Study: Recent strengthening of eastern Pacific ENSO is unprecedented in the last millennium paleorecord (DOI: 10.1126/science.ady2660)
As an El Niño of historic proportions is taking shape in the tropical Pacific, a University of Michigan study has found that El Niño events have become nearly 40% stronger in the last 40 years than they were in the pre-industrial era.
In fact, 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.
"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 U-M Department of Earth and Environmental Sciences. "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."
The findings, published in Science, were supported by the U.S. National Science Foundation and the United Kingdom Natural Environmental Research Council.
A history written in coral
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.
El Niño extremes occur every few years, so the researchers focused on core samples that spanned at least 20 years. Sampling the cores a millimeter at a time, the group measured two aspects of the coral skeleton's chemistry. They analyzed the ratio of the elements strontium to calcium, which depends on the temperature at which the coral grew, and supplemented these results with analysis of the ratio of oxygen isotopes, also a measure of temperature at this site.
They 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."
"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."
Confirming the connection
The researchers then examined whether natural processes could have caused similar intensifications of El Niño over the past thousand years. To do this, they used climate models that simulate the last thousand years using histories of volcanic eruptions and solar variability. The models found no large shifts in El Niño that suggested natural processes could create such large changes.
Cole says their findings underscore the need to mitigate more global warming.
"El Niño is a really big source of climate extremes, and so if it's getting stronger, the impacts are getting stronger. Impacts like droughts, floods, wildfire and food insecurity," she said. "Changes in the hydrologic cycle also lead to issues like damage to infrastructure: floods that wipe out homes, highways or railroads, or to health effects, such as diseases like cholera.
"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. 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 Galápagos National Park and the Charles Darwin Research Station in Galápagos also provided support for the research. Cole's co-authors include U-M researchers Kelsey Dyez, Cameron Tripp, Jonathan Overpeck and alumnus Jake Okun; University of Arizona researchers Diane Thompson and Marcus Lofverstrom; Samantha Stevenson-Karl of the University of California, Santa Barbara; University of Edinburgh researcher Sandy Tudhope; Colorado College researcher Allison Lawman; University of Illinois researcher Jessica Conroy; Gloria Jimenez of Moody's Risk Management Services; and University of Minnesota researcher R. Lawrence Edwards.
