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/Oslo.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
Available from amazon.com or University of Chicago Press.
Webbed Reprint Collection|
William H. Calvin
University of Washington
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
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
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
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
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.
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.
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
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
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
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
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.
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.
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,
On to the NEXT CHAPTER
Copyright ©2002 by
Book's Table of Contents
Calvin Home Page
All of my books are on the web. The six out-of-print books
are again available via Authors Guild reprint editions,
All of my books are on the web.
The six out-of-print books
are again available via Authors Guild reprint editions,