Potsdam Institute for Climate Impact Research
This text was originally published in the collection “Die Schweiz und die Ozeane” (Switzerland and the Oceans) by the Swiss think tank Denknetz.
The researchers compiled proxy data, mainly from natural archives such as ocean sediments and ice cores, dating back several hundred years to reconstruct the history of the AMOC current. They found solid evidence that the weakening in the 20th century was unprecedented in the past millennium—it is likely a consequence of human-induced climate change. The gigantic AMOC ocean circulation is significant for weather conditions in Europe and regional sea levels in the US; an important indication of its slowdown is a “cold bubble” that has formed in the North Atlantic in recent decades.
"The Gulf Stream system functions like a giant conveyor belt, transporting warm surface water from the equator to the north and sending cold, low-salinity deep water back to the south. It moves almost 20 million cubic meters of water per second, about a hundred times the flow of the Amazon River," explains Stefan Rahmstorf, researcher at the Potsdam Institute for Climate Impact Research (PIK) and initiator of the study. The study showed a slowdown in ocean currents of about 15 percent since the mid-20th century and linked this to human-induced global warming. However, a reliable picture of the long-term development has been lacking until now – the researchers provide this with their review study, which compares the results of previous proxy data studies.
“For the first time, we have combined a number of earlier studies and found that they provide a consistent picture of AMOC development over the last 1600 years,” says Rahmstorf. “The study results suggest that the AMOC current was relatively stable until the late 19th century. With the end of the Little Ice Age around 1850, the ocean current began to weaken, followed by a second, even more drastic decline since the mid-20th century.” The 2019 Special Report on the Ocean of the Intergovernmental Panel on Climate Change (IPCC) already concluded with medium confidence that “the Atlantic Meridional Overturning Circulation has weakened compared to 1850–1900.” “The new study provides further independent evidence for this conclusion and places it in a longer-term paleoclimatic context,” Rahmstorf continued.
Why is the Gulf Stream (AMOC) weakening?
Climate models have long predicted a slowdown in circulation as a response to global warming caused by greenhouse gases—and according to a number of studies, this is likely the reason for the observed weakening. Atlantic circulation is driven by what scientists call deep convection, caused by differences in density in the ocean: warm, salty surface water moves from south to north, cooling and becoming denser. When it becomes heavy enough, the water sinks into deeper ocean layers and flows back south. Global warming disrupts this mechanism: increased precipitation and the accelerated melting of the Greenland ice sheet are feeding fresh water into the North Atlantic. This reduces the salinity and thus the density of the water there, which inhibits sinking and thus weakens the AMOC circulation.
The weakening is also associated with a unique and significant cooling of the North Atlantic over the last hundred years. This so-called “cold bubble” was predicted by climate models as a result of a weakening AMOC, which transports less heat to this region.
“If we continue to drive global warming in the future, the Gulf Stream system will continue to weaken – by 34 to 45 percent by 2100, according to the latest generation of climate models,” Rahmstorf concludes. “This could bring us dangerously close to the tipping point where the current becomes unstable.”