Pending AMOC Collapse?

Various news outlets are reporting on a paper published in Nature Communications warning that the Atlantic meridional overturning circulation (AMOC) could shut down this century due to global warming (see, e.g., these stories from USA Today, Axios, and The Guardian). As the USA Today article explains:

    The AMOC is a crucial conveyor belt for ocean water and air, which creates weather. Warm, salty water moves north from the tropics along the Gulf Stream off the U.S. East Coast to the North Atlantic, where it cools, sinks and heads south.

    The faster it moves, the more water is turned over from warm surface to cool depths.

    The cycle keeps northern Europe several degrees warmer than it would otherwise be and brings colder water to the coast of North America.

And from the Axios article:

    The AMOC is a system of ocean currents that transport water and heat between the Southern Ocean and the far North Atlantic Ocean in a years' long cycle that influences weather patterns in both hemispheres.

• The currents are driven by differences in water density regulated by changes in temperature and salt concentration.
• As warm water flows north along the ocean's surface, it becomes denser as evaporation increases salinity levels and temperatures decrease, making it sink deeper.
• It then slowly spreads southwards before rising from the ocean’s depths in the Southern Ocean in a process known as “upwelling,” thereby completing the circuit.

But an influx of fresh water from melting ice caps could disrupt the current, either weakening it or pushing it further south. The result could well spell sharply reduced temperatures in Europe and higher temperatures and less rainfall across the American Southwest, more tropical storms and hurricanes along the East Coast, and disrupt rain patterns in South America, Africa, and, even, India, according to the articles cited earlier. Scientists also predict it would result in higher sea levels along the East Coast.  

    The scientists in this case have used what they describe as "new and improved" statistical tools to predict when this collapse of AMOC could occur. But, as USA Today relates, "[t]he calculations contradict the message of the latest Intergovernmental Panel on Climate Change report, in which an abrupt change in the AMOC is considered 'unlikely' this century."

    Of course, the scientists link the impending collapse due to anthropogenic global warming from increased CO2. It is more likely due to higher temperatures at the pole due to the weakening magnetic field allowing more solar particles to impact and enter the atmosphere--i.e., there is literally more radiation entering at the poles. 

    This is not the first time these events have happened: the Guardian article, for instance, notes that "[t]he Amoc collapsed and restarted repeatedly in the cycle of ice ages that occurred from 115,000 to 12,000 years ago. It is one of the climate tipping points scientists are most concerned about as global temperatures continue to rise." The collapse of the AMOC appears to be connected to Dansgaard–Oeschger (D-O) events. From the Wikipedia article (footnotes omitted):

    The course of a D-O event sees a rapid warming, followed by a cool period lasting a few hundred years. This cold period sees an expansion of the polar front, with ice floating further south across the North Atlantic Ocean.

    D-O events are also believed to cause minor increases in atmospheric carbon dioxide concentrations on the order of around 5 ppm.

* * *

    The events appear to reflect changes in the North Atlantic Ocean circulation, perhaps triggered by an influx of fresh water or rain.

    The events may be caused by an amplification of solar forcings, or by a cause internal to the earth system – either a "binge-purge" cycle of ice sheets accumulating so much mass they become unstable, as postulated for Heinrich events, or an oscillation in deep ocean currents (Maslin et al.. 2001, p25).

    These events have been attributed to changes in the size of the ice sheets and atmospheric carbon dioxide. The former determines the strength of the Atlantic Ocean circulation via altering the northern hemisphere westerly winds, gulf stream, and sea-ice systems. The latter modulates atmospheric inter-basin freshwater transport across Central America, which changes the freshwater budget in the North Atlantic and thus the circulation. These studies corroborate the previously suggested existence of a "D-O window" of AMOC bistability ('sweet spot' for abrupt climate changes) associated with ice volume and atmospheric CO2, accounting for the occurrences of D-O type events under intermediate glacial conditions in the late Pleistocene. 

The article also points out that "[t]he little ice age around 400 to 200 years ago has been interpreted by some as the cold part of a D-O cycle." 

    Ben Davidson at Suspicious Observers discusses this in today's news video at the 2:19 mark: