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/74N.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
Flying above the clouds often presents an interesting picture when there are mountains below. Out of the sea of undulating white clouds, mountain peaks stick up like islands. Greenland looks like that, even on a cloudless day – but the great white mass between the occasional punctuations is not a fluffy cloud layer but a massive ice sheet, miles deep. In places this frozen fresh water descends from the highlands in a wavy staircase, looking far more massive and magisterial than any alpine glacier.
Twenty thousand years ago a similar ice sheet lay atop the
Baltic Sea and the land surrounding it.
Another sat on Hudson Bay, and reached as far west as the
foothills of the Rocky Mountains – where it pushed, head to head,
against ice coming down from the Rockies.
These northern ice sheets were almost as high as Greenland’s
mountains, obstacles sufficient to force even the jet stream to make a
The ice ages provide a valuable perspective on how climates
change, a view of the times in which our brief warm interlude is
situated. Figuring out the
mechanisms for ocean circulation modes depends on the evidence from
paleoclimate, and on the stabilities seen via the great computer
simulations. Most of the
whiplash climate changes of the past were during icy periods that had
ice sheets in Canada and Scandinavia.
Now only Greenland’s ice remains, but the abrupt cooling in
the last warm period shows that a flip can occur in situations much
like the present one. What
could possibly halt the salt-conveyor belt that brings tropical heat so
much farther north and limits the formation of ice sheets?
Oceanographers are busy studying present-day failures of annual
flushing, which give some perspective on the catastrophic failures of
could possibly stop the salty conveyor that brings tropical heat
so much further north, and limits the formation of these ice sheets?
It’s not an idle question, and flushing failures are the
obvious candidate. But,
just as in medicine where we have to ask what the natural history of a
disease is (natural ups and downs can fool you into thinking that a
treatment is working when it’s just happenstance), we need to know
what the natural history of flushing failure is.
In the Labrador Sea, flushing failed during the 1970s, was
strong again by 1990, and then declined.
In the Greenland Sea over the 1980s salt sinking declined by 80
percent. Obviously, local
failures can occur without catastrophe – it’s a question of how
often and how widespread the failures are, and whether they occur
simultaneously in both the Greenland Sea and in the Labrador Sea.
Large-scale flushing at both those sites is certainly a highly
variable process, and perhaps a somewhat fragile one as well.
And in the absence of a flushing mechanism to sink cooled
surface waters and send them southward in the Atlantic, additional warm
waters do not flow as far north to replenish the supply.
There are a few obvious precursors to flushing failure.
One is diminished wind chill, when winds aren’t as strong as
usual, or as cold, or as dry – as is the case in the Labrador Sea
during the negative phase of the North Atlantic Oscillation.
This decade-scale shift in the atmospheric-circulation pattern
over the North Atlantic, from the Azores to Greenland, strongly affects
wintertime downwind. At
the same time that the Labrador Sea gets a lessening of the strong
winds that aid salt sinking, Europe gets particularly cold winters and
Another precursor is more floating ice than usual, which reduces
the amount of ocean surface exposed to the winds, in turn reducing
evaporation. The sea ice
in the major flushing sites is only partly indigenous – much comes
sailing down from farther north to replace what melts.
Fresher water freezes more easily.
Thus an ice lid is one possible candidate for what occasionally
shuts down the highly efficient north-of-sixty-degrees portion of the
Gulf Stream circulation. Arctic
cold could thus cause a much more general worldwide cooling, abruptly.
Yet another precursor, as Henry Stommel suggested in 1961, would
be the addition of fresh water to the ocean surface, diluting the
salt-heavy surface waters before they became unstable enough to start
sinking. Two mechanisms
can be seen when piecing together the past:
icebergs melting in the Heinrich events, and the freshwater
floods seen just before the Younger Dryas and the 8,200 year event.
But there is a third way to get enough fresh water layered on
the ocean surface: more
rain falling into the North Atlantic.
Yes, I know that it sounds like carrying coals to Newcastle. But, at least in certain regions such as the Greenland and
Labrador Seas, rain falling into the ocean is a serious problem.
Lots more rain falling is now what is predicted to happen from
global warming. Even that
which falls on land at higher latitudes tends to be carried toward the
sea on north-flowing rivers, and so into the already somewhat less
salty Arctic Ocean. It can
stop salt flushing. Whereas
the Norwegian coast stays ice-free now, an addition of fresh water
would allow it to freeze more easily and so cap the oceans, preventing
the wind-driven evaporation that makes the sea water dense enough to
There is also a great deal of unsalted water in Greenland’s
glaciers, just uphill from the major salt sinks. The last time an
abrupt cooling occurred was in the midst of global warming.
Many ice sheets had already half melted, dumping a lot of fresh
water into the ocean. The
Greenland Sea got a lot of re-freshing from the east side of Greenland,
making the out-of-the-Arctic Greenland Current even less salty.
The Labrador Sea
not only got the meltwater from Greenland’s west coast but also from
the Hudson’s Strait flow that came off of the great mountain of ice
which accumulated atop Hudson Bay and Labrador.
In the models of greenhouse warming, the gradual dilution of
northern oceans by rain eventually slows the flushing until it is about
25 percent below normal. Further
dilution and flushing fails. And
that doesn’t even consider the melting of the ice sheets (or
additional perturbations like El Niño or the North Atlantic
could steadily dilute the ocean surface, but years’ worth of melting
can also be dumped into the ocean in just one day.
There are already some indications that the Greenland Sea route
for Gulf Stream warming is in trouble.
A fifty-year trend of decline in the deep cold water in one
branch of the conveyor’s return loop has just been discovered,
suggesting (if not compensated elsewhere) a decline in the warm water
surface delivery of more than 20 percent.
stories scientists tell are not simply bedtime tales. They place us in the world, and they can force us to alter
the way we think and what we do.
Thomas Levenson, 1989
living in this modern world who do not know the
basic facts that determine their very existence, functioning,
and surroundings are living in a dream world. Such persons are, in a very real sense, not sane.
We [scientists] . . . should do what we can, or we shall be
pushed out of the common culture. The lab remains our workplace, but it must not become our
Gerald Holton, 1996
Greenland's Maze of Fjords
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All of my books are on the web. The six out-of-print books
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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,