A study conducted by Dr. Aaron Kaufman of the Weizmann Institute's Environmental Sciences and Energy Research Department together with Drs. Miryam Bar-Matthews and Avner Ayalon of the of the Geological Survey of Israel demonstrates that these ancient cave formations are a unique repository of information about climatic fluctuations.

"Past changes in climate are reflected in the way stalactites and stalagmites grow and develop," says Kaufman. "By carrying out isotopic analyses on more than a hundred samples from each individual structure, we were able to piece together an ongoing picture of diverse climatic changes."

The study showed that throughout most of the period between 20,000 and 58,000 years ago, the climate in the region was dry and cool. Afterwards, the annual rainfall fluctuated significantly, and about 6,000 years ago the general climatic conditions became similar to those of today. These changes were reflected in the types of local vegetation.

Stalactites are icicle-shaped formations hanging down from cave roofs, while stalagmites look similar but point upward from the cave floor. Both are created when water picks up calcium carbonate from the soil and rocks surrounding the cave, and then trickles through cracks in the cave roof, leaving some of the mineral behind as it drips down onto the floor. Every hundred years or so a distinctive layer, or ring, is formed, so that the cross section of a stalactite or stalagmite resembles that of a tree trunk.

The scientists performed three types of isotopic analyses on the samples. For purposes of dating, they took advantage of the fact that when stalactites and stalagmites are first formed, they contain a known quantity of one particular radioactive isotope - uranium 234 - but none of a different isotope, thorium 230. Uranium 234 decays at a uniform rate, during which some of it is converted into thorium 230; the older a sample is, therefore, the higher its thorium-to-uranium ratio.

To obtain data about the climate from these same samples, the researchers analyzed the ratios of stable isotopes that are known to be a function of specific temperature and precipitation conditions: the ratios of carbon 13 to carbon 12, and of oxygen 18 to oxygen 16.

The knowledge of past climatic changes provided by this study may lead to a better understanding of today's climate and may also help make climatic projections for the future. These findings were partly reported in Quaternary Research (1997, Vol. 47, no. 2), and will be further described in the Proceedings of the International Symposium on Isotope Technique in the Study of Past and Current Environmental Changes in the Hydrosphere and the Atmosphere.
 

REHOVOT, Israel -- June 24, 1996 -- El Nino, the periodic abnormal warming of surface water in the eastern equatorial Pacific, has long been notorious for the devastation it often inflicts on South America. But now it appears that its effects reach as far as the Middle East -- and surprisingly, they may actually be positive.

By studying tree rings, satellite cloud images and rain water, an Israeli research team, led by the Weizmann Institute's Dr. Dan Yakir, has discovered a striking correlation between El Nino and rainfall in central Israel over the past 20 years. Their results, which may prove valuable in predicting rainfall, are reported in the current issue of Global Change Biology. "It is perhaps befitting that El Nino --Spanish for the Christ Child -- should have a link to the Holy Land," remarks Dr. Yakir.

He conducted this study together with Dr. S. Lev Yadun, then a post-doctoral fellow at the Weizmann Institute's Plant Genetics Department, and Prof. A. Zangvil of the Blaustein Institute for Desert Research in Sde Boker.

The scientists found that between 1975 and 1995, winters with above-average rainfall in central Israel coincided with El Nino events.

For example, the record rainfall in Israel for this century, in the winter of 1991/1992, coincided with one of the most devastating El Nino appearances in recent years. Conversely, relatively dry winters in Israel have coincided with below-average ocean surface temperatures in the eastern equatorial Pacific. El Nino events, which take place on average once in four to five years, are marked by a massive increase in ocean surface temperature, which leads to storms and floods that wreak havoc on the coasts of Ecuador and Peru and may cause other large-scale climatic upheavals, primarily in the tropics.

In Israel, by contrast, heavy rains are a welcome blessing. The researchers also found that El Nino associated variations in Israel's rainfall were significant for local plant growth. They revealed a strong correlation over the past 20 years between El Nino and the width of annual rings of pine trees growing near Jerusalem. (In the normally dry Middle East, even a minor increase in rainfall can produce marked increases in the width of tree rings.)

Going back in history, the scientists noted that no El Nino events were recorded during the crippling, decade long drought that affected Israel in the 1930s.

And even further back in time, "Could Pharaoh's seven bad years have reflected an ancient period devoid of El Nino events?" asks Dr. Yakir, referring to the major famine recounted in the Biblical book of Genesis. To offer an explanation for this long-distance climatic connection between the eastern Pacific and Israel, the scientists proposed a mechanism based on regional satellite cloud images and isotopic analysis of rain in Israel. The satellite images helped identify a previously unnoticed strip of clouds which, under certain conditions, form a nearly straight line connecting equatorial Africa, known to be influenced by El Nino, with Israel.

It is possible that, during El Nino winters, changes in atmospheric circulation over parts of Africa and the adjacent ocean allow more of the high-altitude clouds to cross the Sahara Desert and reach the Middle East, contributing to Israel's rainfall. This hypothesis is supported by the isotopic analysis of local rain water over the past 20 years. The results indicate that during El Nino years, changes in atmospheric circulation over the eastern Mediterranean allow moisture from Africa to penetrate the region and contribute to local rainfall.

If the findings of Yakir and colleagues are substantiated, it may be possible to take advantage of El Nino forecasts, based on advanced computer models developed in the United States, to predict rain patterns in Israel more than a year in advance. This is of great significance for Israel's water management and agriculture, which -- in a region plagued by water shortages -- are critically dependent on careful planning.