A 5,500-mile blob of seaweed in the Atlantic Ocean that has menaced beaches across the Caribbean and Florida in recent years is exploding in size, while a second patch farther north is declining rapidly, driven by rapid changes in the region’s climate.

A study published Thursday in the journal Nature Geoscience finds a big shift in the growth patterns of sargassum, a type of floating macroalgae that provides food and shelter for fish, turtles, seabirds and other marine life.

The southern patch, known as the Great Atlantic Sargassum Belt, has now reached 38 million metric tons, a 40 percent increase from its record year of 2022.

“Usually we have a 10 percent to 20 percent fluctuation year to year,” said Chuanmin Hu, a professor of physical oceanography at the University of South Florida and an author of the paper. “But this year was crazy, and we do not have an answer of why.”

Scientists hadn’t detected sargassum in this equatorial region before 2011.

Since then, winds and ocean currents have pushed blobs of sargassum west toward coastal waters in the springtime. Beaches along Mexico’s Yucatán Peninsula have suffered from smelly piles of seaweed, while the president of the Dominican Republic said in June that his nation faced a “regional emergency” from sargassum and called for action by the United Nations.

In 2023, about 13 million metric tons of sargassum covered some Florida beaches and decomposed into smelly piles before disappearing in the summer. A study that year in The Journal of Global Health by doctors on the island of Martinique reported respiratory ailments from decomposing seaweed that produces hydrogen sulfide gas.

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