Clues of climate and the Bible's seven lean years

When archaeologists sift through the debris of a vanished culture, they should consider the ancient climate. It can shed light on the bygone habitat and give plausibility to old myths. It can also give a useful perspective on our own climatically uncertain times.

Take the biblical tale of Joseph. The famous seven-year cycle of feast and famine appears to be one of Egypt's regular routines, according to Dmitri Kondrashov, Yizhak Feliks, and Michael Ghil at the University of California at Los Angeles.

The scientists used new statistical techniques to fill in gaps in 1,300 years of Nile River water levels recorded from AD 622 through 1922. They then searched these data for climatically significant cycles. Their results, reported in Geophysical Research Letters, suggest "quite strongly" that North Atlantic circulation influences East African climate. The scientists add that "most strikingly," their analysis picked out a North Atlantic driven seven-year cycle of high and low river levels that is "possibly related to the biblical cycle of lean and fat years."

They also note the need for Joseph-like wisdom today. They explain that the "fairly sharp shifts" in river levels that have recurred in the past 1,300 years "support concerns about the possible effect of climate shifts in the not-so-distant future."

The ancient Mayans on the Yucatan Peninsula could have used such wisdom. Their once-flourishing civilization collapsed between AD 750 and 950. Many archaeologists suspect that prolonged drought was the precipitating cause.

Now a remarkable geological record that tracks the relevant climate on a bimonthly basis strongly reinforces that conclusion.

It lies in the sediments in the Cariaco Basin off Venezuela. Undisturbed by currents or burrowing animals, the sediment layers reflect the long-term history of rainfall in exquisite detail, according to Larry Peterson at the University of Miami and Gerald Haug at the University of Potsdam in Germany. This history applies to Yucatan as well as northern South America.

Weather in the region is dominated by the Intertropical Convergence Zone, where the trade winds meet and drive rain-giving atmospheric convection. This zone moves north and south with the seasons. When it is north of Venezuela and Yucatan, the rains come. When it's south, the weather in these lands is dry. Different types of sediment run off the land during the wet and dry seasons.

A study of these sediments reveals that the rain-giving convergence zone stayed farther south than normal during the Mayan decline, explain Professors Peterson and Haug in the current issue of American Scientist. This resulted in several periods of multiyear droughts that correlate nicely with archaeologists' estimates of when various Mayan centers collapsed.

The Mayan land rests on a limestone foundation laced with underground channels and caverns. Little water remains at the surface, so Mayans either saved rainwater in leakproof reservoirs or tapped the underground water table. The civilization's population growth and land development apparently had stretched these water resources to the limit and couldn't stand the additional strain brought by a prolonged drought.

Many parts of the world, including North America, are stretching their fresh water resources to the limit. Even without man-made global warming, the climate could throw another dust bowl at us that we might find very hard to endure.