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Two new studies are adding to concerns about one of the most troubling scenarios for future climate change: the possibility that global warming could slow or shut down the Atlantic’s great ocean circulation systems, with dramatic implications for North America and Europe.

The research, by separate teams of scientists, bolsters predictions of disruptions to global ocean currents — such as the Gulf Stream — that transfer tropical warmth from the equator to northern latitudes, as well as a larger conveyor system that cycles colder water into the ocean’s depths. Both systems help ensure relatively mild conditions in parts of Northern Europe that would otherwise be much colder

The papers offer new insight into how rapidly melting Arctic ice could slow or even temporarily halt the ocean’s normal circulation, with possible effects ranging from plunging temperatures in northern latitudes to centuries-long droughts in Southeast Asia. A fictional version of this scenario was depicted in the 2004 disaster film “Day After Tomorrow, ” although most scientists believe the movie’s vision of ice sheets over New York and Europe are highly unlikely.

One study, by three scientists from Germany’s Alfred Wegener Institute, uses computers to model how Greenland’s rapid thawing could affect the Atlantic meridional overturning circulation, the system that pushes cold, dense saltwater into the deep ocean and helps transport warm water northward, helping to warm Europe’s climate.

Their report, in the journal Geophysical Research Letters, says previous research may have underestimated changes to the ocean from the huge influx of fresh, cold water from melting ice sheets. Using new methods, the German scientists were able to estimate more precisely how much ice would melt and how all that added freshwater would affect ocean circulation. In the ocean, colder water normally tends to sink, but cold freshwater — less dense than saltwater — stays near the surface, disrupting the normal flow.

“Previous studies have generally had to estimate the amount of melting and then insert the meltwater into the ocean simulation by hand, or haven’t included the feedback between ice sheet melting and ocean salinity at all, ” lead scientist Paul Gierz said.

The team’s computer models projected a drop in ocean salinity of about 7 percent in the areas near Greenland’s melting ice sheets, a decline that would alter deep-ocean circulation patterns over time, resulting in “less heat being transported to the high latitudes … which has implications for both North American as well as European weather and climate, ” Gierz said.

Because the climate systems tend to respond slowly to environmental changes, the full impacts may not be felt for decades.

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