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Pinpointing history of droughts through exploration of tree rings: Unexpected complexity in U. S. West's patterns of drought

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  • Roger L. Bagula
    Message 1 of 1 , Jul 2, 2012
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      Pinpointing History of Droughts Through Exploration of Tree Rings:
      Unexpected Complexity in U. S. West's Patterns of Drought

      ScienceDaily (July 2, 2012) — Through an exploration of tree rings and
      oxygen isotopes, researchers at the University of Pittsburgh are now
      able to better pinpoint the history of droughts in the arid and semiarid
      areas of the American West.

      A paper published in the online July 2 issue of the Proceedings of the
      National Academy of Sciences explores the Medieval Climate Anomaly, a
      particularly warm period occurring in the northern hemisphere of the
      American West around 950 to 1250 C.E. While this time period is known as
      being a "dry period," the Pitt researchers have discovered an unexpected
      complexity to the patterns of drought.

      "East of the Cascade Mountains, the Pacific Northwest is now dry and hot
      in the summer and wet in the winter," said Byron A. Steinman, principal
      investigator on the project who earned his PhD in geology from Pitt in
      2011 and is now a postdoctoral researcher at Penn State University.
      "We've found that it may not have been dry in the winter in the Pacific
      Northwest during the Medieval Climate Anomaly."

      Steinman, who worked with Pitt professor of geology and planetary
      science Mark B. Abbott, began by studying tree rings, which often can
      record past precipitation and temperatures. However, tree rings are more
      accurate at recording this information during the spring and summer
      months, when the tree is growing and not lying dormant. To determine the
      validity of the tree-ring data, the researchers decided to undertake a
      study of oxygen isotopes for comparison. They explored isotopes found in
      nearly 1,500 years of bottom-of-lake sediments from two bodies of water
      in Washington state: Castor Lake and Lime Lake. The isotopic composition
      of these sediments, says Steinman, can reflect the amount of water
      entering a lake, especially during the wet season.

      The researchers paid particular attention to the calcium carbonate in
      the water (shown in the form of calcite), as the oxygen in this mineral
      relates directly to the isotope ratio of lake water. Castor Lake is on a
      plateau, and the water inflow comes only from precipitation and
      groundwater. Therefore, no water is lost through evaporation. However,
      Lime Lake loses the majority of its water through a permanent outflow
      stream. By comparing the two lakes, the researchers could determine the
      water balance between evaporation and precipitation.

      To pinpoint the time of the drought, the researchers looked at two
      stable isotopes of oxygen -- oxygen 16 and oxygen 18 -- in the
      sediments. Oxygen 16 is lighter than oxygen 18, and so during
      evaporation more of it is released -- the calcite in the sediments
      containing more of the oxygen 18. If the lakes are full of water,
      however, there will be more oxygen 16 in the calcite. The layers of
      sediments that are laid down each year can be dated either using carbon
      14 dating of organic material or by locating layers of tephra (volcanic

      In the end, however, what they found was a mismatch of data.

      "The tree ring and isotope data matched up on a short-term, decadal
      scale," said Steinman. "However, on a longer-term, century scale, the
      records diverged. The tree-ring data suggests dry conditions during the
      Medieval Climate Anomaly summers while the isotope data suggest
      wetter-than-expected winters."

      In the paper, the researchers suggest a strong centennial relationship
      over the past 1,500 years between winter precipitation and the climate
      variability patterns that shift about every 20 to 30 years in the
      Pacific (known as Pacific Decadal Oscillation-PDO). PDO is linked to the
      El Nino Southern Oscillation, a tropical phenomenon that influences
      global weather patterns.

      "Before and during the Medieval Climate Anomaly, the North Pacific Ocean
      was warmer, and Washington had a greater precipitation than during the
      Little Ice Age, which occurred from 1450 to about 1850 C.E., when there
      was less precipitation," said Steinman.

      Steinman hopes to continue this study, producing additional quantitative
      precipitation records with different lake systems, to better understand
      these climate phenomena.

      Other researchers on this project were Michael E. Mann, professor of
      meteorology and geosciences and director of the Penn State Earth System
      Science Center; Nathan D. Stansell, a former Pitt Ph.D. student who
      graduated in 2009 and now a research fellow at The Ohio State
      University's Byrd Polar Research Center; and Bruce Finney, professor of
      biological sciences at Idaho State University.
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