The climate "switch" in the ocean has finally been flipped: will it lead to cooler temperatures on Earth?

Scientists from the National Oceanic and Atmospheric Administration (NOAA) have announced that La Niña—a cool weather phenomenon—has officially begun. Researchers now state that the weather pattern is finally taking shape, but they warn that it is likely to be significantly weaker and shorter than usual, as reported by Live Science.

La Niña is the cold phase of a natural climate pattern known as the El Niño-Southern Oscillation (ENSO), which describes the periodic changes in atmospheric and sea surface temperatures in the tropical Pacific Ocean. It is also known that the phases of this cycle influence the global weather and climate of the Earth.

According to researchers, during La Niña, the northern U.S. and Canada typically experience colder and wetter winters, while the southern U.S. tends to be warmer and drier. It is also known that La Niña tends to increase hurricane activity over the Atlantic.

Observations indicate that this year La Niña arrived later than researchers expected, and therefore did not have enough time to strengthen before winter began. Scientists believe that conditions for this unusual cool phenomenon began developing back in December of last year and are likely to persist until April. NOAA's statement mentioned that scientists had anticipated the onset of La Niña since last spring, but all necessary conditions came together only in December 2024.

It is noteworthy that ENSO is a multi-year cycle that causes the warm El Niño phase followed by the cooler La Niña phase. This cycle alternates approximately every 2-7 years, with each phenomenon typically lasting up to a year. It is known that El Niño contributed to the extreme heat in 2023 and 2024, leading scientists to believe that La Niña would follow its retreat.

It remains unclear why La Niña developed so slowly, but scientists suspect that a higher average global temperature of the ocean may have played a role in this process.

NOAA has a guideline for determining when La Niña officially begins: the surface temperature of the tropical Pacific Ocean must drop 0.5 degrees Celsius below the long-term average. Observations show that since April of last year, the average temperature in this part of the Pacific Ocean has remained within 0.5 degrees Celsius, only falling below the threshold in December.

Scientists now believe that La Niña will persist from February to April with a 59% probability, and then from March to May, neutral conditions are expected to remain with a 60% probability. The temperature must stay below the threshold for five consecutive seasons—any three-month period—for this La Niña to be included in NOAA's official historical record. Unfortunately, scientists believe this will not happen if La Niña dissipates by March-May.

At the same time, scientists acknowledge that ENSO events are difficult to predict, and the situation can change rapidly at any moment. Researchers will continue to monitor the surface temperature of the tropical Pacific to determine how long La Niña lasts.