Washington: Researchers have found that El Nino - Southern Oscillation (ENSO) that spawns droughts, floods, and other weather disturbances world-wide, was unusually active in the late 20th century.
Tree-rings have been shown to be very good proxies for temperature and rainfall measurements.
An international team of scientists spearheaded by Jinbao Li and Shang-Ping Xie, while working at the International Pacific Research Center, University of Hawaii at Manoa, compiled 2,222 tree-ring chronologies of the past seven centuries from both the tropics and mid-latitudes in both hemispheres.
The inclusion of tropical tree-ring records enabled the team to generate an archive of ENSO 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 ENSO 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.
Lead author Jinbao Li said that in the year after a large tropical volcanic eruption, their record shows that the east-central tropical Pacific is unusually cool, followed by unusual warming one year later.
He asserted that 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.
Co-author Shang-Ping Xie, meteorology professor at the International Pacific Research Center, University of Hawaii at Manoa and Roger Revelle Professor at Scripps Institution of Oceanography, University of California at San Diego, said that many climate models do not reflect the strong ENSO response to global warming that they found.
He explained that this suggested that many models underestimate the sensitivity to radiative perturbations in greenhouse gases.
Xie said that the results now provide a guide to improve the accuracy of climate models and their projections of future ENSO activity, adding that if this trend of increasing ENSO activity continues, we expect to see more weather extremes such as floods and droughts.
The study has been published in the online issue of Nature Climate Change.