Africa's climate has abruptly turned from very wet to very dry in time spans as short as a lifetime, a scientist at Columbia University's Lamont-Doherty Earth Observatory reported today (Dec. 11, 1997) at the American Geophysical Union's fall meeting in San Francisco.
Africa becomes dramatically 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. That cycle of cold-wet, then warm-dry periods exactly matches a pattern of dramatic, abrupt changes in the North Atlantic region reported in the Nov. 14 issue of the journal Science by Gerard Bond, another Lamont-Doherty scientist. Together, the findings link rapid climate changes in the high latitudes and the subtropics, pointing to 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, Dr. 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, Dr. deMenocal said.
Analyzing ocean sediments from the coast of Senegal, Dr. 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. Africa became significantly colder and drier and remained that way for centuries. Then the climate snapped back just as quickly, bringing flooding rains that created large lakes in what is now the Sahara Desert.
Dr. 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," Dr. 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 Dr. deMenocal and Dr. 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.
In the North Atlantic, Dr. Bond estimated that every 1,500 years or so, cold spells, each lasting a few hundred years, chilled ocean temperatures by 2 to 3.5 degrees Fahrenheit. Such changes would freeze harbors, devastate harvests and cause alpine glaciers to advance. But in subtropical Africa, the effects were far more severe, Dr. deMenocal found. Ocean temperatures off Africa dropped 8 to 14 degrees F, as much as during the last ice age. The resulting environmental changes would have been devastating to human populations living in the region.
The mechanism that causes the simultaneous changes in the north and subtropical Atlantic has yet to be determined, but Dr. deMenocal said that the boundary between warm tropical and cooler temperate waters, which is currently located off West Africa, may periodically shift positions, causing very large swings in African temperature and rainfall.
Dr. deMenocal made his findings by analyzing seafloor sediments cored by the Ocean Drilling Program's drill ship, the JOIDES Resolution. The site off the northwest coast of Africa, at 20 degrees north latitude and 18 degrees west longitude, was chosen because it is located in an area where a combination of factors causes sediments to quickly pile up on the ocean floor. As a consequence, each inch of sediment represents about 80 years, so that Dr. deMenocal could identify climate changes that occurred in short time spans.
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, Dr. 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.
The core provides a previously unavailable, highly detailed, continuous record of Africa climate changes, Dr. deMenocal said. On land, scientists can measure when Africa was wetter by finding fossils that accumulated when lakes filled up. But when those lakes dried up during droughts, winds blew away sediments and erased the climate record.
Scientists have shown that as the last ice age faded, Africa became wetter. By 15,000 years ago, Lake Victoria had filled up and started to flow to the Nile River. Fossils of ancient hippopotami and crocodiles have been found in old dried-up lake basins across northern Africa, indicating that the landscape once was filled with lakes of assorted sizes between 15,000 and 7,000 years ago, Dr. deMenocal said. Satellite images show vestiges of ancient lake and river systems running through what is today the Sahara. But Africa began to get drier around 7,000 years ago, as Earth's orbit shifted and Africa began to pass closest to the Sun in winter, rather than in summer as it previously did.
"Most other evidence had suggested that the development of dryness in Africa was progressive, and all our climate models predict that the changes should occur as gradually as the shift in Earth's orbit," Dr. deMenocal said. "But we found that Africa's climate shifted from one regime to another abruptly, in one steep, fast step, rather than a gradual transition."
Dr. deMenocal's research is supported by the National Science Foundation. The Lamont-Doherty Earth Observatory is part of the Columbia Earth Institute, a Columbia University enterprise instituted to conduct research, develop innovations and educate both citizens and world leaders on issues related to the wise stewardship of our planet.