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

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 ash).

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.