Cycles of Drought
For the past 110 years, weather observers have kept track of the climate at thousands of places across the United States and the world. They've recorded temperature, rainfall, winds and other weather patterns day in and day out. From those data, they have described what is "normal" for a given place at a given time of the year. They have also been able to recognize cycles of drought throughout the recorded history.
Every decade or two over the historic record, a given region experienced dry conditions with devastating consequences for agriculture. But there are problems to overcome in this emerging science.
First, how do you actually define a "drought?" The Central Plains states like Nebraska can expect to receive somewhere around 20-inches of rain during a normal year. Alabama can expect to get over 55-inches, and their agriculture and social institutions are geared to that level of rainfall. If Alabama got only 20-inches of rain in a year, it would be a catastrophic drought for them, whereas it would be a normal year in Nebraska.
Scientists have adopted several different definitions of drought to help them understand the forces the produce it and to predict when another cycle may hit. The simplest definition is a deficit in moisture availability due to lower than normal rainfall. Other measures look at vegetative conditions, agricultural productivity, soil moisture, water levels in reservoirs and streams, and economic impacts. Using these measures, the University of Nebraska's Drought Mitigation Center maintains a weekly drought monitoring map, along with a host of other information.
One of the other problems is that the historic record of climate change only goes back 110 years or so. So scientists have begun studying secondary indicators of climate change the width of tree rings, lake and sand dune sediments, archaeological remains and other environmental indicators.
These studies are suggesting some interesting possibilities. For instance, drought studies may suggest why the "Lost Colony" on Roanoke Island was lost. The first English colony in North America was established on Roanoke Island in 1585. But the settlers returned to England a year later. A second group landed on Roanoke in 1587, intending to establish a permanent colony. They mysteriously disappeared by the time a supply ship reached the island in 1591.
New tree ring studies from the region have charted cycles of drought back to A.D. 1185. These studies suggest that the most severe three year period of drought in 800 years happened just as the Lost Colony was trying to get established.
Other studies have suggested that the historic droughts of the 1930s and 1950s may not have been as long or bad as droughts in prehistoric times. The 1930s drought was longer and more extensive than any drought in the last 300 years, but around A.D. 200 some parts of North America experienced drought conditions for several decades.
Paleoclimatic data have also suggested that even longer cycles can change regions from deserts to plains to forests and back over the course of centuries. These 200- and 700-year long cycles go back to the Little Ice Age and may be effected by solar activity or conditions in the ocean and atmosphere.
What all this means for farmers is that they can expect to endure times of drought, but they will have a hard time predicting exactly when or where drought might hit.
Written by Bill Ganzel, the Ganzel Group. First published in 2009. A partial bibliography of sources is here.