Scientists Redefine El Niño Classification Amid Rising Global Temperature Trends

WASHINGTON – The natural climate phenomenon known as El Nino, which significantly influences global weather patterns, is both a contributor to and a consequence of a warming planet, according to meteorologists.

Recent research has provided insight into an unusual alteration within the El Nino and La Nina cycles, shedding light on the scientific puzzle of why Earth’s temperature, already on an upward trajectory, has surged dramatically over the past three years.

Meanwhile, scientists have had to revise their classification of El Nino and La Nina due to the swift changes in weather driven by climate change. With global ocean temperatures rising, the U.S. National Oceanic and Atmospheric Administration recently adjusted its criteria for determining when these weather patterns shift phases. This adjustment might result in more frequent La Nina events and fewer instances of El Nino, given the warming of tropical waters.

In early 2023, Earth’s average monthly temperature saw a significant rise above the existing trend linked to anthropogenic climate change, continuing this pattern through 2025. Scientists propose various explanations for this phenomenon, including intensified greenhouse gas effects, diminished maritime particle pollution, a volcanic eruption beneath the ocean, and increased solar activity.

A study published in Nature Geoscience by Japanese researchers this month examines the increase in Earth’s energy imbalance in 2022. This imbalance, characterized by more heat being trapped, leads to rising temperatures. The researchers estimate that about 75% of the change in Earth’s energy imbalance is due to the combination of ongoing human-induced climate change and the transition from a three-year La Nina cooling phase to a warming El Nino phase.

What’s El Nino vs. La Nina

El Nino is a cyclical and natural warming of patches of the equatorial Pacific that then alters the world’s weather patterns, while La Nina is marked by cooler than average waters.

Both shift precipitation and temperature patterns, but in different ways. El Ninos tend to increase global temperatures and La Ninas depress the long-term rise.

La Ninas tend to cause more damage in the United States because of increased hurricane activity and drought, studies have shown.

Why weather cycles switch from warm to cool

From 2020 to 2023, Earth had an unusual “triple dip” La Nina without an El Nino in between. In a La Nina, warm water sticks to a deeper depth, resulting in a cooler surface. And that reduces how much energy goes out into space, said study co-author Yu Kosaka, a climate scientist at the University of Tokyo.

She compared it to what happens when people have fevers.

“If our body’s temperature is high then it tends to emit its energy out, and the Earth has the same situation happening. And as the temperatures increase, it acts to emit more energy outward. And for three-year La Nina, it’s opposite,” Kosaka said.

So more energy — which becomes heat — is trapped on Earth, she said. La Ninas more typically correspond to a one- or two-year buildup of extra energy imbalance, but this time it was longer so the difference was more noticeable and included hotter temperatures, Kosaka said.

“When there is a transition from La Nina to El Nino, it’s like the lid is popped off,” releasing the heat, explained former NOAA meteorologist Tom Di Liberto, who’s now with Climate Central.

About 23% of the energy imbalance driving the recent higher temperatures comes from this unusually long La Nina pattern, with slightly more than half coming from gases from the burning of coal, oil and gas, the study authors said. The rest can be other factors.

Scientist Jennifer Francis of the Woodwell Climate Research Center, which wasn’t involved in the study, said the research makes sense and explains an increase in energy imbalance that some scientists were attributing to accelerated warming.

Changing how El Ninos and La Ninas are labeled

For 75 years when meteorologists calculated El Ninos and La Ninas, it was based on the difference in temperature in three tropical Pacific regions compared to normal. An El Nino was 0.5 degrees Celsius (0.9 degrees Fahrenheit) warmer than normal and La Nina was cooler than normal by the same amount.

The trouble in a warming world is what’s considered normal keeps shifting.

Until now, NOAA used the 30-year average as normal. It updated the 30-year average every decade, which is how often it updates most climate and weather measurements. Then the water warmed so much for El Ninos and La Ninas that NOAA updated its definition of normal every five years, but that wasn’t enough either, said Nat Johnson, a meteorologist at NOAA’s Geophysical Fluid Dynamics Lab.

So NOAA came up with an El Nino index that’s relative, starting this month. This new index compares temperatures to the rest of Earth’s tropics. Recently that difference between the old and new methods has been as much as half a degree Celsius (0.9 degrees Fahrenheit), and “that’s enough to have an impact,” Johnson said.

That’s because what really matters with El Ninos and La Ninas is the way the waters interact with the atmosphere. And recently the interactions didn’t match the old labeling, but they do match the new method, Johnson said.

This will likely mean a bit more La Ninas and fewer El Ninos than in the old system, Johnson said.

Here comes another El Nino

NOAA’s forecast is for an El Nino to develop later this year in the late summer or fall. If it comes early enough, it could dampen Atlantic hurricane activity. But it would also mean warmer global temperatures in 2027.

“When El Nino develops, we’re likely to set a new global temperature record,” Woodwell’s Francis said in an email. “’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.”

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