IS THERE EVIDENCE OF LONG-TERM DROUGHTS IN THE SOUTHWEST?

The researchers used drought-sensitive tree ring chronologies that extend back before A.D. 1500 from trees in the western United States, the Southeast, and the Great Lakes region. They found that dry conditions extended from the Sierra Madre Occidental in Mexico and the Southwest to the Rocky Mountains and the Mississippi Valley throughout the last half of the 1500s. Severe conditions occurred at times in Mexico, the Southwest, Wyoming, Montana, and the Southeast. Looking back as far as A.D. 1200, no other drought appears to have been as intense, prolonged, or widespread as the 16th century mega-drought, the researchers found.

How do scientists use tree rings?

Tree rings are one of Mother Nature's climatological record books that researchers can examine to see how the climate of the past looked. Every year, a growing tree produces a "ring" which represents the new growth that occurs during the warm season, along with the very slow growth of the trunk that occurs during the cold season. By counting these rings, researchers can find out how old these trees are and therefore know when these rings were created. Favorable growing seasons are represented by wide rings, while poor growing seasons, due to cold summers or due to prolonged drought, cause very tight rings.

The scientists compare the tree ring characteristics to the climate data gathered by humans over the past 100 years. Based on how the rings are oriented and shaped, statistical models are used to reconstruct climate data past the human record, going back hundreds of years. Individual trees have their own personal histories, but a group of 30-40 tree ring samples from trees in the same region form a library with a shared recording of the climatic past. Through this method, scientists have reconstructed the climate pattern back more than 1000 years and unearthed this mega-drought of the 16th century.

Are there any human records to back this up?

Archival records from the Spanish colony of Santa Elena on Parris Island, S.C., indicate a severe drought from 1566-69. In 1587, the year Sir Walter Raleigh's colony on Roanoke Island disappeared, the Parris Island settlers abandoned their colony. Tree ring records show that year was the region's worst drought in 800 years.

The severely dry weather in the Southwest and northern Mexico may explain why some American Indians in these areas abandoned their pueblos between 1540 and 1598, the researchers contend. Records during the Chichimeca War in Mexico, between American Indians and European settlers, indicate drought conditions existing during the 40 years of the war beginning in 1550.

What might have led to the development of this mega-drought?

Ironically, the lack of precipitation may have been linked to ocean currents. Because the drought-affected area looks amazingly like a pattern formed on a smaller scale in today's climate-ocean current phenomenon La Nina, some researchers speculate that a period of colder than normal ocean currents in the equatorial Pacific Ocean may have caused the prolonged drought. When the ocean is colder than normal in the equatorial Pacific Ocean, weather systems tend to stay north of the southern tier of the United States, and any storms that do occur tend to produce less precipitation.

Could this happen again?

Researchers know that this drought pattern was not a consequence of global warming. However, they do not know exactly what caused it, only that it actually happened. If it was indeed caused by ocean currents, these researchers claim that it could return in the future. Further studies of ocean sediments and coral reefs may reveal the ocean's role, if any, in this mega-drought.

If such a drought were to occur today, it would wipe out certain agricultural activities. Economic activities over North America would also suffer dramatically. The good news is that based on the tree ring chronology, this is the only event of this magnitude in the climate records over the past 1000 years. However, the bad news is that numerous drought periods lasting five to ten years or greater have been seen in the tree ring chronologies, indicating that prolonged periods of dry weather conditions can occur across the Southwestern United States.