April 12 (UPI) -- The Gulf Stream has been slowing down since the middle of the 17th century, new research suggests, and is now at its weakest point in 1,600 years.
"Our study provides the first comprehensive analysis of ocean-based sediment records, demonstrating that this weakening of the Atlantic's overturning began near the end of the Little Ice Age, a centuries-long cold period that lasted until about 1850," Delia Oppo, a senior scientist with the Woods Hole Oceanographic Institution, said in a news release.
Scientists believe an influx of freshwater, triggered by the melting of ice sheets and glaciers, began to dilute surface sea water toward the end of the Little Ice Age. As more freshwater was flushed into the ocean, surface water layers became less dense and less likely to sink, slowing what's known as the Atlantic Meridional Overturning Circulation.
To estimate the strength of the circulation through time, scientists analyzed the grain size found in sediment core layers. Larger grain sizes serve as evidence of stronger currents. Using a variety of computer models, researchers were able to estimate ocean temperatures based on the strength of the circulation.
"Combined, these approaches suggest that the AMOC has weakened over the past 150 years by approximately 15 to 20 percent," said lead researcher David Thornalley, a senior lecturer at University College London and WHOI adjunct.
Scientists detailed their analysis of the AMOC this week in the journal Nature.
"North Atlantic circulation is much more variable than previously thought, and it's important to figure out why the models underestimate the AMOC decreases we've observed," said Jon Robson, a senior research scientist from the University of Reading.
Another study published in the latest issued of Nature suggests the weakening of the Gulf Stream and related currents in the Atlantic has accelerated since the 1950. Researchers warn that as the AMOC grinds to a halt, weather patterns across the Northern Hemisphere could be significantly altered.
"What is common to the two periods of AMOC weakening -- the end of the Little Ice Age and recent decades -- is that they were both times of warming and melting," said Thornalley. "Warming and melting are predicted to continue in the future due to continued carbon dioxide emissions."