For over a year, Atlantic Ocean surface temperatures have reached new highs, but that trend has reversed at record speed in recent months, and no one knows why.

In June, temperatures in the Atlantic were 1 to 3 degrees Celsius higher than normal across much of the ocean, with some areas up to 5 degrees Celsius warmer than average. These temperatures were not an isolated incident, as the Atlantic has regularly recorded record highs since March 2023. This year was the fourth year in a row that the world’s oceans set new heat records.

The hot water was partly a result of human-caused climate change, but also due to a particularly strong El Niño phenomenon in 2023 and 2024. But that phenomenon appears to be over, according to the National Oceanic and Atmospheric Administration.

NOAA data shows that sea surface temperatures in the Atlantic have been dropping surprisingly quickly since May. Since early June, temperatures have been one to two degrees Fahrenheit colder than normal for this time of year. That means El Niño is likely being replaced by its counterpart La Niña, a weather system that causes cold water to rise to the surface of the Atlantic sometime between September and November. Both El Niño and La Niña are complex systems driven by trade winds, solar heating and rainfall in the tropics and are difficult to predict. Still, the sudden change in temperature in the Atlantic is a mystery, and no one seems to know why it happened so quickly.

“We’ve gone through the list of possible mechanisms and so far nothing has come up,” Frans Philip Tuchen, a postdoctoral fellow at the University of Miami, told New Scientist.

The ocean heatwave was associated with some devastating environmental impacts, including a global coral bleaching event that put pressure on over 99% of tropical reefs in the Atlantic Ocean. Bleaching occurs when conditions change and the algae that live in the corals are shed. The result is bone-white corals that not only affect biodiversity but also reduce the ability of reefs to mitigate the impacts of coastal storms and reduce flooding.

El Niño and La Niña have global consequences that go far beyond changes in water temperature. A recent study found that El Niño can lead to trillions of dollars in global GDP losses, an effect that can last for years. El Niño can also increase disease rates, as mosquitoes, toxic algae and bacteria thrive during longer rainy seasons.