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Study Of Tree Rings Reveals Climate Change’s Role In El Niño

Aired 7/2/13 on KPBS News.

By looking at tree rings dating back seven centuries, climate scientists have found that man-made climate change has made El Niño worse in recent years.

Wildfires may be wreaking havoc in the Southwest right now, but it's only a matter of time until El Niño returns. And Jinbao Li of the University of Hong Kong thinks the flooding it often causes in Southern California will only get worse.

This Douglas-fir sample from the Southwest has annual tree rings dating back to the year 1527. The narrowing of the rings that formed from the 1560s through the 1590s indicates that the tree grew little during the 16th century megadrought.
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Above: This Douglas-fir sample from the Southwest has annual tree rings dating back to the year 1527. The narrowing of the rings that formed from the 1560s through the 1590s indicates that the tree grew little during the 16th century megadrought.

Li's latest research suggests that global warming has been aggravating El Niño in recent years. To reach this conclusion, Li and his colleagues (including Shang-Ping Xie of the Scripps Institution of Oceanography) looked at thousands of tree rings dating back seven centuries.

In a study published over the weekend, the researchers plotted history's El Niño patterns by studying trees from places as far apart as Indonesia, South America and New Zealand. In places that get lots of precipitation, tree rings grow wider during El Niño years. And they get skinnier in places that dry up.

Li sees these El Niño rings getting wider and skinnier lately, and he says global warming is to blame. El Niño occurs when water heats up in the Pacific Ocean. As temperatures rise, so will the effects of El Niño.

"There will be more severe drought and more widespread forest fires during La Niña years," says Li. "And there will be more storm floods during El Niño years in Southern California."

The 1997 El Niño cycle was one of the worst Li observed. He said years like '97 are becoming the new normal as climate change accelerates.

On Monday, another team of climate scientists proposed a method for predicting El Niño as far as a year in advance. Li thinks that recent research has deepened our understanding of this anomalous weather phenomenon, and predictive models will improve.

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