteen degrees farther north. “Southerly” Rome lies near the same latitude, 42°N, as “northerly” Chicago – and the most northerly major city in Asia is Beijing, near 40°N.  London and Paris are close to the 49°N line that goes through Hudson Bay and, west of the Great Lakes, separates the United States from Canada. Berlin is up at 52°, Copenhagen and Moscow at about 56°. Oslo is nearly at 60°N, as are Stockholm, Helsinki, and St. Petersburg; continue due east and you’ll encounter Anchorage.

     Europe’s climate, obviously, is not like that of North America or Asia at the same latitudes.  For Europe to be as agriculturally productive as it is (it supports more than twice the population of the United States and Canada), all those cold, dry winds that blow eastward across the North Atlantic from Canada must somehow be warmed up.  The job is done by warm water flowing north from the tropics, variously called the Gulf Stream and, when nearing Ireland, the North Atlantic Current.  This warm water then flows up the Norwegian coast, with a westward branch warming Greenland’s tip, at 60°N.  It keeps northern Europe about 5-10°C warmer in the winter than comparable latitudes elsewhere – except, of course, when it fails. 


The whiplash climate changes are appallingly sudden and painful.  And remember that they can happen even in the midst of warm climates like our present one.  There was one cooling back during the previous warm period about 122,000 years ago that lasted for a dozen centuries before rewarming finally occurred.  There was another abrupt cooling at 117,000 years, but with no recovery.  That’s how the last warm period ended – suddenly (ice per se returned more gradually).  There have been dozens of whiplashes since then, but the warm flickers up out of ice age temperatures never lasted more than a few centuries, and were never as warm as now.

     During the last big abrupt cooling, 12,900 years ago, Europe cooled down to Siberian temperatures within a decade (about ten-fold greater than in the Little Ice Age), the rainfall likely dropped by half, and fierce winter storms whipped a lot of dust into the atmosphere.  Such conditions lasted for over 1,300 years, whereupon things warmed back up, even more suddenly.  The dust settled and the warm rains returned, again within a decade.  

     Not only was Europe affected but also, to everyone’s surprise, the rest of the habitable world appeared to be chilled about the same time.  Tropical swamps decrease their production of methane at the same time that Europe cools, and the Gobi Desert whips much more dust into the air.  When this happens something big, with worldwide connections, must be switching into a new mode of operation.

     The North Atlantic Current is certainly something big, with the flow of about a hundred Amazon Rivers (an amount equal to all the rain falling on earth).  And (to give you a little preview of the How, coming up) it sometimes changes its route dramatically, much as a bus route can be abbreviated into a shorter loop.  Its effects are clearly global too, inasmuch as it is part of a long “salt conveyor” current that extends through the southern oceans into the Pacific.

Declare the past, 
diagnose the present, 
foretell the future.
  - Hippocrates

     Even a decade ago, we didn’t know much about the climate flips; we simply thought that climate creep was starting to occur and that we needed to prevent greenhouse gases from slowly ramping up the heat.  That too is still true, but we now know that the biggest threat from global warming is that it could trigger a far worse abrupt cooling, something akin to accidentally shifting into low gear when cruising at a high speed.

     I hope never to see a failure of the northernmost loop of the North Atlantic Current, because the result would be a population crash that would take much of civilization with it, all within a decade.  Ways to postpone such a climatic shift are conceivable, however – old-fashioned dam-and-ditch construction in critical locations might even work. Although we can’t do much about everyday weather, we may nonetheless be able to stabilize the climate enough to postpone an abrupt cooling.  It all depends on developing some wisdom from all the new knowledge.


Sudden onset, sudden recovery – this is why I use the words “whiplash” and “flip-flop” to describe these climate changes.  They are utterly unlike the changes that one would expect from accumulating CO2 or the setting adrift of ice shelves from Antarctica.  Change arising from some sources, such as volcanic eruptions or the West Antarctic Ice Sheet collapsing, may be abrupt – but they don’t flip back just as quickly, centuries later.

     Temperature records suggest that there is some grand mechanism underlying all of this, and that globally it has two major states, warm-and-wet and cool-and-dry-and-windy-and-dusty.  In discussing the ice ages there is a tendency to think of warm as good – and therefore of warming as better. Alas, further warming might well kick us out of the warm-and-wet mode.

     As for the How behind all the transitions, one naturally thinks of the sun first.  The sun’s variability, even though small enough so you’d think it insignificant, does correlate with some monsoon changes.  But there is likely a chain of “causes,” perhaps different for the abrupt warmings than for the abrupt coolings.

     The likeliest reason for the abrupt coolings is an intermittent plumbing problem in the North Atlantic Ocean, one that seems to trigger a major rearrangement of the atmospheric circulation.  North-south ocean currents help to redistribute equatorial heat into the temperate zones, supplementing the heat transfer by winds.  When the warm currents penetrate farther than usual into the northern seas, they help to melt the sea ice that is reflecting a lot of sunlight back into space, and so the earth becomes warmer.  Eventually that helps to melt ice sheets elsewhere.  The major ice sheets take more than 10,000 years to disappear, a time scale a thousand times slower than some of the flip-flops in temperature.

     The warm-and-wet mode of global climate seems to involve ocean currents that deliver an extraordinary amount of heat to the vicinity of Iceland and Greenland.  Like bus routes or conveyor belts, ocean currents must have a return loop.  Unlike most ocean currents, the North Atlantic Current has a return loop that runs deep beneath the ocean surface.  Huge amounts of seawater sink at known downwelling sites every winter, with the water heading south after it reaches the bottom.  When that annual flushing fails for some years, the northern end of the usual conveyor belt stops moving and so heat stops flowing so far north – and apparently we’re popped back into the cool-and-dry mode.

     I have no trouble imagining this circuit because it is just like the Number 12 bus route in Seattle.  A trolley bus heads north out of downtown and turns around near my home on northern Capitol Hill, returning to make a southerly loop through the downtown streets.  But some Number 12 buses instead turn around at the bottom of Capitol Hill.  Indeed, that near-north shortcut is where all of them turn around when ice closes the streets on Capitol Hill.  Well, the warm-and-wet mode of climate corresponds to using the far-north turnaround and the cool-dry-dusty-windy mode to using only that near-north turnaround.  (Imagine, if you like, the return path as taking the bus in a tunnel beneath the street.)

     When ice puts a lid on the far-northern seas, the conveyor belt carrying heat north and salt south turns around well below Iceland, rather than in the far-north sinking sites near the northeast and southwest coasts of Greenland. 

What if our climate jumped to something totally unexpected?  What if you went to bed in slushy Chicago, but woke up with Atlanta's mild weather?  Or worse, what if your weather jumped back and forth between that of Chicago and Atlanta: a few years cold, a few years hot?  Such crazy climates would not doom humanity, but they could pose the most momentous physical challenge we have ever faced, with widespread crop failures and social disruption.

     Large, rapid, and widespread climate changes were common on Earth for most of the time for which we have good records, but were absent during the few critical millennia when humans developed agriculture and industry.  While our ancestors were spearing woolly mammoths and painting cave walls, the climate was wobbling wildly.  A few centuries of warm, wet, calm climate alternated with a few centuries of cold, dry, windy weather. The climate jumped between cold and warm not over centuries, but in as little as a single year.

- Richard B. Alley, 2000



Notes and References
(this chapter
corresponds to 
pages 229 to 236 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