William H. Calvin, A Brain for All Seasons: Human Evolution and Abrupt Climate Change (University of Chicago Press, 2002). See also http://WilliamCalvin.com/BrainForAllSeasons/72N.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
Available from amazon.com or University of Chicago Press.
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William H. Calvin
University of Washington
the what and where of our chattering climate
– and a little of the short-term how.
The sink in the Greenland Sea is simply part of a vast conveyor
belt that carries the North Atlantic’s excess salt to the less salty
southern oceans and eventually to the Pacific Ocean.
The most easily understood part of the return loop is where the
conveyor rises again in the southern oceans.
There is little land to get in the way of winds and wind-driven
waves at the southern latitudes that correspond to Europe in the north;
the tip of South America is about all there is, with a large gap until
reaching the Antarctic peninsula.
The winds reach in a complete circle, making for rough seas.
These westerly winds, thanks to the Coriolis effect, move the
surface waters a bit to the left (the so-called Ekman current), and
this allows waters from intermediate depths to rise to the surface.
Models show that the southern westerlies alone are sufficient to
start a conveyor running; what’s added up in the far north (and in
the narrow North Atlantic alone) is a highly efficient return loop of
sinking water at high northern altitudes.
This up-elevator for deep water (the “Ekman suction”) is the
southern end of my analogy to Seattle’s Number 12 bus route, where
the bus comes out of the tunnel to venture north again on the surface
streets. One of the
far-north turnarounds is right beneath me here in the Greenland Sea.
There’s another major far-north turnaround in the Labrador
Sea. The westbound branch
from Ireland, that hooks around the southern end of Greenland, sinks
off the southwest coast of Greenland.
Sometimes most of the turnarounds occur east of Greenland but, a
decade later, most of the action has shifted to the Labrador site.
All of the offshore Greenland sites qualify as far-north for
purposes of my bus route analogy, but they fail when the winds are
wrong or the floating ice caps them.
The near-north turnarounds, corresponding to cool-and-dry
climate, are in the oceans south of, say, 55°N where winter sea ice is
less likely (see the picture on page
The analogy to the Number 12 is incomplete because oceanographers imagine a third mode of global climate, seen only in the aftermath of Heinrich events, where the Hudson’s Bay ice mountain collapses and iceberg armadas sail from Canada across to France, dropping rocks off their bottoms all the way (which is how they were detected, as layers in ocean-floor cores that get thicker and thicker as you get nearer Hudson Strait). As the icebergs melt along the way, they spread a lot of fresh water over the ocean surface. This may eliminate even the near-north turnaround, making it really cold and dry up north. The far-southern oceans warmed somewhat during Heinrich events, what with the shutdown of the route for exporting heat.
Come to think of it, the Number 12 has a third mode too. The heavy snows from the 1996-1997 La Niña shut down both
the far-north and near-north turnarounds of the Number 12, leaving the
overheated buses accumulating downtown.
the heat conveyor is a process like that bus route, and not a
particular place; it’s a how that feels familiar to a
physiologist like me. And
the why of the conveyor belt is starting to fall into place, and
it has to do with what the conveyor carries south:
excess salt. The
trade winds blow rain clouds across Panama, carrying Atlantic water to
the Pacific without its salt. Without
a means of exporting the Atlantic’s excess salt to the southern
oceans or the Pacific, the Atlantic’s salinity would rise.
While there is a diluted stream into the South Atlantic that
comes around the tip of South America, the salinity problem is mostly
handled by the conveyor, as it carries extra-salty waters south from
the near-Greenland sinking sites. It’s a heat-north/salt-south conveyor.
There used to be another major way of handling the salt buildup
problem. If you’ve
forgotten about that express route from Pacific to Atlantic located in
the tropics, just think of it as the Panama Canal’s predecessor.
Continental drift connected North America to South America about
three million years ago, damming up the easy route for disposing of
excess salt and creating an instability via the salt buildup.
The dam, known as the Isthmus of Panama, may have been what
caused the ice ages to begin a short time later, simply because of the
This major change in ocean circulation, along with a climate
that had already been slowly cooling for millions of years, may be what
led to ice accumulation most of the time – but also to climatic
instability, with flips every few thousand years or so between
warm-and-wet and cool-and-dry. The
shallowing of the “Central American Seaway” probably started to
change ocean circulation at least 4.6 million years ago, intensifying
the Gulf Stream and bringing warm water to the high northern latitudes.
The evaporative cooling of the Gulf Stream added a lot of
moisture to the Northern Hemisphere. This intensification of the Gulf Stream may well have
postponed the onset of the ice ages for awhile. When the whole earth
gradually cooled a little more, this added moisture allowed ice
mountains to build up around the northern Atlantic.
While this Gulf Stream heat-and-moisture exchanger created a
warm-and-wet mode, things can still flicker back into cool-and-dry in
just a few years.
So a few million years ago was an instance of North Atlantic
warming setting up a worldwide cooling.
There is a more immediate prospect too, because there are
well-known mechanisms whereby our current global warming could trigger
a nasty flip to cool-and-dry.
my window on the world, I keep expecting to see a fleet of
oceanographic research ships down in the Greenland Sea, plumbing the
depths with improved instruments, trying to figure out the what-how-why
questions of this salty river. But
oceanographers and paleoclimatologists do not have big budgets, and
they have not been able to co-opt the naval ships of the NATO countries
to tow additional instrument packages around and launch their telemetry
We are over the season’s remnants of sea ice that extend down
the northeast coast of Greenland for much of the year.
They are being pushed out of the Arctic Ocean by a circulation
pattern that starts over near Siberia; occasionally you’ll see a
Siberian tree washed ashore along this coast of Greenland.
Hereabouts, the ice moves southward at about 15 km every day,
carried by the far less salty Greenland Current.
I can now see land ahead, fingers of bare red-orange rock with waves crashing over them, the forerunners of Greenland. We have come to the far shore of the Greenland Sea. The fjords are ahead, and I need to look for bathtub rings with my binoculars.
Greenland fjord with floating sea ice
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All of my books are on the web.
The six out-of-print books
are again available via Authors Guild reprint editions,