An analysis of thousands of tree rings from around the Pacific Rim reveals that El Niño activity is at its highest level in seven centuries, and researchers at the University of Hawaii suggest that is likely due to global warming.

El Niño, which involves warmer sea surface temperatures in the tropical Pacific, can trigger droughts, floods and other weather disturbances worldwide. In Hawaii, it typically leads to drier winters and a more active hurricane season in the summer.

A team of scientists led by Jinbao Li and Shang-Ping Xie at UH-Manoa’s International Pacific Research Center analyzed 2,222 tree-ring growth records from Asia, New Zealand and North and South America. Tree rings are good proxies for rainfall and temperature measurements.

The study, "El Niño modulations over the past seven centuries," was published July 2 in the journal Nature Climate Change.

Because El Niño is a tropical phenomenon, the researchers were eager to include tropical trees, along with other ancient trees from cooler climates that are often studied. But most tropical trees do not have rings. After hiking for days, Li and co-author Edward R. Cook were able to track down some specimens in Indonesia that allowed them to look back in time 700 years, Xie said in a telephone interview.

"We were able to find trees in the tropical regions with clear rings — that’s quite rare," Xie said, noting that the researchers used drills to peer inside so as not to harm the trees.

Evidence of El Niño activity encoded in the tree rings corresponded closely with data from equatorial Pacific corals and other temperature reconstruction data, producing what the authors described as a record of "unprecedented accuracy."

"Trees and corals don’t talk, yet we still find correlations between the two," said Xie, a meteorology professor in UH’s School of Ocean and Earth Science and Technology. "One lives on the mountains and one lives under the sea. The only reason they agree is because they are both influenced by El Niño."

The data indicated that El Niño, known in the scientific world as El Niño-Southern Oscillation or ENSO, has been most active in recent decades, when compared to records dating to the 1300s, Xie said.

"Our data indicate that ENSO activity in the late 20th century was anomalously high over the past seven centuries, suggestive of a response to continuing global warming," the authors wrote.

The scientists analyzed volcanic activity over the centuries to help assess how disturbances in the atmosphere might affect El Niño. Immediately after a major eruption, they found that temperatures dropped because volcanic ash blocked the sun, followed by unusual warming the next year.

"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," said Li, a professor at the University of Hong Kong. "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 warning."

Xie, who is also a climate science professor at Scripps Institution of Oceanography, said climate models diverge and their study adds more evidence to the picture.

"Many climate models do not reflect the strong El Niño-Southern Oscillation response to global warming that we found," Xie said. "Our results now provide a guide to improve the accuracy of climate models and their projections of future ENSO activity. If this trend of increasing ENSO activity continues, we expect to see more weather extremes such as floods and droughts."

"Nobody knows exactly what the future holds," Xie added, "but the nature of science is to look into the past the best way we can and try to improve the model to predict the future."

In related news, the United Nations World Meteorological Organization announced last week that the planet has warmed faster since the turn of the century than ever recorded, Bloomberg News reported. The 2000-10 decade was the warmest yet for both hemispheres, and about 94 percent of countries logged their warmest 10 years on record, according to Bloomberg News.

"Rising concentrations of heat-trapping greenhouse gases are changing our climate, with far-reaching implications for our environment and our oceans," Michel Jarraud, the organization’s secretary general, said, according to Bloom­berg. "Given that climate change is expected to lead to more frequent and intense heat waves, we need to be prepared."