IT’S ONE OF THE MIGHTIEST RIVERS you will never see, carrying some 30 times more water than all the world’s freshwater rivers combined. In the North Atlantic, one arm of the Gulf Stream breaks toward Iceland, transporting vast amounts of warmth far northward, by one estimate supplying Scandinavia with heat equivalent to 78,000 times its current energy use. Without this current — a heat pump on a planetary scale — scientists believe that great swathes of the world might look quite different.

Now, a spate of studies, including one published last week, suggests this northern portion of the Gulf Stream and the deep ocean currents it’s connected to may be slowing. Pushing the bounds of oceanography, scientists have slung necklace-like sensor arrays across the Atlantic to better understand the complex network of currents that the Gulf Stream belongs to, not only at the surface, but hundreds of feet deep.

“We’re all wishing it’s not true,” Peter de Menocal, a paleoceanographer and president and director of the Woods Hole Oceanographic Institution, said of the changing ocean currents. “Because if that happens, it’s just a monstrous change.”

The consequences could include faster sea level rise along parts of the Eastern United States and parts of Europe, stronger hurricanes barreling into the Southeastern United States, and perhaps most ominously, reduced rainfall across the Sahel, a semi-arid swath of land running the width of Africa that is already a geopolitical tinderbox.

The northern arm of the Gulf Stream is but one tentacle of a larger, ocean-spanning tangle of currents called the Atlantic Meridional Overturning Circulation, or AMOC. Scientists have strong evidence from ice and sediment cores that the AMOC has weakened and shut down before in the past 13,000 years. As a result, mean temperatures in parts of Europe may have rapidly dropped to about 15 degrees Celsius below today’s averages, ushering in arctic like conditions. Parts of northern Africa and northern South America became much drier. Rainfall may even have declined as far away as what is now China. And some of these changes may have occurred in a matter of decades, maybe less.

The AMOC is thus a poster child for the idea of climatic “tipping points”—of 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. “It’s a switch,” said Dr. de Menocal, and one that can be thrown quickly.

Which brings us to the cold blob. Almost everywhere around the world, average temperatures are rising— except southeast of Greenland where a large patch of the North Atlantic has become colder in recent years.