Scientists have compiled 2,222 tree-ring chronologies of the past seven centuries in order to achieve an accurate record of the ENSO (El Nino - Southern Oscillation).
The graph below shows the variability of El Nino going back to the 1300s from tree-ring analysis (blue line). Instrumental analysis (red line) was also done for the 20th century. The dashed lines indicate boundaries for natural variability. El Nino behavior is largely beyond natural variability late in the 20th century. Image courtesy of the International Pacific Research Center.
The international research team, led by Jinbao Li and Shang-Ping Xie, found that the ENSO was unusually active late in the 20th century compared to the past seven centuries, implying that this climate phenomenon is responding to ongoing global warming, according to the EurekAlert report.
Key comments from the lead author's of this study (via EurekAlert)
"In the year after a large tropical volcanic eruption, our record shows that the east-central tropical Pacific is unusually cool, followed by unusual warming one year later. Like greenhouse gases, volcanic aerosols perturb the Earth's radiation balance. This supports the idea that the unusually high ENSO activity in the late 20th century is a footprint of global warming," explains Jinbao Li.
If this trend of increasing ENSO activity continues, we expect to see more weather extremes such as floods and droughts, says Xie.
This new study is published in the June 30, 2013, online issue of Nature Climate Change.
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