El Niño Events Nearly 40% Stronger in Last 40 Years, Study Finds

Climate change is intensifying one of Earth's most powerful natural climate patterns, a new University of Michigan study finds. El Niño is a natural warming of the tropical Pacific Ocean that recurs every few years and disrupts weather worldwide. The study found El Niño events have grown nearly 40% stronger over the past 40 years than in the preindustrial era, stronger than any El Niño in the roughly 1,000 years before about 1850, when humans began adding greenhouse gases to the atmosphere. The findings appear in the journal Science.

To reconstruct that history, lead author Julia Cole, professor and chair of earth and environmental sciences at the University of Michigan, and colleagues sampled 13 coral cores from the Galápagos Islands, including living colonies and ancient coral boulders. Corals grow 1 to 2 centimeters a year, laying down calcium carbonate layers whose chemistry forms a coral record of past seawater temperature. The team measured the ratio of strontium to calcium and of oxygen isotopes in each layer, both temperature markers.

The record showed a "pretty consistent pattern of lower intensity El Niños" before roughly the last 40 to 50 years. "We don't see a time in the past where El Niños have been as strong as today, and we show that the strength of El Niño changes in parallel with warming global temperatures," Cole said, adding that the big El Niño events of the last 40 years are "not normal in the context of the last thousand years."

To check whether natural causes could explain the shift, the researchers ran climate models simulating the past 1,000 years using known volcanic and solar history. The models found no natural process that could produce a change this large. "This is such a clear story," Cole said. An El Niño forms when weakening trade winds let warm Pacific water, which usually flows toward Australia, spread back toward South America, lasting one to two years.