When my composure returned I made a feeble attempt to
explain away what I was seeing as mere coincidence. Where Moraga's weaving
had 72 strong millennial cycles, the Castile had 76, a small but possibly
important difference. The precession cycle was not as well defined in her
piece, but clearly, the long, 100,000-year cycle of orbital eccentricity
seemed to be an organizing theme in both creations.
All climate records and geophysical time series oscillate
at several frequencies and these same climate cycles were common at other
times during Earth's history. The 100,000-year eccentricity cycle, for example,
was the pacemaker for episodes of glaciation during the last great Ice Age.
A millennial cycle of about 2500 years dominated the advance and retreat
of glaciers and timed the release of flotillas of icebergs into the North
Atlantic. The same cycle brought the rains that caused the rise and fall
of lakes in western North America and Africa.
These climate cycles we already knew. But how did they wind up in a tapestry
that looked so much like the climate changes the Castile?