Scientists change how El Niño is labeled as warming shifts Pacific patterns

Scientists say the natural El Niño cycle, which can warp weather worldwide, is both adding to and being shaped by a warming world, meteorologists said. The updates come as researchers try to explain why Earth’s already-rising temperatures spiked to a new level over the past three years and as forecasters adjust how they recognize when the Pacific has flipped between El Niño and La Niña.

A study described by Japanese researchers, published in Nature Geoscience, calculated that an unusual recent twist in the warming and cooling cycle—including El Niño and its counterpart La Niña—could help explain part of the scientific mystery of the temperature jump. In a separate development, scientists said the U.S. National Oceanic and Atmospheric Administration has altered how it determines when the weather pattern has entered a new cycle, a change expected to shift how often the Pacific is labeled El Niño versus La Niña as global warming continues.

The Nature Geoscience study examined Earth’s energy imbalance, a measure of how much energy is coming into the planet versus how much is leaving. The researchers said an increased imbalance—more trapped heat—leads to warmer temperatures. They calculated that about three-quarters of the change in Earth’s energy imbalance in 2022 could be attributed to a combination of long-term human-caused climate change and a shift from a three-year cooling La Niña cycle to a warm El Niño one.

Scientists also described why a long La Niña can matter for how energy stays trapped. Yu Kosaka, a climate scientist at the University of Tokyo and a co-author of the study, compared the situation to fevers: if the body temperature runs high, it tends to emit more energy outward, while “for three-year La Nina, it’s opposite,” Kosaka said. She said La Niñas more typically correspond to a one- or two-year buildup of extra energy imbalance, but that the unusual length of this La Niña made the difference more noticeable and included hotter temperatures.

Former NOAA meteorologist Tom Di Liberto, now at Climate Central, described the transition from La Niña to El Niño as releasing heat. “When there is a transition from La Nina to El Nino, it’s like the lid is popped off,” Di Liberto said.

The study authors said about 23% of the energy imbalance driving the recent higher temperatures came from that unusually long La Niña pattern, while slightly more than half of the rest came from greenhouse gases produced by burning coal, oil and gas. The remainder, the authors said, could include other factors.

Why La Niña can dampen growth—and El Niño can boost it

Meteorologists described the basic difference between the phases. El Niño involves cyclical, natural warming of patches of the equatorial Pacific that then changes weather patterns worldwide, while La Niña is marked by cooler than average waters. Both phases shift precipitation and temperature patterns, but in different ways: El Ninos tend to increase global temperatures, while La Ninas depress the long-term rise.

Scientists also said La Niñas tend to cause more damage in the United States because of increased hurricane activity and drought, citing prior studies. And beyond its effects on surface temperatures, the phases also influence how the ocean and atmosphere exchange energy, which can affect how much heat escapes back into space.

Changing how scientists label El Niño and La Niña

Separate from the new research on energy imbalance, scientists said forecasters have had to update how they label El Niño and La Niña because “normal” keeps shifting in a warming world. For about 75 years, meteorologists calculated the phases based on the difference in temperature across three tropical Pacific regions compared with normal. In that approach, an El Niño was 0.5 degrees Celsius (0.9 degrees Fahrenheit) warmer than normal, and La Nina was cooler by the same amount.

Until now, NOAA used the 30-year average as normal, updating that definition every decade. But scientists said the water warmed enough that NOAA updated its definition of normal every five years as well. Nat Johnson, a meteorologist at NOAA’s Geophysical Fluid Dynamics Lab, said that wasn’t enough either, and that NOAA developed an El Niño index that is relative, starting this month.

Johnson said the new index compares temperatures to the rest of Earth’s tropics. He said the difference between the old and new methods recently has been as much as half a degree Celsius (0.9 degrees Fahrenheit), “and ‘that’s enough to have an impact,’” Johnson said. He said the interaction between the ocean and the atmosphere is what matters most and that, recently, those interactions did not match the old labeling but do match the new method.

The updated method, Johnson said, is likely to mean “a bit more La Ninas and fewer El Ninos than in the old system.” The shift matters because the label affects how forecasts and research interpret what comes next in global weather patterns.

What comes next, and how warm water could reshape forecasts

Looking ahead, NOAA’s forecast described by the article said an El Niño is expected to develop later this year in late summer or fall. The article said if it develops early enough, it could dampen Atlantic hurricane activity, but it would also mean warmer global temperatures in 2027.

Jennifer Francis of the Woodwell Climate Research Center, who said she was not involved in the energy-imbalance study, said the research made sense and explained some of the increase in energy imbalance that other scientists had attributed to accelerated warming. In an email, Francis said: “When El Nino develops, we’re likely to set a new global temperature record.” She also said “‘Normal’ was left in the dust decades ago. And with this much heat in the system, everyone should buckle up for the extreme weather it will fuel.”