La Niña has arrived. Here’s what that means.

The National Oceanic and Atmospheric Administration declared the return of La Niña on Thursday, marking the start of an expected period of global cooling that nonetheless carried ominous signs about the trajectory of world temperatures.

The climate pattern is the inverse of the better-known El Niño and is known for cooling a vast swath of the Pacific Ocean. That tends to lower average global temperatures, while also encouraging weather extremes including intense Atlantic hurricanes, East African drought and floods in Indonesia.

But La Niña’s cooling effect will not be enough to prevent this year from becoming another of the warmest in human history, climate scientists predicted. This La Niña could instead demonstrate just how high the baseline of average global temperatures has shifted.

“We’ve reached the stage where every year is an anomalously hot year,” said Gavin Schmidt, a climate scientist and director of the NASA Goddard Institute for Space Studies. “They’re all different, statistically, than the climate we grew up with.”

Though La Niña is arriving later and in a weaker state than scientists had expected, it could still have some “robust” domino effects on the weather, said Nat Johnson, a meteorologist at NOAA’s Geophysical Fluid Dynamics Laboratory.

In the United States, La Niña is known for warm and dry winter conditions across the country’s southern tier, and wet and snowy conditions from the Pacific Northwest to the northern Plains. While recent cold and snow across the East tend to be less likely during La Niña, Johnson said the pattern could bear some responsibility for drought that rapidly developed across the eastern third of the country in the fall of last year, fueling a spate of wildfires across the Northeast.

What’s unusual about this La Niña

The latest episode of La Niña emerged in December, based on observations of slightly cooler-than-normal sea surface temperatures along the equator in the central and east-central Pacific, according to NOAA’s Climate Prediction Center.

And it is forecast to be short-lived, with about a 3-in-5 chance that it persists through April and similar chances that so-called neutral conditions return sometime between March and May. The climate pattern known as the El Niño-Southern Oscillation shifts irregularly between episodes of El Niño, La Niña and neutral conditions, when neither El Niño nor La Niña is present, with each phase typically lasting one or two years. The last La Niña persisted for three years, ending in early 2023.

Scientists track these shifts by monitoring key zones of the central and eastern Pacific. During a strong El Niño that peaked last winter, those waters were warmer than normal, fueling a dramatic surge in global heat that began in 2023 and lasted through 2024.

For the past six months, climate models and scientists have predicted that La Niña would develop - and stem that stretch of global warmth. But its emergence nonetheless came with surprises. It’s unusual for a La Niña to form at this time of year, instead of during the spring or summer. And it never developed the strength that had appeared likely months earlier.

“It’s an open question as to why,” Johnson said.

What La Niña will mean for the globe

This episode of La Niña arrives as the planet experiences persistent temperatures that are the hottest in more than 100,000 years, context that could guide how it affects weather and climate conditions.

The world is so warm, that trend is prone to persisting despite La Niña’s cooling influence, said Nick Dunstone, a fellow at the United Kingdom’s Met Office. The office is forecasting that, while 2025 probably will be cooler than 2023 and 2024, average global temperatures will still surpass every other year on record.

And even with slight cooling due to La Niña, weather extremes will continue largely unchanged, with no noticeable effect from relatively narrow swings in average global temperatures, Schmidt said.

“We’ll keep on seeing this breaking of records and climate impacts worsening as long as we continue to emit carbon dioxide,” he said.

La Niña is, however, likely to end a stretch of global heat that has all but quashed hopes of avoiding a feared warming benchmark, a rise in average temperatures more than 1.5 degrees Celsius above preindustrial levels. A forecast for global surface temperatures produced by the Cooperative Institute for Marine & Atmospheric Studies at the University of Miami and updated Wednesday shows monthly global averages are expected to remain at least a few tenths of a degree below the 1.5C threshold through August.

Schmidt said that possibility offered little comfort as decades of human-caused global warming contribute to deadly heat waves, unprecedented floods and intensifying storms.

“We’re kind of psychologically geared to think that targets and round numbers are important in some way,” he said. “But the climate really doesn’t care too much about that.”