NSF PR 96-82 - December 12, 1996
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Scientists Uncover Link between Tropical and North
Atlantic Climate Change
Scientists working under a grant from the National
Science Foundation (NSF) have unveiled a new theory
that winds in the tropics caused vast iceberg armadas
to surge across the North Atlantic.
In the December 13 issue of Science,
Andrew McIntyre and Barbara Molfino, paleoclimatologists
at the Lamont-Doherty Earth Observatory in Palisades,
New York, report evidence that strong westward-blowing
trade winds died down along the equator at about the
same time that iceberg 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 could shift so dramatically
again in modern times," says Connie Sancetta, program
director in NSF's marine geology and geophysics program,
which funded the research.
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.
McIntyre will present his new theory on December 16
at the American Geophysical Union's fall meeting in
San Francisco. 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 scientists found relatively high levels of F.
profunda every 8,400 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 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.
Explains McIntyre, "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."
EDITORS: Andrew McIntyre will present the new
findings linking tropical and North Atlantic climate
change at the American Geophysical Union's fall meeting
in San Francisco. He is to speak Monday, December
16 at 1:50 P.M. in Room 104 of the Moscone Center.
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