Roman Era Drought in Southwest USA
There have been periodic droughts in the American southwest for millenia. A new study at the UA's Laboratory of Tree-Ring Research has revealed a previously unknown multi-decade drought period in the second century A.D which was in the middle of the Roman era. The findings give further evidence that extended periods of aridity have occurred at intervals throughout our past. Almost nine hundred years ago, in the mid-12th century, there was a better known multi-decade drought in the southwestern US made locally famous by the Anasazi. This was the most recent extended period of severe drought known for this region.
The second century A.D. saw an extended dry period of more than 100 years characterized by a multi-decade drought lasting nearly 50 years, says a new study from scientists at the University of Arizona.
The study was done in the southern San Juan Mountains in south-central Colorado. The region serves as a primary drainage site for the Rio Grande and San Juan rivers.
The study will be published in the upcoming Geophysical Research Letters."
Dendrochronology is the science of using annual growth rings of trees to understand climate in the past. Because trees add a normally clearly defined growth ring around their trunk each year, counting the rings backwards from a tree's bark allows scientists to determine not only the age of the tree, but which years were good for growth and which years were more difficult.
"If it's a wet year, they grow a wide ring, and if it's a dry year, they grow a narrow ring," said Routson.
Darker wood, referred to as latewood because it develops in the latter part of the year at the end of the growing season, forms a usually distinct boundary between one ring and the next. The latewood is darker because growth at the end of the growing season has slowed and the cells are more compact.
Nothing is much older than a bristlecone pine tree: The oldest and longest-living species on the planet, these pine trees normally are found clinging to bare rocky landscapes of alpine or near-alpine mountain slopes. The trees, the oldest of which are more than 4,000 years old, are capable of withstanding extreme drought conditions.
To sample the trees without damaging them, the dendrochronologists used a tool like a metal screw that bores a tiny hole in the trunk of the tree and allows them to extract a sample, called a core. "We take a piece of wood about the size and shape of a pencil from the tree," explained Routson.
"In our chronology for the south San Juan mountains we created a record that extends back 2,200 years," said Routson.
The chronology extends many years earlier than the medieval period, during which two major drought events in that region already were known from previous chronologies.
"The medieval period extends roughly from 800 to 1300 A.D.," said Routson. "During that period there was a lot of evidence from previous studies for increased aridity, in particular two major droughts: one in the middle of the 12th century, and one at the end of the 13th century."
"Very few records are long enough to assess the global conditions associated with these two periods of Southwestern aridity," said Routson. "And the available records have uncertainties."
"There was another period of increased aridity even earlier," said Routson. "This new record shows that in addition to known droughts from the medieval period, there is also evidence for an earlier megadrought during the second century A.D."
"What we can see from our record is that it was a period of basically 50 consecutive years of below-average growth," said Routson. "And that's within a much broader period that extends from around 124 A.D. to 210 A.D. — about a 100-year-long period of dry conditions."
The prolonged drought in the 12th century and the newly discovered event in the second century A.D. may both have been influenced by warmer-than-average Northern Hemisphere temperatures, Routson said: "The limited records indicate there may have been similar La Nina-like background conditions in the tropical Pacific Ocean, which are known to influence modern drought, during the two periods."
For further information: http://www.uanews.org/node/43022
Photo: Cody Routson