November 5, 2011
Evidence Of Previously Unknown 2nd Century Drought Discovered
Researchers with the University of Arizona (UA's) have discovered evidence of a formerly unknown, decades-long drought that occurred in the southwestern United States in the second century, the school announced in a November 3 press release.
The study, which was the work of geoscientists Cody Routson, Connie Woodhouse and Jonathan Overpeck of the university's Laboratory of Tree-Ring Research, involved a study of the southern San Juan Mountains in Colorado.
The region is said to be a primary drainage site for both the Rio Grande and San Juan Rivers -- the latter of which is a tributary for the Colorado River, meaning that any climate changes that affect it would also have an impact on the river it flows into.
By using annual growth rings of trees to understand past climate, a science identified as dendrochronology, they were able to determine the age of the trees, as well as which years were better for growth and which years were less conducive for growth, the UA scientists said.
"If it's a wet year, they grow a wide ring, and if it's a dry year, they grow a narrow ring," Routson said in a statement. "If you average that pattern across trees in a region you can develop a chronology that shows what years were drier or wetter for that particular region."
Routson and his colleagues combed the area, looking for climate indicators in the growth rings of the oldest trees in that region, and attempted to complete a long-term record for conditions in the San Juan area.
They focused on bristlecone pine trees, since they are said to be the oldest and longest-lived species on the planet, with some having lived for more than 4,000 and surviving extreme drought conditions. They then used a special tool, which resembles a metal screw, to bore a small hole in the tree's trunk and remove a sample without causing too much damage to the tree itself.
"We took our samples back to the lab where we used a visual, graphic technique to match where the annual growth patterns of the living trees overlap with the patterns in the dead wood," Rouston said. "Once we have the pattern matched we measure the rings and average these values to generate a site chronology“¦ In our chronology for the south San Juan mountains we created a record that extends back 2,200 years. It was pretty profound that we were able to get back that far."
They found evidence of major droughts in the mid-12th century and the late-13th century, but they also discovered "evidence for an earlier megadrought during the second century A.D.," Rouston added, including a period of dry conditions lasting from 124 A.D. to 210 A.D.
"We're showing that there are multiple extreme drought events that happened during our past in this region," Routson said. "These megadroughts lasted for decades, which is much longer than our current drought. And the climatic events behind these previous dry periods are really similar to what we're experiencing today."
The UA team's research has been published in the journal Geophysical Research Letters.
Image 2: Dendrochronologists extract a small, pencil-shaped sample of wood from a tree with a tool called an increment borer. The tiny hole left in the tree's trunk quickly heals as the tree continues to grow. (Photo by Daniel Griffin/Laboratory of Tree-Ring Research)
Image 3: Twisting with time, bristlecone pine trees live for thousands of years on high mountain slopes and record each passing year with a new ring of growth. (Photo by Cody Routson)
Image 4: A cross section of wood shows the annual growth rings trees add with each growing season. Dark bands of latewood form the boundary between each ring and the next. Counting backwards from the bark reveals a tree's age. (Photo by Daniel Griffin/Laboratory of Tree-Ring Research)
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