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/78N.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
of expensive undertakings of the past, I see one far below, though it
hardly qualifies as a pyramid or a cathedral.
It’s the first sign of civilization since that lonely weather
station on Jan Mayen Island. We are now over the fjord system of northwest Greenland, just
950 miles south of the North Pole, and there’s a big airport below,
considerably larger than anything that the Eskimo lifestyle requires.
It is surely Thule, the U.S. air force base built in the summer
of 1951 to refuel long-range bombers in the scary days of Josef Stalin. American air force personnel out of favor with their
superiors worried about being assigned to Thule, just as their Cold War
counterparts worried about being sent to Siberia.
Thule is well above the Arctic Circle and the sun simply
doesn’t rise for long months.
The most detailed evidence for climate change once came from
Camp Century, a research camp atop the ice cap near Thule.
Ice cores from there in 1966, and others from the Dye 3 site in
southern Greenland in 1981, suggested some abrupt changes but various
objections were raised to parts of the data.
So two independent deep cores were drilled between 1989 and 1993
starting at the highest part of Greenland’s ice cap down south, where
the sideways movement of ice was minimal and unlikely to contaminate
the chronology (the “GISP2” American team led by Pieter Grootes
drilled 30 km from the “GRIP” European team led by Willi
Dansgaard). And this way,
they also got the longest possible record, striking bedrock after
seeing ice that was 250,000 years old, the date of the warm period
before last, the one that melted much of Greenland’s ice sheet.
Having two nearby cores has allowed confidence in the
interpretation of the last 115,000 years, though the lack of agreement
between them during the last warm period 130,000 to 117,000 years ago
has raised some questions about how much fluctuation there was in that
warm period. There is
little doubt that there was at least one “mid-Eemian cooling event”
about 122,000 years ago, because it is seen in ocean-floor cores, coral
reefs, and the pollen cores of European lakes. It lasted perhaps
fifteen centuries before recovering to temperatures even warmer than
But only the ice core methods have the resolution to say if
there were other still-briefer events – and so the paleoclimate
researchers are now looking at sites in North Greenland and Antarctica
that might be capable of yielding undisturbed cores.
As I mentioned back in Copenhagen, they’re now drilling North
GRIP. It’s particularly important data to get, as you want to
know how the whiplashes work in a warm period like today’s, when
Greenland and Iceland are the main source of North Atlantic meltwater
and the sea level is at a modern height.
Speaking of meltwater, the results of floods may linger for
decades, simply because the ocean waters mix so slowly.
Nothing on this multidecade scale happens in the atmospheric
circulation. But the
oceans are sluggish enough so that we might say that they have a
Great Salinity Anomaly, a pool of water less salty than normal,
was tracked moving around the North Atlantic between 1968 and 1982.
It probably started in the Arctic Ocean, but it nicely
illustrates what would happen if a fjord dam collapsed as it is derived
from about 500 times as much unsalted water as that released by the
four-month accumulation at Russell Fjord.
Two years after it was detected off Greenland’s east coast, it
arrived in the Labrador Sea, where it prevented the usual salt sinking. By 1971-1972 the semi-salty blob was off Newfoundland.
It then crossed the Atlantic and passed near the Shetland
Islands around 1976. From
there it was carried northward by the warm Norwegian Current, whereupon
some of it swung west again to arrive off Greenland’s east coast –
where it had started its inch-per-second journey.
This counter-clockwise “sub-polar gyre” thus has a loop time
of more than a dozen years.
So freshwater blobs drift, sometimes causing major trouble, and
a Greenland flood thus has the potential for some years afterward to
stop the enormous heat transfer that keeps the North Atlantic Current
going strong. It shows us
the lingering effects of a freshwater excess from any source.
They circulate, causing trouble in a way that any engine
mechanic can instantly appreciate, from water blobs in the fuel line.
There is potentially a longer version of this as well, since the
conveyor belt takes about a thousand years to cycle.
Remember that fresh water freezes more easily than the ocean’s
usual salt water, so if downwelling fails locally, a puddle of fresher
water may form from the rains or floods – and it will freeze more
easily, preventing the winds from doing their evaporation job that
might restart the downwelling. Once downwelling stops over a large area, it is hard to
restart. Sea ice may form,
stopping the winds from stirring the surface, stopping evaporation,
leaving the passing winds as cool and dry as when they left Canada.
abrupt coolings of the past may have been triggered by various
causes at various times, and I will devote the next several pages to
sketching them out for the fans of causation.
Causes in the real world tend to come in layers, and there is an
underlying 1,500-year cycle in North Atlantic ice rafting that goes
back 100,000 years, in both our warm period and the antecedent ice age
. The isotope record of
the last 12,000 years of solar activity parallels it, suggesting a
solar pacemaker. Stephen
Porter sees the 1,500-year cycle in the layers of wind-blown silts in
called loess, even in warm
periods like today. The
latest episode in the 1,500-year cycle was probably the Medieval Warm
Period that started about 1,200 years ago, dipping down into the Little
, and now warming again – though augmented by the extraordinary
twentieth-century carbon dioxide rise, now well outside the bounds of
carbon dioxide variation during the prior three ice ages.
This minor solar cycle may run all the time.
But its effects may be amplified by sea salt. Waves bury air and when bubbles rise to the surface, they
pop, spraying salt into the atmosphere.
The salt ions provide nuclei for condensation, augmenting the
number of water droplets that form in the air from the water vapor
. This changes the
brightness of the atmosphere (many small droplets reflect more than a
few larger droplets made from the same amount of water vapor).
From this, you get more clouds.
Sunlight returned to space doesn’t heat the earth.
The cool-and-dry periods have a lot more of these airborne
sodium and chloride ions. High
winds not only produce more salt spray, but much of the dust falling on
came from near large salt
flats in China
. So there is no lack of
windy sources for elevating salt.
Salt is involved in another way as well:
there ought to be a slow buildup of salinity in the Atlantic
Ocean (the Atlantic exports more fresh water as rain than it gets back
from rivers returning to the Atlantic) – unless, of course, long
loopy currents between Atlantic and Pacific Oceans serve to even things
The geochemist Wallace Broecker, to whom we owe a number of the
important ideas about abrupt climate change, speculates that there is a
chain of causation starting with more far-northern winter sea ice and
(because of the ice preventing the winds from stirring up waves and
evaporation and salt excess) thereby fewer sinks for the Gulf Stream
, which in turn diminishes the big conveyor
loop of currents linking
the North Atlantic to the Pacific. This decreases the export of excess salt to the Pacific; it
also makes the high latitudes colder and the tropics warmer.
This temperature contrast makes for more high intensity storms,
which leads to more atmospheric dust and sea salts, which reflects more
sunlight back into space and cools the earth further.
As salt builds up in the Atlantic, it makes it easier to restart
the far-northern downwellings.
So this system could oscillate, producing a slow cycle in Atlantic salinity and far-northern ice, even in warm-and-wet times like today. But much stronger albedo effects (a measure of how much sunlight is simply reflected back out into space) might be generated by the high winds of the glacial era, giving 10°C temperature changes rather than the 1°C excursion of the Little Ice Age . Since the 1,500-year period remains relatively unchanged, whether in an ice age or warm period, the sun’s variability – though small – is an attractive explanation. Many people are looking for what amplifies the sun’s effects on earth climate.
Something like albedo might explain the 1,500-year cycle without
a two-state mechanism; the D-O flips might arise from an abrupt
atmospheric reorganization triggered by accumulating regional
differences in sea surface temperatures.
(Anyone who works on nerve and muscle excitability will
immediately recognize an analogy: we often see a modulation of the amplitude of analog
processes so that they occasionally cross a threshold to trigger a
flip.) I like it, and it
really doesn't conflict with Broecker’s earlier idea that atmospheric
water vapor levels changed (presumably secondary to an atmospheric
cellular circulation change). A
one-two punch of albedo enhancement followed by less tropical
evaporation might carry things from a warm-and-wet regime to a
What, someone asked by email, causes carbon dioxide to decrease
during the cold-and-dry periods? Dust
storms. While weathering
of rock is a long-term answer, it’s probably not what causes the
variations within an ice age. But
soil, liberated by vegetation failures in the cool-and-dry mode’s
droughts, is carried by the winds into the ocean, fertilizing the
plankton and so a whole food chain. A lot of carbon falls to the ocean floor in the form of fecal
pellets, and so is taken out of circulation for a while.
this seems like a lot of “causes,” it is.
In biology, we have even longer Rube Goldberg chains of
medicine, this gives us multiple opportunities to interrupt a chain of
The staged “causes” of an abrupt cooling are going to
include proximate causes – the coup de grace to the old
climate might come from too much rain in the wrong place from a
particularly severe El Niño finally tripping an atmospheric cell
reorganization. But some
near-proximate cause might set the stage, say the loss of crucial
whirlpools that sink surface waters with high efficiency.
That in turn might be set up by the albedo or whatever else
amplifies the 1,500-year cycle, thus poising the climate near the
tipping point, setting things up for a quick slide.
Underlying that would be whatever creates the 1,500-year cycles
(maybe Broecker’s salt oscillator, maybe something else).
And you’d expect even more ultimate “causes” such as
continental drift making the North Atlantic narrow enough that the
Norwegian Current is “boxed in,” unable to turn right from the
Coriolis effect, so it goes even farther north, hugging Europe’s
continental shelf – or uplifting Panama 3 million years ago to close
off the old tropical route for equalizing salinity.
A greenhouse warming may reduce the 1,500-year cycle aspects,
but this provides us with no comfort regarding future prospects since a
warming can shortcut the usual circuit, bypassing the usual
stage-setting by amplification of the 1,500-year cycle.
No one is saying that greenhouse warming was the typical cause
of climate flips – but clearly Greenland still has the fresh water
supply that is capable of suppressing flushing.
Clearly some of Greenland’s fresh water will be released by
any further warming of our climate, adding to the sea surface dilution
produced by global warming’s increased rainfall.
This assessment hinges on several things.
First, that the oceanographers are correct in their present
hypothesis, that a persistent failure of late winter sinking of the
ocean surface near Greenland and Iceland is a likely cause of most of
the abrupt cooling episodes. And
second, that massive continental ice sheets (as existed over Canada and
Scandinavia, when most of the abrupt coolings occurred) aren’t needed
to provide the necessary dilution of the salty surface – that rain or
Greenland’s meltwater will do, as in that abrupt cooling of the last
warm period which occurred when the two continental ice sheets were
Let me try to summarize the state of affairs before I speculate
a little, based on what I know from somewhat analogous nonlinear
systems seen in biophysics and neurobiology that also flip.
have come a long way from the chimplike ancestor 5-6 million
years ago. And we’ve
come a long way in this e‑seminar, up through our ancestral
climates and changing ways of making a living.
We’ve seen both the gradual and the abrupt.
Where possible, I’ve tried to sneak in some of the mechanistic
aspects, whether vertical curtains of air rising into the heavens or
salt-heavy cold water sinking to the abyss.
But we’re now up to the present, and wondering what the future will bring. I’ll try to break the news gently, but what it boils down to is that the future is not an exact science. Neither is medicine. You’re always having to act on incomplete data, because the failure to act can prove fatal.
Of this much we’re sure:
global climate flip-flops have frequently happened in the past, and
they’re likely to happen again.
It’s also clear that sufficient global warming could trigger
an abrupt cooling in at least two ways – by increasing high-latitude
rainfall or by melting Greenland’s ice, both of which could put
enough fresh water onto the ocean surface to suppress flushing.
To that you can probably add a third, capping evaporation with
Further investigation might lead to revisions in such
mechanistic explanations, but the result of adding fresh water to the
ocean surface is pretty standard physics.
In the four decades of subsequent research, Henry Stommel’s
theory has only been enhanced, not seriously challenged.
Up to this point in the abrupt climate change story, none of the
broad conclusions is particularly speculative.
But to address how all these nonlinear mechanisms fit together
– and what we might do to stabilize the climate – will require some
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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,