Is the earth's climate changing?

But there are other periodicities. A spinning top does not keep its axis in a constant direction; it wobbles. If it had a flashlight built into its axis, it would trace a circle on the ceiling. The earth's axis now points to the North Star, but that is not where it pointed when Hipparchus of Rhodes first noted the wobble in about 120 B.C. Scientists, who do not use words like "wobble" (for everybody understands them), call this motion precession; it takes about 25,700 years to complete one wobble-circle. Actually, it is not pure snobbism that makes scientists eschew such words, for there is a lot of wobbling going on (as can be found under precession and nutation in a good encyclopedia), and the different types need to be distinguished. But it is the basic type of precession (and slight changes in the distance from the sun) that very probably cause the recurrent ice ages. And there is about 9,000 years to go before it gets cold again from that quarter.

Another periodicity is that of solar activity, which moves from relative quiescence to high activity in the form of sunspots, flares, protuberances, and other forms of gigantic upheaval, and back again to low activity. The period is not regular, but has an average of 11 years. (These are half-cycles, if you want to quibble; the solar goo in the spots whirls clockwise for 11 years, and reverses for the next half-cycle.)

What does that do to the weather? For an answer, scientists have been looking to tree rings, which record annual rainfall by their widths, but until recently the evidence supplied by the rings has been very confusing.