One in two El Niño events could be extreme by mid-century

In a new study published Sep. 25 in the journal Nature, a University of Colorado Boulder climate scientist and collaborators reveal that the planet could see more frequent extreme El Niño events by 2050 if greenhouse gas emissions continue to increase.

"It's pretty scary that 2050 is not very far away," said Pedro DiNezio, the paper's co-lead author and associate professor in the Department of Atmospheric and Oceanic Sciences. "If these extreme events become more frequent, society may not have enough time to recover, rebuild and adapt before the next El Niño strikes. The consequences would be devastating."

Shifting wind and soaring temperatures

El Niño occurs when water temperatures along the equator in the Pacific Ocean rise by at least 0.9 °F above average for an extended period.

The seemingly marginal temperature change can shift wind patterns and ocean currents, triggering unusual weather worldwide, including heat waves, floods and droughts.

When the area warms by 3.6°F above average, scientists classify the El Niño event as extreme. Since the U.S. National Oceanic and Atmospheric Administration began collecting data in the 1950s, the agency has recorded up to four extreme El Niño events.

During an extreme El Niño, the impacts on global weather tend to be more severe. For example, during the winter of 1997-98, El Niño brought record rainfall to California, causing devastating landslides that killed more than a dozen people. Over the same period, the planet lost about 15% of its coral reefs due to prolonged warming.

Last winter El Niño almost reached extreme magnitude, DiNezio said.

"El Niño events are difficult to simulate and predict because there are many mechanisms driving them. This has hindered our ability to produce accurate predictions and help society prepare and reduce the potential damage," they said.

Prior research suggests that climate change is intensifying and increasing the frequency of extreme weather events, possibly linked to changes in El Niño patterns. However, due to limited data, scientists have yet to confirm whether El Niño will strengthen with warming.

DiNezio and their team set out to simulate El Niño events in the past 21,000 years -- since the peak of Earth's last Ice Age -- using a computer model.

The model shows that during the Ice Age, when Earth's climate was colder, extreme El Niño events were very rare. As the planet warmed since the end of the Ice Age, the frequency and intensity of El Niño have been increasing.

The team validated the model by comparing the simulated data with past ocean temperature data retrieved from fossilized shells of foraminifera, a group of single-celled organisms ubiquitous in the oceans long before human existence. By analyzing the type of oxygen compounds preserved in these fossilized shells, the team reconstructed how El Niño drove ocean temperature fluctuations across the Pacific Ocean for the past 21,000 years. The ancient record aligned with the model's simulations.

"We are the first to show a model that can realistically simulate past El Niño events, enhancing our confidence in its future predictions. We are also proud of the robust technique we developed to evaluate our model, but unfortunately, it brought us no good news," DiNezio added.

The model predicts that if society continues to pump greenhouse gases into the atmosphere at the current rate, one in two El Niño events could be extreme by 2050.

The control knob

Despite El Niño's complexity, the model reveals that a single mechanism has controlled the frequency and intensity of all El Niño events as the planet has warmed since the last Ice Age.

When the eastern Pacific Ocean water warms from natural fluctuation, the winds that always blow east to west over the equatorial Pacific weaken due to changes in air pressure above the ocean. But during an El Niño, weakened winds allow warm water to flow east, and the warmer water weakens the winds even more, creating a feedback loop known as the Bjerknes feedback.

DiNezio's research suggests that as the atmosphere warms rapidly from greenhouse gas emissions, the planet experiences a stronger Bjerknes feedback, leading to more frequent extreme El Niño events.

With the most recent El Niño now in the past, DiNezio emphasized that society needs to focus on taking measures to reduce the impact from future extreme El Niño events, including cutting emissions and helping communities, particularly those in the developing countries, become resilient to extreme weather.

"We now understand how these extreme events happen, and we just need the will to reduce our reliance on fossil fuels," they said. "Our findings emphasize the urgent need to limit warming to 1.5 °C to avoid catastrophic climate impacts."