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/65N.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
leaving Europe’s shoreline now, though it would have extended
farther west back in the icy periods when the sea level was a lot
lower because ice sheets locked up so much water. Except for the ice, you could have built a forty-story hotel
at the seashore about 16,000 years ago; by 12,000 years ago, the waves
would have been lapping at the windows halfway up.
By 8,000 years ago, they’d have been over the top.
A number of river valleys, carved in glacial times, were
drowned when the sea level rose, such as the Thames below London, San
Francisco Bay, and Chesapeake Bay.
The warm waters of the North Atlantic Current now flow up the
west coast of Scotland and then continue all the way up the Norwegian
coast; Oslo’s harbor is ice-free all year.
The warm current splits, some heading west toward Greenland,
and the rest continuing north into the Greenland Sea, keeping a large
area free of sea ice. I
mentioned that sea ice can cap the ocean surface and prevent the winds
from evaporating water and leaving salt behind. And that, along with rain and meltwaters, can interfere with
sinking the surface waters. Let
me warn you that the “how” of things is going to be pretty salty.
If you want to see cold sinking in action, pour some hot coffee
into a tall transparent mug. Wait
for a few minutes for the motion to settle.
Then get some very cold cream (not half-and-half) and, using a
spoon lowered to its rim in the coffee, gently pour the cream into the
spoon, allowing it to overflow the rim and layer out onto the surface
of the coffee. If you do
this right, as my friend Dan Hartline showed me a decade ago, you will
soon be rewarded by the sight of a column of cream plunging to the
bottom. Though cream often floats because of its fat content, its
density increases when cold, enough to sink through hot coffee.
Indeed, the density buildup from salt excess and evaporative
cooling is what causes the North Atlantic surface waters to sink so
dramatically. (Unlike the
thermohaline circulation, Dan’s
trick has a reversing feature: Once
the cream warms down on the bottom, it will come geysering up to the
top again in some very pretty turbulent plumes, finally ending up in a
layer on top.)
Besides downwelling, you can create upwelling, by blowing
gently from one side of the rim of the coffee cup.
You’ll push a wave of coffee across the surface, making room
for deeper coffee to rise to the surface nearest your lips.
Winds cause oceans to upwell in many places, such as off the
north coast of South America.
waters are flushed regularly, even in lakes. Twice a year they sink, carrying their load of atmospheric
gases downward. That’s
because water density changes with temperature.
Fresh water is densest at about 4°C (a typical
refrigerator setting; anything that you take out of the refrigerator,
whether you place it on the kitchen counter or move it to the freezer,
is going to expand a little). A
lake surface cooling down in the autumn will eventually sink into the
less dense (because warmer) waters below, mixing things up.
Seawater is more complicated, not expanding much below about 5°C
but with the salt content becoming very important in determining
whether water floats or sinks. Because
surface water that evaporates leaves nearly all of its salt behind,
the surface becomes saltier – and if it becomes more dense than the
underlying water, it sinks, sometimes in great blobs that do not mix
very well with underlying waters, just like Dan’s cream.
The fact that excess salt is flushed from surface waters has
global implications, some of them recognized two centuries ago.
Salt circulates, because evaporation up north causes it to sink
and be carried south by deep currents.
That the cold waters of the ocean depths came from the Arctic
was posited in 1797 by the Anglo-American physicist Sir Benjamin
Thompson (the Count Rumford that I mentioned back in Germany
), who also posited that, if merely to compensate, there would
have to be a warmer northbound current as well.
By 1961 the oceanographer Henry Stommel was beginning to worry
that these warming currents might stop flowing if too much fresh water
was added to the surface of the northern seas.
By 1987 the geochemist Wallace Broecker was piecing together
the paleoclimatic flip-flops with the salt-circulation story and
warning that small nudges to our climate might produce “unpleasant
surprises in the greenhouse.”
are not well mixed at any time.
Like a half-beaten cake mix, with strands of egg still visible,
the ocean has a lot of blobs and streams within it. When there has been a lot of evaporation, surface waters are
saltier than usual. Sometimes
they sink to considerable depths without much mixing, as happens in
the eastern Mediterranean where the surface gets salty because
evaporation exceeds the input from rivers.
The salty bottom water flows west and out the bottom of the
Strait of Gibraltar into the Atlantic Ocean.
This water is about 10 percent saltier than the ocean’s
average, and so they sink into the depths of the Atlantic.
A nice little Amazon-sized waterfall flows over the ridge that
connects Spain with Morocco, 800 feet below the surface of the Strait.
Another underwater ridge line stretches from Greenland to
Iceland and on to the Faeroe Islands and Scotland.
It, too, has a salty waterfall, which pours the hypersaline
bottom waters of the Greenland Sea and the Norwegian Sea south into
the lower levels of the North Atlantic Ocean.
This salty waterfall is more like thirty Amazon Rivers
combined. Why has the
eastern branch of it declined more than 20 percent in the last fifty
Indeed, why does it exist?
The cold dry winds blowing eastward off Canada evaporate the
surface waters of the North Atlantic Current, and leave behind all
their salt. In late
winter the heavy surface waters sink en masse.
These blobs, pushed down by annual repetitions of these
late-winter events, flow south, down near the bottom of the Atlantic.
The same thing happens in the Labrador Sea between Canada and
the southern tip of Greenland.
Salt sinking on such a grand scale in the Nordic Seas allows
warm water to flow much farther north than it might otherwise do.
It has been called the Nordic Seas heat pump.
Nothing like this happens in the Pacific Ocean (which is, in
consequence, about 5°C cooler), but the Pacific is
nonetheless affected, because the sink in the Nordic Seas is part of a
vast worldwide salt-conveyor belt.
Such a conveyor is needed because the Atlantic is saltier than
the Pacific (water which evaporates from the Atlantic is carried by
the trade winds across Central America to fall as rain in the
The Atlantic would be even saltier if it didn’t mix with the
Pacific, in long, loopy currents.
These carry the North Atlantic’s excess salt southward from
the bottom of the Atlantic, down into the southern oceans, and some
continues into the Pacific Ocean.
A round trip on a grocery-checkout conveyor belt takes less
than a minute. This
conveyor takes more than a thousand years to make a complete loop.
floor of the North Atlantic Ocean showing the major far-north
The Gulf Stream
can also sink at “near-north” sites near the bottom of the
picture, especially when floating ice caps the far-northern sinks.
else being equal, the Coriolis effect tends to make the major currents
turn towards the right in the Northern Hemisphere.
But shorelines and continental shelf walls may prevent right
turns, as when the Norwegian Current continues north until attracted
to the left by the sinking sites.
Gyres are counterclockwise, however; as surface waters are
attracted toward sinking sites, they will turn right.
the last million years, the pattern recorded in cores of Greenland ice
has occurred over and over: a long stagger into an ice age, a faster
stagger out of the ice age, a few millennia of stability, repeat.
The current stable interval is among the longest in the record.
Nature is thus likely to end our friendly climate, perhaps quite
soon – the Little Ice Age may have been the first unsteady step down
Richard B. Alley, 2000
climate record kept in ice and in sediment reveals that since the
invention of agriculture some 8,000 years ago, climate has remained
remarkably stable. By contrast, during the preceding 100,000 years, climate
underwent frequent, very large, and often extremely abrupt shifts.
Furthermore, these shifts occurred in lockstep across the globe.
They seem to be telling us that Earth’s climate system has
several distinct and quite different modes of operation and that it can
jump from one of these modes to another in a matter of a decade or two.
So far, we know of only one element of the climate system which
has multiple modes of operation: the
oceans’ thermohaline circulation.
Numerous model simulations reveal that this circulation is quite
sensitive to the freshwater budget in the high-latitude regions where
deep waters form.
S. Broecker, 1997
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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,