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“Until now, models have predicted something for the future . . . but it was something that seemed very distant,” said Oltmanns, the lead scientist behind the research, which was published this week in Nature Climate Change.

“But now we saw with these observations that there is actually freshwater and that it is already affecting convection, and it delays convection quite a lot in some years,” she continued.

Photo by Drew Angerer / Getty Images

One caution is that this is an observational study, not a prediction for the future – and Oltmanns said “nobody really knows” how much freshwater is enough to significantly slow or shut down the circulation, which is technically called the “Atlantic meridional overturning circulation,” or AMOC. Still, it suggests that key processes that have raised long-standing concern are already happening.

To collect the data, Oltmanns and her team ventured by ship out into the Irminger Sea to the southeast of Greenland. There, they read data from ocean moorings that take measurements of the character of the waters in key regions of ocean convection. The researchers now have a 13-year record to draw upon from this area.

In winter, cold air chills the northward-flowing surface water in this region enough to cause it to become denser and sink. But meltwater interferes with and delays this process because, lacking salinity, it is less dense and so less prone to sink.

In the high meltwater years, the ocean is also just warmer overall, the study found. That also delays the onset of convection because it is harder for the ocean surface layer to lose enough heat to sink, Oltmanns said.