WASHINGTON (AP) — A new study finds that climate change and El Niño combined to rev up last month's explosively intensifying Hurricane Polo, which was the longest-lasting Category 5 storm ever to churn in the Pacific.

Global warming caused by the burning of coal, oil and gas was the bigger of the two culprits, adding about 6 mph (nearly 10 kph) to its already monstrous strength, according to a joint study published Thursday by the science teams World Weather Attribution and Climate Central. El Niño contributed a bit less, about 5 mph (8 kph), a study co-author said.

Climate change made the conditions that fueled the hurricane 23 times more likely, while El Niño — the natural and temporary warming of parts of the Pacific — made it 14 times more likely, the study found. It all has to do with the 88-degree Fahrenheit (31.2 degrees Celsius) waters that were as much as 3.6 degrees Fahrenheit (2 degrees Celsius) warmer than normal, the study said.

This year, with a super strong El Niño, has set records for strength and longevity of Pacific storms. Polo set the record for time spent at Category 5 in the Pacific at two-and-a-half days.

“El Niño and climate change both made this hurricane stronger than it would have otherwise been, but the effect of climate change is even bigger than the one of El Niño,” said climate scientist and study co-author Friederike Otto, who coordinates the international science team behind World Weather Attribution.

“El Niño comes and goes, but climate change only gets worse. That means that now every hurricane that will occur in the East Pacific has at least this climate change signal making it stronger. And as long as we keep burning fossil fuels, that will only get bigger,” Otto added.

Researchers found that without climate change and El Niño, Polo would have been a monster storm anyway. But every bit of additional wind speed can dramatically increase the damage.

While Hurricane Polo ultimately weakened considerably before it hit a sparsely populated part of Mexico, scientists remain fascinated by how it turbocharged from a tropical storm to a top-of-the-chart Category 5 hurricane in about 24 hours.

The study puts the different atmospheric dangers in perspective, said Kristina Dahl, a climate scientist and vice president of science at Climate Central, a science and journalism nonprofit out of New Jersey.

“It’s a powerful reminder that even though El Niño right now is shifting hazards around the globe and changing where it’s flooding and getting dry, the really dominant signal behind all of this remains climate change,” she said.

The study looked at the warm water that fuels storms, using a measure of how strong they can get. But the study didn't focus on the upper level wind shear where El Niño has a huge footprint. El Niño usually increases those storm-decapitating high altitude winds in the Atlantic. However, in the Pacific because of changes in wind patterns, El Niño decreases the winds that weaken storms.

The two sets of researchers measured conditions slightly differently and used different techniques but came to the same general conclusion, making them more confident in the result. However, the rapid study has not had time to be peer-reviewed, which is the gold standard of science.

To calculate the impact of climate change on Polo, both teams used the scientifically accepted method of comparing what happened in our warmer real world to a hypothetical world that hasn't warmed by 2.5 degrees (1.4 degrees Celsius) since pre-industrial times.

Study co-author Daniel Gilford, a Climate Central scientist, said the process they looked at was similar to cars on a road. Wind shear from El Niño can be a red light, as it is in the Atlantic, or a green light, as it is in the Pacific. In the Pacific that green light says go, and then the water warmed by both climate change and El Niño acts like a driver flooring the accelerator.

University at Albany atmospheric scientist Brian Tang, who wasn't part of the study, said he was initially surprised that the research found climate change to be the bigger influence on Polo, “given how extreme this El Niño is, but from a long-term standpoint, perhaps the result isn’t as surprising as it might seem.”

But three other outside scientists — Jeff Masters of Yale Climate Connections, University of Pennsylvania's Michael Mann and North Carolina State University's Carl Schreck — said they believe the study underestimates how much climate change has added to Polo's strength. Masters pointed to Gilford's own 2024 study that found climate change added 16 mph (30 kph) to hurricanes in the Atlantic from 2019 to 2023.

Gilford acknowledged that Thursday's report could be underestimating climate change's impact a bit, but said he was factoring in other conditions and being conservative.

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