The surge in commodity prices as illustrated by the Continuous Commodity Index caught many analysts by surprise earlier this year. Anyone that is a student of the long cycles in the broad commodity indices shouldn’t have been caught off guard.

There’s a long history of these indices putting in a minor low or a bottom every three years. This one stands out particularly vividly because the decline into the low had been so long and persistent. The strength of the rebound out of that low might also have been more dynamic with the bottom coming toward the end of its window.

The first tip of a change in the trend of the index came from the action in the crude oil market, a commodity that also has this...