A key current in the planet’s ocean circulatory system, including the Atlantic’s Gulf Stream, is the weakest in at least 1,600 years ― a decline that could significantly worsen the effects of climate change, according to new research.
Two studies published in the journal Nature this week say the global system of ocean currents known as the Atlantic Meridional Overturning Circulation, or AMOC, has dropped in strength by some 15 percent since the mid-20th century to a record low.
The currents, part of the so-called global ocean conveyor belt, transport warm water from the equator to the North Atlantic, where heat released into the atmosphere warms western Europe. The cooler water then sinks and travels south in the deep ocean to Antarctica, and eventually back up to the equator.
A disruption in the system could have cataclysmic effects on weather patterns, including hurricanes, swings in temperatures and ocean levels, and even survival. The 2004 science fiction film “Day After Tomorrow” addressed catastrophic weather events triggered by a collapse of the AMOC.
The strength of the AMOC strength has plunged in the last 150 years, according to one of the studies by researchers from the University College London and the Woods Hole Oceanographic Institution in Massachusetts.
The currents began to weaken near the end of the Little Ice Age in 1850 as melting freshwater in the Arctic diluted saltwater and made it less dense, so it didn’t sink as usual to help drive the circulatory pattern.
The other new study, by the Potsdam Institute for Climate Impact Research in Germany and the U.S. National Oceanic and Atmospheric Administration, found that the circulation has weakened since 1950 because of global warming due to the use of fossil fuels.
“Warming and melting are predicted to continue in the future due to continued carbon dioxide emissions,” David Thornalley of London, who led the first study, said in a statement.
The findings are especially troubling because the AMOC decline is so dramatic, and because the extent of the slowdown isn’t explained by current climate models.
“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 from the University of Reading, a co-author of the first study.
Thornalley told The Guardian that the AMOC has “played an important part in abrupt climate change in the past.” He said that while current climate models don’t predict a circulation shutdown, “the problem is, how certain are we it is not going to happen? It is one of these tipping points that is relatively low probability, but high impact.”
Stefan Rahmstorf of the Potsdam Institute told The Washington Post it’s still unclear whether a slowdown was “really happening.”
He added: “I think it is happening. And I think it’s bad news.”