The Hopi tribesmen of northern Arizona are born meteorologists.

When snake weed blooms in the spring, they know they’re in for bumper summer rains. When the desert stays largely barren, they prepare for drought. As far back as tribal lore goes, Hopi farmers have sustained themselves and their crops by diligently reading their arid mesa surroundings.

This summer, however, their millennia-old forecasting techniques failed them, and not for the first time in recent years. The weeds sprouted in great numbers in April. The usual rains in August did not come at all. Were it not for local grocery stores and the seed stockpiles they maintain in anticipation of the occasional bad year, many Hopi might well have gone hungry.

“These indicators have always