Memories from Africa


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Past weather patterns


If the earth of Africa could speak, specifically, sediments over 2,000 years old, what could they tell us about the state of our lives in this era? Weizmann Institute scientists scaled the snow-capped mountains and valleys of Kenya to listen to the earth "remember" its past and more importantly, to impart its wisdom to our present and future.

Prof. Aldo Shemesh, head of the Environmental Sciences and Energy Research Department, together with Prof. W. Karlan from Stockholm University, Sweden, literally hit the ground of Africa trekking, in search of answers to the past trends of global warming.

But what does the purity of an ancient African landscape have to do with today's car exhaust and industrial fumes being spewed into the atmosphere?

Just as global warming suddenly crept up on us in the late twentieth century, so too were the scientists trying to identify past periods of rapid, sudden warming in our planet's history. They've been studying the historic climatic changes which took place in equatorial regions, of great importance in our understanding of the world's current climate transformation.

They travelled thousands of miles in distance to Mount Kenya, thousands of feet above sea level (14,268 feet/4,350 meters) and thousands of years back in time to look for abrupt climate change in the past ­ without any relationship to contemporary human activity.

After a long and tiring climb by foot, the scientists, accompanied by local porters, reached the upper valleys of Mount Kenya. In breathtaking surroundings replete with dramatic peaks and valleys, there are lakes in whose depths are found siliceous algae. Using specialized equipment brought from Sweden, they drilled and removed sections of the sediments from the bottom of several of the lakes.

There were two major thrusts of their investigation: dating the sediments, and attempting to learn about the climate prevalent when the algae were alive. Back from the field, Shemesh, joined by research student Miri Rietti-Shati, evaluated the age of the sediments by accelerator mass spectroscopy according to the measurement of the quantity of the radioactive carbon 14 within them. These measurements indicated that the sections brought back from the Mount Kenya lakes contain sedimentation dating from between 2,250 BCE and 750 CE.

That would place its age during the era of the establishment of the Greek Bronze-age Minoan, as well as Mexican Olmec civilizations and the Chinese Chou dynasty.

The next stage of their research was based on the quantitative proportions between the isotopes of oxygen found in the siliceous algae's skeletons, which would make it possible to determine what climate was then in existence.

An isotope is a particular version of an atom of an element. Isotopes of the same element are almost identical to each other from the aspect of their chemical properties. They are different from each other only by their weight and other physical characteristics. One example: The most common isotope of oxygen is oxygen 16, but in nature the (heavier) oxygen 18 isotope also exists.

The quantitative relationship between the oxygen isotopes found in the siliceous algae skeletons depends on climatic conditions. Significantly, when the climate cools, the quantity of oxygen 18 isotope increases relative to the quantity of oxygen 16 isotope in the algae skeletons.

Therefore, when the quantitative relationship between these two isotopes is tested in a sample of the sediment composed of the siliceous algae skeletons, it's possible to infer what the climate was like at the time the sediments were formed.

This exploration, carried out using a unique technique developed by Prof. Shemesh, showed that in Central Africa between 350 BCE and 450 CE, there occurred a swift and significant climatic warming. Prof. Shemesh: "This finding offers evidence that sudden climatic warming has happened naturally, without any connection to human activity. The documenting of climatic changes which have occurred in the past in various areas on Earth is likely to help in more accurately forecasting the outcome of human activity since the beginning of the Industrial Revolution."