The tropics throw their weight around.

Despite its frosty name, the ice age was not a period of unrelenting cold. Global temperatures in the glacial epoch swung up and down radically from one millennium to the next. During the warmest of these spans, vast armadas of icebergs sailed across the northern Atlantic Ocean for reasons that scientists have not yet discovered.

Most researchers have searched for answers in the far north, the site of the iceberg flotillas, which are called Heinrich events. But the explanation for Heinrich events may actually lie much farther south, propose Andrew McIntyre and Barbara Molfino of the Lamont-Doherty Earth Observatory in Palisades, N.Y.

McIntyre and Molfino came to this conclusion after studying seafloor sediments extracted from drill holes in the equatorial Atlantic. They examined fossilized remnants of the marine alga Florisphaera profunda, which grows near the ocean surface but below most other algae. Because F. profunda flourishes whenever westward-moving equatorial winds slacken, the scientists used the alga as a record of past winds.

The sediment records revealed that all Heinrich events during the last 45,000 years occurred during times of extremely weak equatorial winds. When winds were strong or normal, warm water in the Caribbean and Gulf of Mexico was largely contained in those regions. When the winds died, the warm water flooded into the Atlantic, where it was carried north by the Gulf Stream. Once it reached the far north, this water could have caused the great discharges of ice into the Atlantic, the researchers proposed at the meeting and in the Dec. 13, 1996 Science.

What controls the equatorial winds? Because the winds appear to follow specific cycles over thousands of years, McIntyre and Molfino hypothesize that slow changes in Earth's orbit drive the wind variations and therefore deserve ultimate blame for the Heinrich events.