Earth’s oceans are heating up, but one patch in the North Atlantic has cooled by about 1 degree Celsius since the 19th century. Scientists now think they know why: The “cold blob” is the result of slowing ocean heat transport in the Atlantic, researchers report in the June 16 Geophysical Research Letters.
Prevailing wisdom has been that the patch was getting colder because less heat was coming in via ocean currents — specifically, a massive current known as the Atlantic Meridional Overturning Circulation, or AMOC. This current brings heat from the tropics northward toward Europe, shaping temperature and precipitation in Europe, North Africa and beyond. Another possibility was that the area was losing large amounts of heat to the atmosphere, leaving colder waters behind.
To sort this out, physical oceanographer Stefan Rahmstorf and his colleagues reanalyzed decades of North Atlantic temperature and heat flux data. Temperature records there go as far back as 1870, with satellite records kicking in around 1993. If the AMOC was holding steady, and the surface was the source of the heat loss, the data should show an uptick in heat flux to the atmosphere over time.
That’s not what they found. Instead, this patch of water has shown a marked decrease in the amount of heat escaping to the atmosphere over the last half-century, especially since 1993. And the largest drop in heat content has been in the top 1,000 meters — coinciding with the AMOC’s location. That suggests the AMOC’s heat supply to this region has been declining over the last few decades, the study concludes.
The finding is one more warning sign that the AMOC is slowing down and possibly nearing a tipping point, says Rahmstorf, of the Potsdam Institute for Climate Impact Research in Germany. The current’s shutdown could bring more cold and hot extreme temperatures to Europe and be devastating for agriculture.
“For thirty years of my career studying this, I considered the #AMOC tipping risk a high impact but low probability risk for the future of humanity,” Rahmstorf wrote June 9 on X. “Recently, I’ve changed my mind.”
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