Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 101

detailed history of the climate of the North Atlantic region, which
may be read from the chemical changes in the water molecules that
make up the ice layers. Reading this record from ice cores drilled
from the Greenland ice cap has shown that, in the approximately
100,000-year-long span of the last great glaciation, the temperature
seemed to flicker up and down about every 15,000 years. It took a lit-
tle while for the significance of this pattern to sink in, and for it to be
corroborated with the patchier record of climate change preserved in
ocean floor sediments and lake muds. The temperature change in
these flickers was not quite as great as that between full glacial and
interglacial phases. But it was quite enough to make vivid, life-or-
death differences to the early humans then hunting and gathering in
Europe. The climate swung from modestly warm to bitterly cold and
back again, each 50 human generations or so. The changes, too, could
be fast . That is clear from the transition that we know best: the most
recent one.
The latest of these abrupt swings between cold and warm marks
the end of the last great glaciation and the beginning of our warm
epoch, the Holocene, and it took place 11,600 years ago. The cold
phase is called the Younger Dryas, after a mountain plant called Dryas
octopetala , more commonly named the white dryad. It has pretty little
white flowers, which are unusual in its family (the Rosaceae) for
having eight petals. When the weather gets cold the white dryad
descends from the mountains on to the plains, and its pollen spreads
far and wide (to be detected in layers of pond mud, much later, by
scientists).
How long did it take to go from the bitter cold of the Young Dryas
to the warmth of the Holocene? This can only be worked out by look-
ing ever more closely at the strata, whether of ice or of mud, that cap-
tures this change. The warming used to be thought to be encompassed
in a couple of decades—within a human lifetime. One of the latest