Recent research confirms something that climatologists have long been worrying about, namely that the Gulf Stream (more formally known as the Atlantic Meridional Overturning Circulation (AMOC)) is slowing down. This could have major effects on climate in the Northern hemisphere.
“The Gulf Stream System works like a giant conveyor belt, carrying warm surface water from the equator up north, and sending cold, low-salinity deep water back down south. It moves nearly 20 million cubic meters of water per second, almost a hundred times the Amazon flow,” explains Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research PIK, initiator of the study published in Nature Geoscience. Previous studies by Rahmstorf and colleagues showed a slowdown of the ocean current of about 15 percent since the mid-20th century, linking this to human-caused global warming, but a robust picture about its long-term development has up to now been missing: This is what the researchers provide with their review of results of proxy data studies.
“For the first time, we have combined a range of previous studies and found they provide a consistent picture of the AMOC evolution over the past 1600 years,” says Rahmstorf. “The study results suggest that it has been relatively stable until the late 19th century. With the end of the little ice age in about 1850, the ocean currents began to decline, with a second, more drastic decline following since the mid-20th century.” Already the 2019 special report on the oceans of the Intergovernmental Panel on Climate Change (IPCC) concluded with medium confidence ‘that the Atlantic Meridional Overturning Circulation (AMOC) has weakened relative to 1850–1900.’ “The new study provides further independent evidence for this conclusion and puts it into a longer-term paleoclimatic context,” Rahmstorf adds.
Gizmodo's Earther blog also has an article on the subject.
“The AMOC is at risk of collapsing when a certain level of freshwater flow into the North Atlantic from increasing ice melt in Greenland is reached,” Johannes Lohmann, one of the authors of the study, said in an email. “These tipping points have been shown previously in climate models, where meltwater is very slowly introduced into the ocean. In reality, increases in meltwater from Greenland are accelerating and cannot be considered slow.”
The study modeled the increase in freshwater flowing. Lohmann said using “a large ensemble of simulations, we systematically varied the rate of change and the ocean’s initial conditions, and investigated how the collapse of the AMOC depended on these factors.”
The models ended up showing that in some cases with a more rapid rate of change, the AMOC actually collapsed before previous predictions indicated it would. If we stick to the cup of water analogy, previous studies essentially found a full cup of hot water needed to be added to the bucket for collapse, but the new findings show dumping in the water faster means you need less than a cup to trigger the collapse. The study shows that “the safe levels of global warming before such a collapse occurs may be smaller than previously thought, and may also be difficult to predict with certainty,” Lohmann said.
Finally, if you're not depressed enough, here's an article from The Guardian, via Nature's Nature Briefing newsletter.
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