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Saturday, October 25, 2014         

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Trees provide natural record of El Niño trends

By Jim Borg

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Studying treering records from the southwestern United States, University of Hawaii scientists have helped assemble a 1,100-year historical picture of the climate phenomenon known as El Niño, offering an avenue to understanding how weather patterns could change in a warming world.

El Niño, associated with warmer than usual sea surface temperatures in the eastern tropical Pacific, typically produces a wide array of violent weather, including more rain and intense storms in some areas, less rain in others. The massive El Niño of 1997-98, for instance, caused flooding and landslides in Northern California, drought and famine in Bangladesh and drought and forest fires in the Philippines; 2,100 people died worldwide.

UH scientist Jinbao Li said by email that the record implies that warmer oceans will lead to more severe El Niños and the opposite phenomena, La Niñas, and more extreme climate conditions around the globe. But a final verdict awaits better climate models, he said.

Formally known as the El Niño Southern Oscillation, the phenomenon also brings rainy winters to the U.S. Southwest, where tree rings are wide in wet years and narrow in dry years.

UH researchers Li, Shang-Ping Xie, Fei Lui and Jian Ma analyzed the tree-ring records available in a database called the North American Drought Atlas and found that the results correlated closely with the 150-year sea surface temperature records in the tropical Pacific. The tree-ring data also matched the record provided by the study of oxygen isotopes in coral around Palmyra Atoll south of Hawaii.

"Our work revealed that the towering trees on the mountain slopes of the U.S. Southwest and the colorful corals in the tropical Pacific both listen to the music of El Niño, which shows its signature in their yearly growth rings," said Li, an environmental scientist and postdoctoral fellow with the UH International Pacific Research Center. "The coral records, however, are brief, whereas the tree-ring records from North America supply us with a continuous El Niño record reaching back 1,100 years."

The research was published Friday in the journal Nature Climate Change. Scientists from China and Columbia University's Lamont-Doherty Earth Observatory also took part.

The research shows high variability in the intensity of El Niños over the centuries.

The strongest activity has been since the 18th century, the scientists said.

The weakest activity was during the 11th century, a period known as the Medieval Climate Anomaly, which saw warm temperatures in Europe, Greenland and Asia and bone-dry conditions in the American West. That was followed by a centuries-long cold period known as the Little Ice Age.






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