Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 102

studies now narrows this change to just three years . It was a hemi-
spheric climate revolution, and it was just like switching a radiator on.
Literally. Over 1,000 years before, to trigger the beginning of Younger
Dryas times, the radiator appears to have switched off just as abruptly.
What happened?
The great oceanographer Wallace 'Wally' Broecker has spent a
good deal of his career puzzling this problem through. The obvious
radiator in that part of the world is the Gulf Stream. Stop that flowing
somehow, and those 1.3 petawatts of heat it carries would also be
stopped. But the Gulf Stream is enormous , as we have seen: how can
you block such a thing? No dam could hold it back, and its immediate
source, in the Gulf of Mexico, lies close to the tropics where tempera-
ture changes in the Ice Ages were small. However, the Gulf Stream is
part of a circulatory system that encircles the Earth—so there may be
other places where a spanner might be tossed into the works to dis-
rupt its mechanism. Wally Broecker reckoned that the roots of this
system, in the far north, might be vulnerable.
This is where the salty Atlantic water is chilled so much that it
becomes sufficiently dense to sink, the downpouring water forming
the North Atlantic Deep Water, at one major beginning of the oceanic
conveyor belt. If the Atlantic brine becomes less salty, though, then
the water will not sink, no matter how cold it gets. And if that hap-
pens, this part of the global oceanic belt can stop.
Making the Atlantic less salty on regular occasions turned out to be
commonplace. The great North American ice sheets, as they waxed
and waned, formed barriers to hold back great lakes of meltwater
(larger by far than the current Great Lakes) on the North American
continent. At times the ice barriers broke down, and the lake water,
suddenly released, flowed in brief but biblical-scale torrents that
scoured the landscape into deep gully systems and then gushed out
into the North Atlantic. Sufficient amounts could be released in a