What If the Ocean's Climate-Controlling 'Conveyor Belt' Came to a Halt?

Freak floods drown buildings, bone-chilling air flash-freezes pedestrians and ice encases the Statue of Liberty. It sounds like a disaster movie, and well, it is: In 2004's "The Day After Tomorrow," the collapse of an ocean current in the North Atlantic sends the world into a whirlwind climate doomsday.

And while that ocean current has not actually collapsed, scientists reporting in two new studies have found that it's weakening, by a lot. In fact, the current hasn't been this sluggish in 1,500 years — a finding that could carry serious (although not disaster-movie serious) repercussions for weather and sea-level rise in locations around the world.

In the Atlantic Ocean, the current known as the Atlantic Meridional Overturning Circulation (AMOC) ferries warm surface waters northward — where the heat is released into the atmosphere — and carries cold water south in the deeper ocean layers, according to the National Oceanic and Atmospheric Administration. Its circulation transports heat around the globe like a conveyor belt, and if its movement were to stop, that heat would not get distributed, and weather havoc could ensue.

But the AMOC has been getting weaker, and cold, freshwater infusions by the runaway melting of glaciers, sea ice and permafrost are to blame, and the AMOC may weaken even further if temperatures on Earth continue to rise and ice reserves continue to melt, scientists reported in the two studies.

Written in sand

In one study, published yesterday (April 11) in the journal Nature, researchers analyzed ocean sediments in a core sampled off the eastern coast of the U.S., from depths where most of the water originated in the North Atlantic's Labrador Sea. They examined positions of different-size sand grains in the geologic record, to reconstruct how the flow of the currents that carried the grains may have changed over time, said study co-author Delia Oppo, a senior scientist in the geology and geophysics department at the Woods Hole Oceanographic Institution.

The researchers traced the start of the current's weakening to the mid-19th century at the end of the Little Ice Age, a centuries-long period of extreme cold that froze northern Europe. When temperatures began warming up, freshwater from melting ice that flowed into the Nordic Seas would have diluted salty seawater near the surface. This weakened the current and prevented it from carrying bigger grains of sand as far as it used to, which told the scientists about differences in the current's strength, Oppo told Live Science.

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Kate McCartyenvironment