Scientists have discovered a key warning sign before a crucial Atlantic current collapses and plunges the Northern Hemisphere into climate chaos.

The Atlantic Meridional Overturning Circulation (AMOC) carries warm water north from the Southern Hemisphere, where it releases heat and freezes. The freezing process concentrates salt in the non-frozen portion of the ocean water; this extra-saline water sinks, travels back south and picks up heat again, restarting the conveyor belt. (The Gulf Stream is part of this belt.)

This release of heat helps keep Europe, and to some extent North America, balmier than it otherwise would be. But sediment records over the past 100,000 years suggest that, at times, the AMOC has shut down abruptly, leading to major climate shifts over mere decades. Scientists believe we could be veering towards this scenario once again — potentially as early as 2025 — as a result of climate change. However, until now researchers had no way of telling if the current is on the path toward one of these tipping points.

In a new study, published today in the journal Science Advances, scientists found that the flow of fresh water into the Atlantic Ocean at a latitude of 34 degrees south (the latitude where South Africa sits) may indicate a key warning sign for an impending AMOC collapse. The team found that about 25 years before the AMOC collapses, this flow reaches a minimum).

Scientists don’t have a long enough record of observations of freshwater flow at this spot to predict how far away the AMOC is from a tipping point right now. However, they do know that this flow has been declining. “We are approaching the tipping point, but we cannot deduce the distance to the tipping point,” study first author René M. van Westen, a postdoctoral researcher in marine and atmospheric science at Utrecht University, told Live Science.

Because the rising and sinking of the AMOC depends on the salinity of the water, this circulation is very sensitive to influxes of fresh water, van Westen said. As the climate warms and precipitation patterns change, the patterns of freshwater flow into the ocean change, too. It’s difficult to predict the outcomes, though, and finding the AMOC’s tipping point requires simulating a gradual increase in freshwater flow in the northern Atlantic over more than 2,000 years, van Westen said. This is a long and computationally expensive process, but trying to cut corners by simulating large freshwater pulses is not as realistic or precise.