The water in the Atlantic is constantly circulating in a complex pattern that influences weather on several continents. And climate scientists have been asking a crucial question: Whether this vast system, which includes the Gulf Stream, is slowing down because of climate change.

If it were to change significantly, the consequences could be dire, potentially including faster sea level rise along parts of the United States East Coast and Europe, stronger hurricanes barreling into the Southeastern United States, reduced rainfall across parts of Africa and changes in tropical monsoon systems.

Now, scientists have detected the early warning signs that this critical ocean system is at risk, according to a new analysis published Thursday in the scientific journal Nature Climate Change.

“I showed that this gradual slowing down of the circulation system is associated with a loss of stability,” said Niklas Boers, a researcher at the Potsdam Institute for Climate Impact Research in Germany, “and the approaching of a tipping point at which it would abruptly transition to a much slower state.”

Alex Hall, the director of the Center for Climate Science at the University of California, Los Angeles, who was not involved in the study, said that although the findings did not signal to him that any collapse of that ocean system might be imminent, the analysis offered a crucial reminder of the risks of interfering with currents.

“We’re poking a beast,” he said. “But we don’t really know the reaction we’ll cause.”

Studying ocean systems is difficult for many reasons. One challenge is that there’s only one Earth, said Andrew Pershing, director of climate science at Climate Central, an organization of scientists and journalists focused on climate change. Consequently, researchers can’t easily compare two oceans — one ocean dealing with the effects global warming caused by increased carbon dioxide in the atmosphere, and another ocean that hasn’t had to contend with that problem.

Dr. Pershing praised the analytical workarounds that the scientists came up with in order to study the ocean-spanning tangle of currents, which are known as Atlantic Meridional Overturning Circulation, or AMOC. By parsing more than a century of ocean temperature and salinity data, Dr. Boer showed significant changes in multiple indirect measures of AMOC’s strength.

“The work is fascinating,” he said.

Dr. Pershing said that analysis supported the idea that the AMOC has gotten weaker over the course of the 20th century. It’s a critical area to study because AMOC epitomizes the idea of climatic “tipping points” — hard-to-predict thresholds in Earth’s climate system that, once crossed, have rapid, cascading effects far beyond the corner of the globe where they occur.

“The big challenge is, what do we do with that information?” he said of the new study.

Susan Lozier, a physical oceanographer and dean at the College of Sciences at Georgia Tech, said that there was no doubt that climate change is affecting oceans. There is wide consensus in her field that sea levels are rising and oceans are warming, she said.

She also called Dr. Boers’ study “interesting,” but said she wasn’t convinced that the findings showed that circulation in that ocean system is slowing. “There are lots of things to worry about with the ocean,” she said, such as the more definitive concerns involving sea-level rise.



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