VOL. 23, NO. 14	FEBRUARY 6, 1998

frica’s climate has abruptly turned from very wet to very dry in time spans as short as a lifetime, a scientist at Columbia’s Lamont-Doherty Earth Observatory reported recently at the American Geophysical Union’s fall meeting in San Francisco.

  Africa becomes significantly colder and wetter every 1,500 years, and stays that way for centuries, according to the new research by Peter deMenocal, a paleoclimatologist at Lamont-Doherty, Columbia’s earth sciences research institute in Palisades, N.Y.

  Analyzing ocean sediments from the coast of Senegal, deMenocal and colleagues found that Africa experienced surprisingly harsh and abrupt climate changes, even after the last ice age ended, a period in the past 10,000 years that until recently scientists had thought was stable and benign.

  The Lamont-Doherty paleoclimatologist found that every 1,500 years or so, within periods as short as 80 years, ocean temperatures off Africa plummeted and seasonal rains essentially ceased over the continent. Then the climate snapped back just as quickly, bringing flooding rains that created large lakes in what is now the Sahara Desert.

  That cycle matches a pattern of dramatic, abrupt changes in the North Atlantic region reported in a recent issue of the journal Science by Gerard Bond, another Lamont-Doherty scientist.

  Together, the findings point to rapid climate changes in both the high latitudes and the subtropics, indicating a persistent, naturally occurring cycle of abrupt climate change that may affect the entire planet.

  The new findings offer the potential of fundamental new understanding of how Earth’s climate system works, and what causes it to shift abruptly, deMenocal said. And they offer new insights into whether the buildup of greenhouse gases in Earth’s atmosphere threatens to destabilize the climate and whether cold spells and droughts may have wreaked havoc on nascent cultures in the cradle of civilization, Africa and the Middle East, he said.

  Such quick, severe climate changes in the subtropics—each launching centuries-long periods of extreme drought or rainfall—may have shifted the course of budding human civilizations there, and similar jumps in the future would create enormous difficulties for today’s societies, deMenocal said.

  DeMenocal found that larger-scale, longer-term climate change, such as the one that turned northern Africa from a landscape dotted with crocodile-filled lakes 9,000 years ago into the vast Sahara today, took not thousands of years but less than a century.

  The prevailing theory is that such long-term changes, which are governed by 20,000-year cycles in Earth’s orbit that affect the amount of solar radiation received by the planet, should occur slowly and progressively.

  Instead, the new discoveries add mounting evidence that Earth’s climate system reaches certain thresholds, then switches gears relatively quickly from one distinct operating mode to another, spawning dramatic climate changes that occur precipitously, he said.

  “The transitions are sharp,” deMenocal said in an interview. “Climate changes that we thought should take thousands of years to happen, occur within a generation or two.”

  The new discoveries by deMenocal and Bond are the latest important clues indicating that Earth’s climate over the past 10,000 years—after the last ice age ended and human civilization began to flourish—was not nearly as stable or resistant to change as previously believed.

  Abrupt changes occurred even in relatively warm, ice-free conditions such as today’s.

  DeMenocal made his findings by analyzing seafloor sediments cored by the Ocean Drilling Program’s drill ship, the JOIDES Resolution.

  The sediments contain the skeletal remains of marine plankton that fell to the seafloor, as well as tiny particles of soil and organic matter (collectively called dust) that were blown off the African continent more abundantly during droughts.

  By measuring relative amounts of fossil plankton that thrive in cold and warm waters, deMenocal and his colleagues could discern when and by how much waters off the African coast cooled.

  By measuring relative levels of dust, they could identify when dry spells occurred and how intense they were.

  The study documented that African dust levels and offshore ocean temperatures rose and fell synchronously and very rapidly.

  DeMenocal’s research is supported by the NSF.