ALL DRIED UP.

Was last summer a scorcher where you live? Could it be a sign of drier times to come?

Until last summer Tyler Gillaspie, 12, of Huntsville, Texas, never gave much thought to drought. But when the lake behind his house shriveled up, drought struck home. "I usually go fishing every weekend during the summer," Gillaspie says. "But with no rain, the lake was down so low you couldn't even put a boat in it."

Gillaspie is one of millions of Americans affected by recent droughts--a climate condition that results when rainfall is less than 0.2 millimeters (.008 inch) over an extended time period, according to the U.S. National Weather Service. Then abnormally dry weather threatens human and animal water sources, crops, and wildlife. And last summer, like a grim reaper, drought cut a swath through much of the U.S., with central and southeastern states among those hardest hit.

In Georgia, stream and river levels plummeted to record lows. In Nebraska, farmers watched more than 20 percent of their corn crop wither; ranchers saw 93 percent of cattle pastures become so barren that some animals starved to death. Wildfires raged in tinderbox forests from New Mexico to Montana. And in Louisiana, wetlands shrank so much that alligators fled their swamps and hit the highways in search of water.

Last June, the U.S. Department of Agriculture declared parts of Alabama, Florida, Nebraska, and New Mexico agricultural disaster areas, making farmers eligible to receive emergency federal aid. But drought knows no borders, and right now in East Africa and southern Asia, tens of millions of people live at risk from persistent droughts: crops have wasted away, wells are bone dry, and vast tracts of land have turned to dust. "Drought is a chronic force of nature," says Nebraska State climatologist (weather scientist) Allen Dutcher. "No place is immune to it."

What causes drought? Are parts of the U.S. in the midst of a lasting dry spell? Could we ever experience a megadrought, one that can last a half-century? Scientists are trying to find out.

HOT AND DRY

It may be hard to believe a baked Nebraska or South Carolina could be caused by a complex weather cycle in the Pacific Ocean, but that's what many climatologists think. They point to a phenomenon called La Nina ("the little girl") that can play havoc with global weather.

During La Nina--a climate cycle that usually lasts between one and two years--strong trade winds and ocean currents drive cold, deep Pacific waters to the surface and toward the coasts of North and South America (see diagram, p. 19). Because cold water evaporates less than warm water, fewer storm clouds form and rain decreases. This creates a ridge of high pressure, or dense dry air, which rolls over the U.S., explains Skip Ely, a National Weather Service meteorologist (weather forecaster). In turn, that high-pressure ridge can cause scorching, rainless summers in the Western and Southeastern U.S.

Some parts of the country have sweated under dry spells from La Nina since summer 1998. However, about every two to seven years, La Nina gives way to her badboy brother El Nino, a cycle that triggers the opposite effect, causing warmer-than-normal Pacific Ocean air and water currents. El Nino often produces precipitation and wild storms over most of the country. The last period of El Nino occurred in 1997-98; in the last 50 years climate conditions in the Pacific Ocean have generated La Nina 23 percent of the time and El Nino 31 percent of the time.

Another potential drought culprit: global warming, or the general heating of Earth's climate due to increased greenhouse gases like carbon dioxide in the atmosphere. Some scientists think global warming could spell more erratic worldwide weather on a regular basis, with more torrential rainfall in some areas and severe drought in others. A new report by Harvard University's Center for Health and Global Environment claims that continual global climate change could lead to more droughts and significantly reduced crop yields in some U.S. agricultural regions.

WHEN DROUGHT LASTS

When drought is severe enough, it can lead to a megadrought, which lasts from 10 to 50 years or longer. David Stahle, a geography professor at the University of Arkansas, studies megadroughts through tree rings. By boring small holes into tree trunks and extracting cross-sections of wood, Stahle studies the rings that indicate a tree's age. When water is plentiful, tree rings are thick; during droughts, rings are narrow. "Trees provide an unwritten history of climate," Stahle says. "By examining trees at different sites, we can map out the history of droughts."

A devastating drought cooked North America from 1540 to 1590, Stahle claims, killing some of America's first European settlers in what is now Virginia. Nearly 400 years elapsed before another megadrought even approached that intensity in North America. In the 1930s, the "Dust Bowl" drought lasted nearly 10 years, and in 1934 affected 65 percent of the U.S.--the worst drought of the 20th century in North America. A major drought struck 40 percent of the U.S. in 1988, but lasted four years--too short for a megadrought.

While parts of the U.S. now suffer through their third year of drought, it's unlikely this marks the onset of a megadrought, says Stahle: "That type of drought is a rare event." Also, La Nina is now weakening, and El Nino is predicted to return this fall.

The long-range forecast for this winter: wetter than normal.

WATER FALL

"Unlike floods or tornadoes, you can't see the effects of a drought overnight," says Donald Wilhite, director of the National Drought Mitigation Center at the University of Nebraska. And periodic droughts rarely cause human death or destruction of buildings. Therefore, disaster planners tend to pay little attention to drought--though on average, drought damage costs the U.S. $6 to $8 billion a year. "By the time you realize you're in a drought, it's often too late--the damage is already done," Wilhite says.

Is there anything you can do to help prevent drought? "Don't waste water," says Stahle. "You never miss something until it's gone." If you and your family conserve water, drought won't pose as much of a threat as it does now. "We can't change the world immediately," he says, "but we can do little things."

THINK ABOUT IT

What are five ways you and your family can conserve water?

Drought in the U.S.

DROUGHT SEVERITY INDEX

This map shows the severity of drought in the U.S. as of August, 2000. The driest Western states, from Montana to New Mexico, experienced raging wildfires due to drought. Complex weather cycles that cause dry spells also triggered wetter than normal conditions in the Northeast. How did your state fare?

KEY

extreme drought

severe drought

moderate drought near normal

unusual moist spell

very moist spell

extremely moist

[CHART OMITTED]

Cross-Curricular Connection

History: Read about other severe 20th century droughts: the Dustbowl in the U.S. in the 1930s and current drought in northwest Africa, which began in the 1980s. What were/are the effects of these droughts?

Did You Know?

* Megadroughts have been implicated in the downfall of the Mayan Empire in Central America a millennium ago--and in the fall of the world's first empire, the Akkadian in Mesopotamia 4,200 years ago. That drought lasted 300 years.

* In 1984-85, drought in Ethiopia created a famine that killed 1 million people.

* Dry conditions in the Western U.S. last summer fueled the worst fire season in decades, charring millions of acres. On August 8, at least 66 major fires raged in 11 states, all having burned at least 100 acres.

National Science Education Standards 

Grades 5-8: natural hazards * populations, resources, and environments * Earth's history

Grades 9-12: natural resources * natural and human-induced hazards * personal and community health * historical perspectives

Resources

"Persistent and Severe, Drought Strikes Again," The New York Times, 5/25/00, p. F1 "Parched Earth," Time, 5/8/00, p. 54 "Year to Date: Hot and Dry--Forecast: Hot and Dry," Environmental News Service: ens.lycos.com The most recent worldwide weather statistics are available at: www.ncdc.noaa.gov/ol/climate/research/2000/apr/a pr00.html