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, says a new study from scientists at the University of Arizona.
UA geoscientists Cody Routson, Connie Woodhouse and Jonathan Overpeck conducted a study of 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.
"These mountains are very important for both the San Juan River and the Rio Grande River," said Routson, a doctoral candidate in the environmental studies laboratory of the UA's department of geosciences and the primary author of the study, which is upcoming in Geophysical Research Letters.
The San Juan River is a tributary for the Colorado River, meaning any climate changes that affect the San Juan drainage also likely would affect the Colorado River and its watershed. Said Routson: "We wanted to develop as long a record as possible for that region."
Dendrochronology is a precise 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. "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."
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.
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. "We drove around and looked for old trees," said Routson.
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.
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.
"We also sampled dead wood that was lying about the land. 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. 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," said Routson. "It was pretty profound that we were able to get back that far."
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.
"And the available records have uncertainties."
But the chronology from the San Juan bristlecone pines showed something completely new:
"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."
"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."
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."
Although natural climate variation has led to extended dry periods in the southwestern U.S. in the past, there is reason to believe that human-driven climate change will increase the frequency of extreme droughts in the future, said Routson. In other words, we should expect similar multi-decade droughts in a future predicted to be even warmer than the past.
Routson's research is funded by fellowships from the National Science Foundation and the Science Foundation Arizona. His advisors, Woodhouse of the School of Geography and Development and Overpeck of the department of geosciences and co-director of the UA's Institute of the Environment, are co-authors of the study.
Study report: Routson, C. C., C. Woodhouse, and J. T. Overpeck (2011), Second century megadrought in the Rio Grande headwaters, Colorado: How unusual was medieval drought?, Geophys. Res. Lett., doi:10.1029/2011GL050015, in press:
UA Department of Geosciences: http://www.geo.arizona.edu
UA School of Geography and Development: http://geography.arizona.edu
UA Institute of the Environment: http://www.environment.arizona.edu
Daniel Stolte | University of Arizona
Europe’s Demographic Future. Where the Regions Are Heading after a Decade of Crises
10.08.2017 | Berlin-Institut für Bevölkerung und Entwicklung
Scientists reveal source of human heartbeat in 3-D
07.08.2017 | University of Manchester
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
18.08.2017 | Life Sciences
18.08.2017 | Physics and Astronomy
18.08.2017 | Information Technology