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A foraminifera called Quinqueloculina impressa The dark brown structure is the microscopic foram's shell. Radiating from the opening are fine, hairlike reticulopodia, which the foram uses to find and capture food.

© Karen L. Wetmore Grycewicz, Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. (More foram info.)

A piece of drill core taken from sediment nearly 1000 meters, or about half a mile, below the ocean floor.
Courtesy Ocean Drilling Program.

Climate changes everything: When the North Atlantic was cool, the Sahara Desert turned green. A space shuttle photo shows Lake Chad today, inside the dotted line (the smaller circular shape). The solid line (bigger area) outlines the lake when the North Atlantic was cool (from 9,000 to 6,000 years ago). The photo was taken from the north, looking across the arid sub-Sahara and Sahel. The humid period reflects changes in Earth's orbit that intensified incoming sunlight, increasing the amount of rain in today's Sahara Desert, and also reducing sea-surface temperatures in the nearby Atlantic.

Courtesy Peter deMenocal.

The worrisome idea that temperature gyrates rather than moving gradually has been emerging for a decade or so from studies of the ancient climate in the North Atlantic Ocean. Now, new research on the tropical Atlantic confirms that the temperature plunged every 1,500 years over large parts of the ocean during the Holocene period. The Holocene started about 11,700 years ago, when climatic warming began melting Ice Age glaciers.

While the new study focused on cooling rather than warming, it implies that climate is less like a steady, slow-moving lorry than an uninsured roller coaster. "The main lesson I learn, over and over, is that the order of the day is change, not stability," says Peter deMenocal, a climatologist at Lamont-Doherty Geological Observatory in New York.

Microscopic accountancy
For a study just published in Science, deMenocal and colleagues counted populations of tiny animals called foraminifera that live mainly near the ocean surface. Like the different species of palms and pines, various forams prefer different climatic conditions. Thus the numbers of different species that died and fell to the ocean floor reflects past sea-surface temperatures.

It sounds dreary -- distinguishing one species from the next so you can bean-count them. Because the free-floating animals are a couple hundred microns across -- about the size of a pencil dot on paper -- the work entails serious squinting through a microscope.

Still, thermometers were kinda rare five or ten thousand years ago, and in the ancient-climate business, you take whatever records you can get. Among the 29 species of forams, four are enough to diagnose sea-surface temperature, says deMenocal.

The research team studied a drill core taken from the ocean floor offshore of Northwest Africa. A steady stream of wind-blown dust from the Sahara Desert gives the core plenty of detail; the layers can be accurately dated because 22 centimeters of sediment accumulate every 1,000 years.

The painstaking counting showed that every 1,500 years or so, the top layer of the ocean abruptly cooled for a few hundred years, then warmed back up. During some of the cool periods, temperatures plunged five degrees Celsius -- to ice-age conditions -- in a century or less. Because the ocean stores so much heat, it's safe to assume that air temperature varied along with water temperature. Although the rapid change was a cooling, it is still faster than computer models predict for the ongoing global warming due to greenhouse gases.

How widespread?
The repeated cooling and subsequent warming are probably due to changes in the amount of sunlight reaching Earth's surface, a change that reflects changes in the planet's orbit.

By finding the familiar oscillations around the Tropic of Cancer, the research indicates that the entire North Atlantic basin has a habit of quickly cooling every 1,500 years or so.

One explanation for some of those cold snaps comes from Wallace Broecker of Lamont-Doherty, who attributes quick changes to a reversal of the gigantic "ocean conveyor" that moves heat from the tropics toward the pole.

It's unclear whether the temperature swings covered the entire globe, or just the regions where ancient climate is heavily studied -- Europe and the North Atlantic. Similar data are not yet available for the Southern or Western Hemispheres, and ice cores from Greenland show some, but not all, of the oscillations.

Not ancient history
If the cause of the periodic coolings and warmings is unclear, the implications for future climate change are, can we say, chilling? "The Holocene was warm and relatively unglaciated, like now," says deMenocal, and thus it seems more relevant to our current situation. Until recently, the Holocene (our present climate regime) was considered stable. But the new information makes it seem variable, like the ice age that preceded it. During the Ice Age, topsy-turvy temperatures propelled the growth and defrosting of glaciers around the globe.

The sea surface temperature has been rocking and rolling over the past 2,500 years in the Atlantic off North Africa. Courtesy Peter deMenocal

The North Atlantic coolings of the Holocene haven't quit: The most recent episode, called the Little Ice Age, reduced the sea-surface temperature around Europe by 1 or 2 degrees Celsius between 1300 and 1870. That sudden big chill changed history by freezing out the Danish colony in Greenland and allowing Sweden's independence after ice trapped the Danish Navy in port. "It's a wonderful example of the relationship between climate and civilization," says deMenocal.

The new study indicates that islands of climatic stability exist amid a churning river of climatic change, casting an ominous shadow on our immediate future. "We have this great increase in greenhouse gases, and we expect a gradual response," says deMenocal, "but history -- paleoclimatology -- says it may not be that way. The climate tends to switch between two stable states. I don't want to be an alarmist, but climate transitions tend to be really abrupt."

--David Tenenbaum

Bibliography
Coherent High- and Low-Latitude Climate Variability During the Holocene Warm Period, Peter deMenocal et al, Science, June 23, 2000, pp. 2198-2202.