Scientists believe El Niño’s end could break a string of off-the-charts heat that has persisted within the world’s oceans and across the planet’s surface for more than a year. A La Niña pattern, El Niño’s planet-cooling inverse, is forecast to develop by summer’s end, and scientists say it will probably slow, but not reverse, the heating of the globe.

But in a world that has warmed close to 1.5 degrees Celsius (2.7 degrees Fahrenheit) in less than two centuries, that prediction isn’t guaranteed — because not even a strong El Niño can explain the scale by which global temperatures have surged recently.

“Even when you factor that in … it’s surprising how warm it has been,” said Nathaniel Johnson, a National Oceanic and Atmospheric Administration meteorologist. “The degree to which we’ve shattered records recently has been surprising.”

This El Niño was historically strong

In a monthly update on what is known as the El Niño Southern Oscillation, scientists at NOAA wrote Thursday they know El Niño is over because unusual warmth that developed in Pacific Ocean surface waters last year has waned. El Niño is in place when warm waters pool along the equator in the eastern and central Pacific, triggering changes in trade wind speeds and direction and releasing heat and moisture into the atmosphere.

Those Pacific waters got as much as 2 degrees C (3.6 degrees F) warmer than normal this winter, an indication of what climate scientists consider to be a historically strong El Niño episode. During the 2015-2016 El Niño, which pushed the planet to what were then record-setting average temperatures, Pacific waters warmed as much as 2.6 degrees C (4.7 degrees F).

That ocean warmth has domino effects on weather patterns around the world, causing extreme heat waves and drought in some areas, and flooding storms in others. The effects were evident across much of the world over the past year.

For example, the hot, humid air rising from that region of the Pacific is known for sending wet and stormy weather eastward — something linked to a surge in dengue cases in Peru and to a second-straight rainy winter in California. Other extremes El Niño is known for also developed in earnest: Floods in Kenya that killed scores of people and extreme heat waves across Southeast Asia.

Now, sea surface temperatures in the El Niño linked zones of the Pacific have been steadily cooling and are close to average.

El Niño is likely to flip quickly into La Niña

The world could soon switch to a set of opposite extremes. In the Thursday update, NOAA’s Climate Prediction Center estimated about a 2-in-3 chance that La Niña forms sometime between July and September.

La Niña, as the inverse of El Niño, is tied to cooler-than-normal waters along the equator in the central and eastern Pacific.

Strong episodes of El Niño frequently transition into La Niña because the oceanic heat associated with El Niño transfers into the atmosphere, and that eventually causes the Pacific waters to rapidly cool, said Johnson, a meteorologist at NOAA’s Geophysical Fluid Dynamics Laboratory in New Jersey.

La Niña has its own set of domino effects around the world, including wetter-than-normal conditions across Southeast Asia and Indonesia, dry conditions across the southern tier of the United States and cooler-than-normal temperatures in southern Asia, eastern Africa and western South America.

Perhaps among the most significant La Niña impacts: An intensified Atlantic hurricane season. La Niña tends to decrease wind shear — differences in wind speeds and direction at varying altitudes — and that can allow tropical storms to more easily form and strengthen.

An expectation that La Niña is likely by fall prompted NOAA forecasters to warn this year’s hurricane season could be among the most active on record.

Questions remain about global warming trends

Whether La Niña’s global cooling influence can counteract the recent spike in record-setting global heat could become clear in the coming months, scientists said.

So far, that appears to be the case, said Gavin Schmidt, director of the NASA Goddard Institute for Space Studies. Global temperature trends suggest a streak of record-hot months could grow to 13 once June is up, or perhaps end at the one-year mark, observed in May, he said.

Analyses suggest the warming trend may be realigning with what climate models tell scientists to expect, based on what they know about the influence of rising greenhouse gases on planetary temperatures, Schmidt said.

But if global temperatures continue to surpass scientists’ expectations, “that would be very indicative of a more systematic issue,” Schmidt said in an email — meaning some influence on global warming that scientists have not yet been able to pinpoint. And that would mean a strong likelihood that 2024 replaces 2023 as the planet’s hottest year on record.