Aug. 19, 1998 REHOVOT, Israel -- August 14, 1998 -- A sudden warming of climate lasting several centuries took place in equatorial Africa some 2,000 years ago, according to a new study reported by a Weizmann Institute of Science-led team in the August 14 issue of Science.
The scientists performed an isotopic analysis of the sediments from Hausberg Tarn, a small lake at an altitude of 4,350 meters on a slope of Mt. Kenya, a dormant volcano in East Africa whose top (at 4,600-4,700 meters) is covered by permanent glaciers. They found that a rapid and significant warming of lake water -- by about 4 degrees C -- took place between the years 350 BCE and 450 AD, reflecting a warming of climate in equatorial East Africa.
This study helps determine how the climate fluctuated naturally long before modern industries began releasing large quantities of greenhouse gases into the atmosphere. Such research may, in turn, throw light on today's climate. It may allow scientists to distinguish between natural climate variability and the global warming believed to be affecting our planet in recent years due to man-made factors.
"Our findings show that the climate can warm up suddenly without any connection to human activity," says research leader Prof. Aldo Shemesh, head of the Environmental Sciences and Energy Research Department at the Weizmann Institute of Science. He conducted the study with Prof. Wibjorn Karlen of the University of Stockholm in Sweden and Weizmann Institute graduate student Miri Rietti-Shati.
"Documenting climatic changes that took place in the past in various parts of the globe may help scientists make more precise predictions about the potential effects of modern activity on the climate of the future," Shemesh says.
While periods of ancient warming have been identified in numerous parts of the world, the new study makes a unique contribution to this area of research because it was conducted on the equator, a region that plays a crucial role in determining the climate system throughout the planet. Moreover, it's the first quantitative assessment of a past warming period on the equator to be performed at such a high altitude, where evidence of past climate changes is particularly direct due to the close proximity of mountain glaciers.
Mt. Kenya's hidden "archive"
The scientists, accompanied by local porters, reached Hausberg Tarn following a long and arduous hike lasting several days. Using boats and special drilling equipment they carried with them, they obtained a nearly 2-meter-long core of sediment from the bottom of the lake. This sediment, containing fossil algae deposits, was later analyzed at the Weizmann Institute.
Using carbon-14 dating, the scientists first determined that the core contained deposits which had accumulated over 3,000 years, between 2,250 BCE and 750 AD.
Then, using a new method developed by Prof. Shemesh, the researchers studied the ratio of oxygen isotopes in the remains of the algae skeletons, called biogenic opal, which accumulated in the sediment.
Isotopes are versions of the same element that are almost identical in their chemical properties but differ in weight and in other physical properties. Thus, for example, the most common isotope of oxygen is O-16, but there is also a heavier oxygen isotope, O-18.
The relative quantities of these two oxygen isotopes in biogenic opal are the result of climatic conditions prevalent in the area when the sediment formed; they reflect the lake's temperature and the isotopic composition of its water at the time of sediment deposition. Thus, when the water was cooler, the opal contained relatively more O-18 compared with O-16. By studying the ratio of the isotopes, the scientists were able to identify the period of sudden warming.
This research has also established that the measurement of biogenic-opal oxygen isotopes in lake sediments is a unique and valuable way to investigate past climate.
"Our research has shown that sediments from high-altitude lakes provide a unique isotopic archive of climatic changes," Shemesh says.
The scientists note in their paper that the warming on Mt. Kenya may have been part of a more global climatic phenomenon because a warm period occurring during approximately the same period had been recorded in two other parts of the world -- in the Swedish part of Lapland and in the northeastern St. Elias Mountains (southern Yukon Territory and Alaska).
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