March 30, 2012

More Years Of Drought According To The Trees

The National Science Foundation has funded a study to determine past climates by studying tree rings. Environmental students from Arizona studied bristlecone pine trees to determine aspects about the Earth´s climates during the Paleo age.

The study of tree growth rings to understand Earth´s past climate is called dendrochronology. As each year passes, trees add growth rings around their trunks. These dendrochronologists study these rings to gain all sorts of information not only about the tree itself but also about the environment surrounding the tree. For example, by counting the rings backwards from the center, scientists can determine the age of the tree. By measuring how far apart the rings are from one another, the scientists can determine how much the tree grew in a given year. In wetter years with ample rain fall, these rings will be wide while smaller, narrower rings indicate a dry year. By averaging these rings for a pattern specific to pattern, these dendrochronologists can determine the past climate of the area where the tree resides.

Cody Routson, a doctoral candidate in the environmental studies lab at the University of Arizona´s (UA) geosciences department, and associate professor Connie Woodhouse and professor Jonathan Overpeck, both from the geosciences department at UA, studied the tree rings to determine the climate during the Paleo age in the Southwestern US. In particular, these students wanted to determine the history of the North American summer monsoon. As they worked, the team created a new tree ring record to compare with existing regional records to evaluate periods of extreme drought over the past two millennia.

The University of Arizona students used bristlecone pine trees, the longest living and oldest species on Earth (some are even over 4,000 years old) to conduct their research. As these trees are so old and of so much value to the research, the team used an increment borer to obtain samples. This machine extracts only a small sample-about the size of a pencil- from the center of the tree, leaving the majority of the tree unharmed. Samples were collected from both dead and living trees and were analyzed at the University lab. Once there, the students used a visual graphic technique to match the overlapping in growth between the live tree samples and the dead tree samples. Once these patterns were matched, the team then set out to calculate an average create a chronology for the site. Once implemented, they had developed a chronology for the south San Juan Mountains extending back 2,200 years. During this time frame, 2 major drought events had occurred and have been documented in other studies.

Speaking to reporters for NSF.gov, Rouston had this to say: “The medieval period extends roughly from 800 to 1300 AD. 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, and the available records have uncertainties.”

The bristlecone chronology told another story, however.

“There was another period of increased aridity even earlier,” says 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 AD. 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 AD to 210 AD–about a 100-year-long period of dry conditions.”

“We´re showing that there are multiple extreme drought events that happened during our past in this region,” said Routson. “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.”


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