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posted: 1/2/2013 6:14 AM

Kids ask about history of droughts

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"What was the biggest drought in the world?," asked students in Jen Janik's third grade class at Big Hollow Elementary School in Ingleside.

Drought is a weather condition in which there is very little or no rainfall.

The National Drought Mitigation Center in Lincoln, Neb., defines drought as "A deficiency of precipitation over an extended period of time -- usually a season or more -- resulting in a water shortage for some activity, group, or environmental sector."

Dry conditions become droughts when they extend numbers of years.

Terry Jones, anthropology professor and chair of the Department of Social Sciences at California Polytechnic State University, said drought is a relative term depending on where you live and what you're accustomed to.

"Drought can be defined differently from region to region and between cultures. In some parts of the world, the north coast of Peru, for example, it can be common for no rainfall at all to occur for perhaps five or six years in a row, but that's expected. Here, in the United States, we get nervous if we have two to three years of subnormal rainfall."

Jones' work as an archeologist centers on the Medieval Drought that drove social changes in the western part of North America. There were two significant droughts -- one lasting 25 years and another spanning 100 years.

"The longer droughts that we know about seem to have taken place in the prehistoric past rather than in more recent times," Jones explained. "For prehistory we don't have instrument records; we can recognize dry years in things like tree rings, but we cannot necessarily distinguish a year with a little bit of rain from one with none whatsoever."

Jones describes his research findings, "This interval known as the Medieval Climatic Anomaly was a period during which global temperatures seem to have been a little warmer than today, so there were more severe climatic events including unusually long droughts."

Counting tree rings -- a science called dendrochronology -- examines the rings that appear in a tree stump as a result of the tree's annual growth. Slim rings indicate low moisture levels during a particular year.

How do scientists know these droughts caused social change?

"We can determine that settlements were abandoned when things stopped being built and artifacts stopped being deposited," Jones said. "It's a little harder to detect resettlement, but we can recognize new settlements being established after the old ones are abandoned. People would potentially consider moving when droughts lasted for an unprecedented period of time."

During the dry time period, there seems to have been an increase in violence, Jones noted. But there were some positive outcomes -- an increase in trade between neighboring communities.

The drought in the Southwest did not follow a global pattern of low precipitation, Jones said.

"Droughts are regional. Even during the Medieval Period, certain areas were affected while others were not," he said. "Major rivers in California, for example, did not stop flowing. The entire planet was not subjected to one simultaneous global drought."

What causes drought? Changes in the jet stream, the wind that carries rain-bearing clouds, can cause changes in weather patterns. Why does it change? Scientists are still trying to uncover exact answers. The National Drought Mitigation Center identifies several possible reasons for weather changes: "Differences in the amount of snow and ice cover, changes in the amount of vegetation covering the land, the moisture in the soil, and ocean surface temperature and currents can cause these patterns to change."

The organization says anyone can make a difference by taking measures to conserve water. Tips for reducing drought's effects include turning off the water while brushing your teeth, xeriscaping, recycling water and installing low-water use shower heads and toilets.

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