In the Dec. 13 issue of Science, Andrew McIntyre and Barbara Molfino, paleoclimatologists at Lamont-Doherty, reported evidence that strong westward-blowing trade winds died down along the equator at about the same times that icebergs flotillas sailed across the North Atlantic during the last ice age -- in approximately the same 7,000- to 10,000-year cycles.
Ever since these cyclic iceberg pulses were first discovered in the early 1990s, scientists have tried to understand what caused them and have wondered whether Earth's climate system can shift so dramatically again in modern times.
The Lamont-Doherty scientists theorize that the equatorial winds relaxed periodically, allowing a large reservoir of warm tropical waters --which had been pushed into the Caribbean Sea and the Gulf of Mexico by the winds -- to flow back eastward. The waters were whisked by the Gulf Stream to the north, where they warmed the North Atlantic region and triggered melting along the edges of the massive ice sheets that covered the Northern Hemisphere.
According to McIntyre and Molfino's theory, the strength of Atlantic equatorial trade winds over long intervals of time is governed by Earth's orbital cycle, which alters the seasonal intensity of solar radiation reaching the planet. The researchers found that over the past 45,000 years, a suborbital rhythm of 8,400 years has produced variations in the strength of the tropical winds.
On Monday (Dec. 16) at the American Geophysical Union's fall meeting in San Francisco, McIntyre will present his new theory, which has already stirred spirited debate among scientists who have reviewed the Science report before publication. Some are skeptical that changes in the tropics could be the primary cause of climate shifts in the North Atlantic region; they have looked for possible causes in the northern hemisphere, including ice sheet dynamics and shifts in ocean circulation. Others question McIntyre and Molfino's proposed link between climate and Earth's orbital cycles, or their mechanism involving the release of a warm water pool in the Atlantic. Nevertheless, the new research does show a provocative link between climate phenomena at the top and middle of the globe.
Supported by grants from the National Science Foundation, McIntyre and Molfino analyzed three ocean sediment cores taken near the equator just below the bulge of Africa. They looked for the fossilized plates of a marine alga called Florisphaera profunda. Unlike other algae, F. profunda lives further below the surface where sunlight levels are lower. When westward trade winds are strong, deeper nutrient-filled waters come nearly to the surface to replace the waters blown westward, and all algae thrive. But when the winds diminish, surface waters don't receive as many nutrients, surface algae don't fare well and the ratio of F. profunda to its cousins goes up.
The Lamont-Doherty scientists found relatively high levels of F. profunda every 8,4000 years, in a regular cycle stretching back 45,000 years. The high points of F. profunda along the equator correlated with times when other scientists have found high levels of pulverized rock in North Atlantic ocean sediment cores. This rock, which had been frozen into the bases of glaciers and carried out to sea in icebergs, is the signature of the great iceberg launchings. Scientists have yet to determine whether these iceberg pulses are a response to climate changes, or a result of them -- or both.
In previous research, McIntyre and Molfino had shown that F. profunda levels correlate with tropical wind strength. Even today, relative F. profunda levels decrease seasonally when the tropical winds diminish. Seasonally, a temperature contrast is created, which draws colder northern hemispheric air into a gyre over the northern part of Africa, where it is heated and blown out again along the equator. This, along with the planet's eastward rotation, creates the strong westward trade winds that push water warmed by the sun toward the Caribbean and Gulf of Mexico.
The scientists say the same phenomenon occurs on a much larger scale, when an orbital cycle, called the precession of the equinoxes, brings the Earth to a point farthest away from the sun at a time when the northern hemisphere is also titled away from the sun in winter. When those coincide, northern winters are colder, the temperature contrast between the two regions is greater, the trade winds blow harder, and relative F. profunda levels decrease.
"Our concept is that the seasonal wind-forced phenomena we see today and the long-term precessional phenomena act the same way," McIntyre said in an interview. "Our hypothesis is based on the premise that the equatorial bulge -- which receives the most solar radiation of anywhere on Earth -- is the central boiler that drives heat and energy around the planet. Small variations in received energy per unit area in the tropics translate into major changes in the total energy of the Earth's system."
McIntyre and Molfino say that whenever trade winds diminish, whether seasonally or on a larger orbitally-induced time scale, it creates a scenario analogous to a long-term El Niño -- the equatorial Pacific Ocean phenomenon in which westward winds slacken to allow a warm pool pushed toward Australia to move back eastward toward South America. Today, trade winds push water into the Caribbean and Gulf of Mexico to a height 3 feet greater than in the eastern Atlantic and 6 feet greater than in the North Atlantic. When the trade winds die down, warm waters stored in this reservoir are released. Unlike the Pacific, the waters don't flow back along the equator. Instead, the warm waters are swept up into the Gulf Stream current and delivered to the North Atlantic.