ancient tree

Ancient trees, such as Polylepis tarapacana growing in rocky soils in the South American Altiplano, are sensitive to climate anomalies associated with large-scale climate patterns stemming from the El Niño–Southern Oscillation. (Photo courtesy of Duncan Christie)

An international team of scientists spearheaded by former Postdoctoral Fellow Jinbao Li and Professor Shang-Ping Xie of the University of Hawaiʻi at Mānoa’s International Pacific Research Center has compiled 2,222 tree-ring chronologies of the past seven centuries from both the tropics and mid-latitudes in both hemispheres. Tree-rings have been shown to be very good proxies for temperature and rainfall measurements. Their work was published in the June 30 online issue of Nature Climate Change.

The inclusion of tropical tree-ring records enabled the team to generate an archive of El Niño–Southern Oscillation activity of unprecedented accuracy, as attested by the close correspondence with records from equatorial Pacific corals and with an independent Northern Hemisphere temperature reconstruction that captures well-known teleconnection climate patterns.

These proxy records all indicate that El Niño–Southern Oscillation was unusually active in the late 20th century compared to the past seven centuries, implying that this climate phenomenon is responding to ongoing global warming.

“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 El Niño–Southern Oscillation activity in the late 20th century is a footprint of global warming,” explains lead author Li.

“Many climate models do not reflect the strong El Niño–Southern Oscillation response to global warming that we found,” says co-author Xie. “This suggests that many models underestimate the sensitivity to radiative perturbations in greenhouse gases. Our results now provide a guide to improve the accuracy of climate models and their projections of future El Niño–Southern Oscillation activity. If this trend of increasing El Niño–Southern Oscillation activity continues, we expect to see more weather extremes such as floods and droughts.”

Adapted from a International Pacific Research Center news release.

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