The El Niño–Southern Oscillation (ENSO) is one of the most significant, but variable, climate patterns in the world. This tropical Pacific Ocean phenomenon affects weather in South America, Australia, Asia, and beyond. During an El Niño event, the sea surface temperature of the eastern tropical Pacific Ocean warms, and trade winds weaken. El Niño’s associated weather extremes often have dramatic implications for public health and global supply chains, setting off natural disasters like flooding in Central and South America and droughts in southeast Asia.
ENSO is so influential that climate scientists have dedicated decades to tracking and predicting its irregular cycles. Researchers are also studying how ENSO will be affected by climate change. Now, new research published in Nature Climate Change has used cutting-edge climate models to predict that by 2040, El Niño events will become more frequent because of changes to the climate. These events are already in motion and will happen regardless of short-term emissions mitigation efforts, according to the authors.
“This [finding] is another layer on a growing pile of work that is pointing quite conclusively to ongoing changes to ENSO related to greenhouse gases,” said Kim Cobb, a climate scientist at the Georgia Institute of Technology who was not involved in the new research.
Modeling a Future of More Frequent Extremes
Thanks to a combination of atmospheric science studies based on sophisticated climate modeling and geoscience studies based on paleoclimate data derived from fossil coral records, researchers generally agree that human-caused climate change will soon increase El Niño frequency and intensity. Some studies even indicate that climate change could already be having an effect on El Niño. However, because the ENSO pattern is so variable, it has been difficult for scientists to decipher climate change signals from natural variability.
In search of answers, a team of experts turned to state-of-the-art climate models based on the same code used to make weather forecasts, said Mat Collins, a climate change researcher at the University of Exeter in the United Kingdom and a coauthor of the paper. “When you see the weather map on the TV, that’s the same principle,” Collins said.
The team analyzed how four possible future emissions scenarios might affect ENSO-related sea surface temperature and rainfall between 2015 and 2099. In the most sustainable future scenario they modeled, carbon dioxide emissions reach net zero around 2050 and then begin to decline by 2099. In the most fossil fuel intensive scenario, carbon dioxide emissions more than double by 2099. These are the same Shared Socioeconomic Pathways used to inform the recent Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report. The researchers found that regardless of any emissions mitigation efforts taken in upcoming decades, climate change will lead to an increase in El Niño–associated rainfall patterns by 2040.
Climate change’s impact on ENSO-related sea surface temperature was predicted to emerge slightly later, becoming clear by 2070. The difference in timing of emergence is likely because there is less background noise in the rainfall data compared to the temperature data, Collins said. In other words, sea surface temperature in the tropical Pacific isn’t necessarily being affected more slowly than rainfall; rather, the climate change signal must be stronger before it can be differentiated from natural variability in the data. “It’s the relative change in the sea surface temperatures that matters, not the absolute change,” he said.
Some Consequences Are Already Locked In
Not only are these findings consistent with takeaways from the IPCC Sixth Assessment Report, but they are also in line with thousands of years of paleoclimate data from the central tropical Pacific, which indicate that El Niño extremes have been stronger in the past 50 years than they were in the preindustrial era, said Cobb.
“We are a day late and a dollar short,” she said. “We are locked into certain increments of warming no matter what.”
More frequent El Niño events will likely mean more floods, droughts, wildfires, and coral bleaching episodes, Collins said. These effects, driven by a more localized climate pattern, will be felt on top of the effect of a steady rise in global temperatures and showcase how climate change will have compounding impacts on global weather patterns.
“There’s this idea that climate change is something that’s going to happen in 100 years’ time, but really, it’s happening now,” Collins said. “We’re likely to see potentially big changes in the climate system in the next few decades.”
—Rachel Fritts (@rachel_fritts), Science Writer