Ancient drought and tropical glaciers collide.

• Position: Earth Science

Two abrupt and drastic climate events, 700 years apart and more than 45 centuries ago, are teasing scientists who now are trying to use ancient records to predict future world climate. The events--one, a massive, long-lived drought believed to have dried large portions of Africa and Asia and, the other, a rapid cooling that accelerated the growth of tropical glaciers--left signals in ice cores and other geologic records from around the world.

Lonnie Thompson, professor of earth sciences at Ohio State University, Columbus, and researcher with the Byrd Polar Research Center, has led more than 50 expeditions to drill cores through ice caps on some of the highest and most remote regions of the planet. He believes that the records from the tropical zones on Earth are the most revealing and "that the last 1,000 years are most critical from the perspective of looking at the future."

The first of the two tantalizing events is apparent in an ice core drilled in 1993 from a field in the Peruvian Andes called Huascaran. Within that core, researchers found a thick band of dust particles, most smaller than a micron in diameter, the concentration of which was perhaps 150 times greater than anywhere else in the core. That band dated back 4,500 years. "Dust that small can be transported great distances--the question is, where did it come from?" Thompson relates. "I believe that record accurately reflects drought conditions in Africa and the Middle East and that the dust was carried out across the Atlantic Ocean by the northeast trade winds, across the Amazon Basin, and deposited on the Huascaran ice cap."

Thompson explains that other re cords, including an ice core taken from glaciers atop Tanzania's Mt. Kilimanjaro, also show a dust event dating to a time when there was substantive drying up of lakes in Africa. He divulges that it is the only such huge event that the ice core records show for the past 17,000 years.

The other mystery surrounds a major cooling event that Thompson believes happened about 700 years earlier. During a 2002 expedition to the Quelccaye ice cap in Peru, the largest tropical ice field in the world, Thompson and colleagues discovered patches of ancient wetland plants that had been exposed as the edge of the ice cap retreated. When carbon-dated, the...