Researchers Associate Unusual El-Nino Activity of 20th Century with Global Warming

Researchers have found that the last few years of the 20th century saw unusually high El-Niño activity when compared with climate records of the past seven centuries. The drastic change in climate is thought to be a result of global warming.

The El Niño - Southern Oscillation (ENSO) is a band of warm ocean water that develops off the coast of South America ever five years. During El-Niño, the amount of rainfall in Hawaii decreases while Pacific winter storms shift downwards, increasing odds of floods in California. Predicting ENSO activity is nearly impossible because it is a natural phenomenon and changes over time. Researchers can't rely on climate records alone as they are insufficient to find if the increase in activity is due to natural causes or man-made ones.

A study group led by Jinbao Li and Shang-Ping Xie, at the International Pacific Research Center, University of Hawaii at Manoa, have now completed over 2,200 tree-ring records of reflecting ENSO activity of the past seven centuries. The tree-rings are supposedly efficient instruments used to measure temperature and rainfall across a region. The research team has complied data from both the tropics and mid-latitudes in both hemispheres, giving an in-depth look at the changes in climate over the past few centuries.

Their records show that ENSO was unusually active in the 20^th century when compared to the rest of the seven centuries. They attribute the changes in ENSO activity to the rise of greenhouse gases which has led to 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 ENSO activity in the late 20th century is a footprint of global warming," Jinbao Li, the lead author of the study said in a news release.

"Many climate models do not reflect the strong ENSO response to global warming that we found," said Shang-Ping Xie, meteorology professor at the International Pacific Research Center and c-author of the study. "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 ENSO activity.

Xie added that if the ENSO activity keeps increasing, more drastic floods and droughts can be expected.

The study is published in the journal Nature Climate Change.