Tree Rings Point to Historic, Long-Term Aridity in Western United States

FAYETTEVILLE, Ark. - While the Western states remain gripped in a severe drought, a group of researchers has studied the tree ring record and found that the current dry spell pales in comparison to the aridity of the same region during medieval times. The researchers believe that the drought patterns outlined in the tree rings could indicate a long-term increase in drought over Western North America.

"The data show that the 20th century has been relatively wet, despite the drought calamities that have befallen us," said David Stahle, professor of geosciences at the University of Arkansas.

Stahle, Edward R. Cook of Lamont-Doherty Earth Observatory, Connie Woodhouse and C. Mark Eakin of the National Oceanic and Atmospheric Administration's National Climatic Data Center, and David M. Meko of the Laboratory of Tree Ring Research, University of Arizona, report their findings in the current online issue of Science Express. Science Express is an electronic publication that highlights selected upcoming papers that will appear in the journal Science.

The researchers examined 602 tree ring chronologies from the Western United States, with the tree rings representing the past 245 to 2,000 years. Scientists have calibrated the tree-ring chronologies with the Palmer Drought Severity Index (PDSI), a water balance model based on precipitation and temperature using modern-day instrumentation that dates back to the early 20th century. The PDSI is based on grid points located at weather stations throughout the United States. This calibration has allowed researchers to reconstruct past rainfall using tree rings. In this instance, the researchers looked at the tree-ring records dating back 1,200 years.

Stahle and his colleagues used the Drought Area Index (DAI) to examine the incidence of drought during that time period. The index is a count of PDSI grid point reconstructions that indicates either a wet or dry year.

Their results show that, when compared to other time periods in the last 1,200 years, the West has enjoyed a relatively wet time period in the 20^th century. The most arid times were centered around AD 936, 1034, 1150 and 1253, a 400-year span that corresponds to a period called the "Medieval Warm Period" (MWP), a European phenomenon.

Shortly after 1300, things began to get wetter again, until about 1920. From 1300 to 1920, the times with the moistest conditions centered around 1321, 1613, 1829 and 1915. Since that time, overall drought conditions have increased to a level not seen for several hundred years, but still wetter than the climate of the MWP.

The researchers point out that the western North American drought conditions during medieval times corresponded with a well-documented warming period in Europe, indicating a possible link between warmer temperatures and drier weather in the West. If the global temperatures continue to rise, it could lead to a long-term increase of drought conditions in western North America.

"The evidence is speculative, but sobering," Stahle said. "We have engineered a water supply system in the West that is resilient. But a return to prolonged drought would challenge even the massive water supply systems of the West." If drought conditions become more persistent, Western states will need to re-think their water usage in order to preserve it as an essential commodity, he said.

To put their findings in context, the researchers also looked at other biological and geological records that indicate climate change, including fire scars, charcoal levels, lake salinity and sand dune activity, all of which provide paleoclimatologists with evidence of precipitation conditions from the past. These records supported the evidence for the existence of a megadrought during the AD 900 to 1300 era.