Atmospheric Dust, Ice, and Heat

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“Now that all the records have been shown to
coincide, “it suggests that the whole world
hydrologic cycle varies in unison, on a pretty
rapid time scale,” said Gisela Winckler …”
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Public release date: 28-Feb-2008
The Earth Institute at Columbia University

Each year, long-distance winds drop up to 900
million tons of dust from deserts and other parts
of the land into the oceans. Scientists suspect
this phenomenon connects to global climate-but
exactly how, remains a question. Now a big piece
of the puzzle has fallen into place, with a study
showing that the amount of dust entering the
equatorial Pacific peaks sharply during repeated
ice ages, then declines when climate warms. The
researchers say it cements the theory that
atmospheric moisture, and thus dust, move in
close step with temperature on a global scale;
the finding may in turn help inform current ideas
to seed oceans with iron-rich dust in order to
mitigate global warming. The study appears in the
Feb. 28 edition of Science Express, the advance
online edition of the leading journal Science.

In the past decade, scientists have documented
similar dust peaks in polar ice cores, and in
sediments from the Atlantic and Indian oceans,
but records from Pacific were contradictory. Now
that all the records have been shown to coincide,
“it suggests that the whole world hydrologic
cycle varies in unison, on a pretty rapid time
scale,” said Gisela Winckler, a geochemist at
Columbia University’s Lamont-Doherty Earth
Observatory and lead author of the paper. “It
gives us the information from where it
matters-where people live, and where the real
engine of climate probably lies.” Changes in the
atmosphere over the Pacific, and the tropics in
general, are thought to affect huge areas of the
world.

The researchers studied cores of seafloor
sediment representing 500,000 years of
deposition, spanning about 6,000 miles of the
Pacific equator, from near Papua New Guinea to
near Ecuador’s Galápagos Islands-nearly a quarter
of the globe’s girth. In each, they found the
same thing: at the height of each of five known
ice ages, accumulation of the isotope thorium
232, a tracer for land dust, shot up 2.5 times
over the level of warmer “interglacial” times.
The peaks appear about every 100,000 years, with
the last one at 20,000 years ago-culmination of
the last glacial age. Through other isotopes, the
scientists traced the dust on the western side to
Asia, and that on the eastern side to South
America. The reasons for the lockstep peaks are
probably complex, but in general scientists say
that colder air holds less moisture than warmer
air, and that cold periods tend to be windier;
this means both dustier land, and more dust
getting blown away.

The dust probably helped make climate even colder
for a while, and this has implications for the
current day, said Robert F. Anderson, head of
Lamont-Doherty’s geochemistry division and a
coauthor. Many types of dust transported at high
altitudes tend to reflect sunlight, thus lowering
the energy reaching earth, said Anderson. And,
when it settles into the ocean, there could be an
intriguing further effect. Rich in the plant
nutrient iron, the dust could have fertilized
near-surface plankton on a massive scale. Like
other plants, plankton uses the greenhouse gas
carbon dioxide for photosynthesis; thus,
theoretically, fertilization could have caused
the ocean to take larger amounts of CO2 from the
air, and entomb it in the ocean. Lowering of
atmospheric CO2 in turn would reduce the air’s
capacity to hold heat-the opposite of what is
currently happening, as the globe warms due to
elevated CO2 levels from burning of fossil fuels
and other human activities.

Lately, a growing number of scientists have been
advocating research to see if massive, manmade
iron fertilization of the oceans might induce
such blooms, and thus mitigate warming. A dozen
early experiments in different regions have shown
that plankton growth increases when iron is
artificially added, but scientists have yet to
show that this could lock significant amounts of
CO2 into the ocean; carbon from the plants would
have to sink to the bottom for this to happen.
“The new data gives us a natural experiment to
see what might have happened in the past,” said
Winckler. The researchers’ next step will be to
analyze their cores for signs of such sunken
carbon during the ice ages; they hope to do this
within a year or two.

Anderson and Winckler caution that the idea of
iron fertilization remains deeply complex and
controversial. “Assessing the past response to
natural variability of iron will enable
scientists to develop more quantitative
predictions about the possible efficacy of adding
it ourselves in the future,” said Winckler.

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INFORMATION FOR JOURNALISTS

To contact authors:
Gisela Winckler Winckler@ldeo.columbia.edu 212-866-0156
Robert Anderson boba@ldeo.columbia.edu 845-365-8508

Copies of the paper, “Covariant
glacial-interglacial dust fluxes in the
equatorial Pacific and Antarctica”: Science
magazine scipak@aaas.org 202-326-6440

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