What a Serbian cave tells us about the weather 2,500 years ago

If you live in northern Europe or North America, your weather depends in part on what happens with the north polar jet stream. Jet streams are strong and steady winds that form around the Earth from west to east a few miles above the surface. The wandering north polar jet stream can bring cold air from the Arctic to the U.S. Midwest or send waves of Atlantic storms to Ireland or Scandinavia.

As with most weather, scientists believe that the jet stream is affected by climate change. Data from the last one hundred and fifty years indicate that the northern jet stream has gotten stronger during that time. But in climatecentury is not that long, and it is not clear whether amplification is a natural phenomenon.

New data from Serbia

In a paper published in the journal Geology, Miaofa Li of Fujian Normal University and Slobodan Markovic of the University of Novi Sad in Serbia provide new data on the issue. Climatologists regularly study air trapped in polar ice to get an idea of the state of the climate hundreds or even thousands of years ago. The researchers point out that very similar results can be obtained by studying the chemical composition of rock formations in a pair of Serbian caves.

Like many caves, the bottom of Cerjanska Cave and Prekonoska Cave, located in southeastern Serbia, is dotted with thin stalagmites. They are formedvery slowly as the water drips from the top of the mountain. Each drop carries dissolved minerals. If water drips into the same spot over many years, the stalagmite will gradually form as the minerals are deposited. This is important because analyzing the minerals that make up a stalagmite can reveal information about the water that formed it.

Researchers were interested in Serbia specifically because of how its location affects the weather. The type of precipitation the country receives depends on the strength of the north polar jet stream. When it blows strong, it brings clouds that have formed over the Atlantic Ocean. When it is weak, the clouds are more likely to come from the Mediterranean Sea.

Analysis of stalagmites

These two water masses haveunique chemical fingerprints. Water from the Mediterranean Sea contains more of a rare oxygen isotope known as oxygen-18, in which eight protons of this element are joined by ten neutrons instead of the usual eight.

Dr. Lee and Markovic and their colleagues studied two stalagmites, one measuring 380 mm from Cerjanska and one measuring 238 mm from Prekonoska. Using traces of two other elements, uranium and thorium, they were able to determine the age of both stalagmites. Toth,that formed at Cerjanska grew between 434 B.C. and 1913, while the one at Prekonoska Cave was formed between 798 B.C. and 404. They then analyzed 581 samples of calcium carbonate mineral containing oxygen.

The researchers concluded that the North Atlantic jet stream appears to be stronger today than at any time in the past 2,500 years. Their confidence in their method was boosted when they were able to see climate events in the rock formations that are known from other sources, such as the Roman Warm Period, which lasted from 300 B.C. to 200, and the Cold Ages Period, from about 300 to 700. What's interesting is that the data shows that the jet stream was stronger during the colder periodsand weaker during warmer times - in contrast to the observed trend today.

The mystery of the modern jet stream

Why the modern jet stream seems to be doing the opposite is not yet clear. Researchers suggest as one possibility the atmospheric influence of something called the North Atlantic cooling region. This is a persistent area of unusually cold water stretching from the Hudson Strait in northern Canada almost to the west coast of Ireland. It is thought to be caused by the melting of the Greenland glaciers and the weakening of ocean conveyor belts that transport warmer water from the tropics to more northern latitudes. Perhaps additional data obtained from other stalagmites in otherof the caves will help solve this mystery.