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/76N.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
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William H. Calvin
University of Washington
of fresh water might also prevent flushing, long before the average
salinity was altered year-round. In
the mid-1990s, we saw floods burst forth from the ice sheets of
Iceland. The mid-Atlantic
Ridge goes through Iceland, and there are a number of volcanos and hot
springs. Some are under
the ice sheets and, when they heat up, they melt ice near the bottom of
the ice sheet. Eventually,
when the pocket of meltwater becomes large enough, its roof may
collapse and the steam plume alerts the local pilots.
But even before then, the hot water may find its way downhill,
melting the ice in its path, and finally burst forth as a sudden flood,
sweeping down the valleys into the sea.
The fjords of Greenland, because they are occasionally dammed
up, offer additional
and even more dramatic examples of the possibilities for
freshwater floods. Fjords
are long, narrow canyons, little arms of the sea reaching many miles
inland; they were scoured by great glaciers when the sea level was
lower. Greenland’s east
coast has a profusion of fjords between 70°N and 76°N, including one
that is the world’s biggest. If
blocked by ice dams, fjords make perfect reservoirs for meltwater.
Glaciers pushing out into the ocean usually break off in chunks.
Whole sections of a glacier, lifted up by the tides, may snap
off at the “hinge” and become icebergs.
But sometimes a glacial surge will act like an avalanche that
blocks a road, as happened when Alaska’s Hubbard glacier surged into
the base of the Y-shaped Russell fjord in May of 1986.
Its snout ran into the opposite side, blocking the fjord with an
ice dam. Any meltwater
coming in behind the dam stayed there.
At Russell Fjord, it was a serious matter for the seals
remaining behind the dam, as they are used to salt water – and all
the additional water coming in was unsalted, diluting the original sea
water. A lake formed,
rising higher and higher – up to the height of an eight-story
building. That’s why I
look for bathtub rings and trimlines on the sides of Greenland fjords,
just in case some floods of past centuries went unreported.
Eventually such ice dams break, with spectacular results. Once the dam is breached, the rushing waters erode an ever wider and deeper path. Thus the entire lake can empty quickly. Five months after the ice dam at the Russell fjord formed, it started leaking about midnight and dumped a cubic mile of fresh water in the next twenty hours. Since the North Pacific Ocean lacks the downwelling so characteristic of the northernmost Atlantic, no great harm was done. Such an outburst flood from a fjord in Norway, Iceland, or Greenland might be a serious matter.
Worse, such a flood can break other ice dams in the vicinity. The Greenland fjords are long, with many side branches.
As water rushes toward the sea, it also has a backwash up other
branches, where it can weaken any ice dams there.
So when warming conditions have produced glacial surges damming
a number of nearby fjords, there is a domino effect:
all of the meltwater reservoirs can be dumped within a few days.
Were such a freshwater flood to occur from either the Greenland
or Iceland coastlines, it might well prevent the flushing that makes
room for more tropical water to flow northward.
Even ordinary flatland lakes sometimes find a new path to the
sea, emptying over a century’s time.
The most recent abrupt cooling, a half-sized one about 8,200
years ago, appears to be due to a meltwater lake inland in Labrador
about the size of present-day Lake Superior, which found a path into
Hudson Bay, its waters thus coming into the Labrador Sea from Hudson
Strait. This cooling was
brief, with an exponential recovery over several centuries (unlike the
usual sudden warming of the classic flip-flop events), and the flooding
perhaps affected only half of the usual flushing sites (the Greenland
Sea sites might have continued to flush the Gulf Stream).
There was also a major outburst flood (from the huge meltwater
Lake Agassiz) that came down the St. Lawrence, just before the Younger
Dryas, 12,900 years ago.
We’re nearing the northwest coast of Greenland.
Somewhere south of here is that ex-lake in Labrador.
But we’re far north of it now; take a Copenhagen-Chicago
flight if you want to see the scene of the most recent major climate
crasher. You wouldn’t
think that rain falling in the ocean – nice, clean drinkable water
– could cause so much trouble. Of
course, no one thought that water would turn out to be comprised of two
Broecker is worried about the world's health. Not so much about the
fever of global warming
but about a sudden chill. For
more than a decade, the marine geochemist has been fretting over the
possibility that a world warming in a strengthening greenhouse might
suffer a heart attack, of sorts: a sudden failure to pump vital
heat-carrying fluids to remote corners of Earth.
If greenhouse warming shut
down the globe-girdling current that sweeps heat into the
northern North Atlantic Ocean, he fears, much of Eurasia could within
years be plunged into a deep chill.
A. Kerr, 1998
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The six out-of-print books
are again available via Authors Guild reprint editions,