Washington: Researchers at the University of Arizona have revealed a previously unknown multi-decade drought period in the second century A.D after studying oldest pine trees.
Almost nine hundred years ago, in the mid-12th century, the southwestern U.S. was in the middle of a multi-decade megadrought. It was the most recent extended period of severe drought known for this region. But it was not the first.
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, said scientists at the University of Arizona’s Laboratory of Tree-Ring Research.
UA geoscientists Cody Routson, Connie Woodhouse and Jonathan Overpeck conducted a study of the southern San Juan Mountains in south-central Colorado.
To develop their chronology, the researchers looked for indications of climate in the past in the growth rings of the oldest trees in the southern San Juan region.
Literally nothing is 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.
“We did a lot of hiking and found a couple of sites of bristlecone pines, and one in particular that we honed in on,” said Routson.
“In our chronology for the south San Juan mountains we created a record that extends back 2,200 years,” he stated.
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.
“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,” revealed Routson.
“What we can see from our record is that it was a period of basically 50 consecutive years of below-average growth. 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,” he added.
The study will be published in Geophysical Research Letters.