As an El Niño of historic proportions is taking shape in the tropical Pacific, a study published in Science has found that El Niño events have become nearly 40% stronger in the last 40 years than they were in the pre-industrial era.

The study, which examined modern and ancient corals in the Galápagos Islands, found that El Niño events in the last four decades were stronger than any in the 1,000 years prior to about 1850, when humans began impacting the climate with greenhouse gases. Additional work by researchers at the University of Arizona leveraged computer simulations to probe whether the wide variability of El Niño in and by itself could account for observed increase in strength.

Image

"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 the warming of global temperature," said lead author Julia Cole, professor and chair of the Department of Earth and Environmental Sciences at the University of Michigan. "Our findings tell us that the big El Niño events of the last 40 years are not normal in the context of the last thousand years."

El Niño is a natural phenomenon that, every few years, causes the tropical Pacific to become warmer than usual. Under normal conditions, trade winds blow along the equator from east to west. This pushes warm water from South America toward Australia. But when the trade winds weaken, warm water spreads eastward, back towards South America, kicking off an El Niño event.

The atmospheric circulation responds, as strong rainfall shifts from Indonesia into the central Pacific, leaving the western Pacific in drought. This weakens the trade winds further, locking in El Niño conditions that can persist for one to two years. These temperature and precipitation fluctuations originate in the tropical Pacific, but they affect weather all over the world.

Storm tracks over the U.S. shift southward, bringing more rain to the desert southwest and less to the northwest. Similar shifts around the world lead to droughts and flooding that bring disease, crop failure, wildfires and other crises that impact people's health and well-being.

"The question is not whether the current El Niño is going to happen, but how bad is it going to be, and how bad will the impacts be?" Cole said. "This event is superimposed on global warming, and it's likely to supercharge the temperature increase that we would normally see from greenhouse gases. There are forecasts that we might get as hot as 1.7 or 1.8 degrees Celsius above pre-industrial temperatures, which is quite a bit higher than the current record."

A history written in coral

Image

To examine historic El Niño patterns, Cole and fellow researchers sampled cores from 13 corals, including living colonies and boulders of ancient coral, from the Galápagos Islands. Corals grow one to two centimeters per year by secreting layers of calcium carbonate. The chemistry of these layers provides a record of the seawater temperature in which the corals grew.

The study fills a critical gap in the spatial understanding of what is happening with El Niño patterns, according to Diane Thompson, one of the study's co-authors who is an associate professor in the department of Geosciences and director of marine research at the University of Arizona's Biosphere 2.

"As the world's largest ocean basin, the Pacific has an oversized impact on the Earth's climate," Thompson said. "If we are going to understand how the Pacific is changing, we need to look in places that are right in the center of action, like the Galapagos."

The researchers were left with a history of variations in temperature in the Galápagos – a location, Cole says, where El Niño has its largest impact. They saw that strong El Niños occurred in the last 40 to 50 years, whereas the El Niños prior to that time period showed "a pretty consistent pattern of lower intensity El Niños."

Confirming the connection

Attributing the changes in variation to climate change is an important but difficult problem because El Niño naturally is a varying system, Thompson explained.

"If you take hundreds of years, you're going to get decades where it is more active, and you're going to get decades where it's less active," she said. "Sometimes it's strong, sometimes it's weak, all of that is normal." 

To solve this conundrum, Thompson and Marcus Lofverstrom, associate professor in the U of A Department of Geosciences, compiled and analyzed simulations from climate models that allowed them to disentangle El Niño's natural, inherent variability from what climate scientists call "forcings" – things like volcanic activity, changes in the orbital configuration between the sun and the Earth or atmospheric warming driven by fossil fuel burning. In short, they examined whether natural processes could have caused similar intensifications of El Niño over the past thousand years.

"What we're seeing is the last few decades being pretty exceptional relative to what we'd expect if El Niño was just behaving as it has in the past," Thompson said.

"We kept adding records thinking well, this is going to get more complicated, but it really didn't," Cole said. "This is such a clear story."

"We believe global warming is supercharging El Niño, and if that's true, then we expect stronger climate extremes that will amplify ecological, infrastructural and human losses," Cole said. "No country has the resources to be fully protected from these impacts. This is one more reason we need to move away from fossil fuels, the root cause of the problem."

The findings were supported by the U.S. National Science Foundation and the United Kingdom Natural Environmental Research Council. The Galápagos National Park and the Charles Darwin Research Station in Galápagos also provided support for the research.