The Atlantic Overturning Current is part of a worldwide twisted loop of ocean water, called the thermohaline cycle (thermo = heat, haline = salt), which emerges very salty and warm out of the Gulf of Mexico, travels north as a surface current along the east coast of North America, veers east in the North Atlantic toward Europe, then loops back west to a region just south of Greenland where it cools and sinks to the ocean floor – because it has become denser than the surrounding and less salty North Atlantic waters (colder water is denser than warmer water, and saltier water is denser than fresher water of equal temperature). The dense highly salted descending water then runs as a cold deep ocean current south along the east coast of South America, and continues in a complicated path along the ocean floor into the Pacific Ocean, where it warms and eventually rises to become a surface current of more buoyant less salty water. This current distributes solar heat collected by ocean waters in tropical latitudes to higher latitudes (closer to the poles).
In 2004, Peter Schwartz and Douglas Randall described the thermohaline cycle this way: “In this thousand-year cycle, water from the surface in tropical areas becomes more saline through evaporation. When it circulates to the poles and becomes cold (“thermo”), the greater density still present from higher salt (“haline”) concentration causes the water to sink to great depths. As with most large-scale geological processes, the thermohaline cycle is not thoroughly understood. Wallace Broecker has been studying the cycle for decades and, according to the December 1996 issue of Discover magazine, he has shown that the thermohaline cycle has not always been in operation, and that it has a strong effect on global climate.”
In 2003-2004, the US Department of Defense commissioned a secret study of what might be the worst possible effects of Global Warming triggering an “abrupt climate change” in the near future, in order to estimate the potential liabilities that military planning would have to consider (to maintain US security, and global power). This study was conducted during the climate-change-denying George W. Bush Administration. When the existence of the resulting report, produced by independent researchers Peter Schwartz and Douglas Randall, became publicly known there was such a public outcry (bad PR for the DOD) that the report was declassified and made publicly available.
The Schwartz-Randall report pointed to the abrupt onset of a significantly colder, dryer climate in the Northern Hemisphere as the most perilous possible consequence of Global Warming up to about 2010, because such warming (the trapping of incoming solar radiation and outgoing infrared radiation from the land and oceans, by greenhouse gases in the atmosphere) might cause the thermohaline cycle to stop. How? Global Warming causes glaciers and ice caps to melt, and such fresh (unsalted) meltwater from Greenland floods into the North Atlantic where the thermohaline current dives to the ocean floor. This fresh surface water dilutes the high salinity of the presently descending thermohaline current, making its waters less dense (less heavy) and so less likely to sink. Sufficient freshening of the thermohaline current would cause it to stop entirely, shutting off this global conveyor belt of climate-regulating oceanic solar heat.
Though abrupt climate change is a less likely and worst case scenario as compared to gradual climate change, Schwartz and Randall concluded that such an occurrence would “challenge United States national security in ways that should be considered immediately.” The climatic cooling that might occur in the Northern Hemisphere as a result of a collapse of the thermohaline cycle could be like the century-long period 8,200 years ago with temperature 5 °F (2.8 °C) colder, or the 13 century-long period 12,700 years ago with temperature 27 °F (15 °C) colder. The shift to colder climate could occur as rapidly as 5 °F (2.8 °C) of cooling per decade. So, the world could plunge into a new Ice Age within a period of twenty years. In their 2004 report, Schwartz and Randall showed data on the salinity of the North Atlantic since 1960; the trend was a steady freshening. (I wrote about the above in an article for the Internet, in July 2004).
A 2015 scientific publication of new observations on the “Atlantic Meridional Overturning Circulation” (the Atlantic part of our thermohaline cycle) concluded that “the melting Greenland ice sheet is likely disturbing the circulation.” The Phys.org news article about this study [Rahmstorf, S., Box, J., Feulner, G., Mann, M., Robinson, A., Rutherford, S., Schaffernicht, E. (2015): “Evidence for an exceptional 20th-Century slowdown in Atlantic Ocean overturning.” Nature Climate Change (the journal)] concluded:
“The scientists certainly do not expect a new ice age, thus the imagery of the ten-year-old Hollywood blockbuster ‘The Day After Tomorrow’ is far from reality. However, it is well established that a large, even gradual change in Atlantic ocean circulation could have major negative effects. ‘If the slowdown of the Atlantic overturning continues, the impacts might be substantial,’ says Rahmstorf. ‘Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in that region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe.’ If the circulation weakens too much it can even break down completely – the Atlantic overturning has for long been considered a possible tipping element in the Earth System. This would mean a relatively rapid and hard-to-reverse change.”
On April 11, 2018, an article titled “Stronger evidence for a weaker Atlantic overturning” appeared at Phys.org. This article notes:
“The Atlantic overturning—one of Earth’s most important heat transport systems, pumping warm water northward and cold water southward—is weaker today than any time before in more than 1000 years. Sea surface temperature data analysis provides new evidence that this major ocean circulation has slowed down by roughly 15 percent since the middle of the 20th century, according to a study published in the highly renowned journal Nature by an international team of scientists. Human-made climate change is a prime suspect for these worrying observations. There have been long debates whether the Atlantic overturning could collapse, being a tipping element in the Earth system. The present study does not consider the future fate of this circulation, but rather analyses how it has changed over the past hundred years. Nevertheless, Robinson cautions: ‘If we do not rapidly stop global warming, we must expect a further long-term slowdown of the Atlantic overturning. We are only beginning to understand the consequences of this unprecedented process—but they might be disruptive.’ Several studies have shown, for example, that a slowdown of the Atlantic overturning exacerbates sea-level rise on the US coast for cities like New York and Boston. Others show that the associated change in Atlantic sea surface temperatures affects weather patterns over Europe, such as the track of storms coming off the Atlantic. Specifically, the European heat wave of summer 2015 has been linked to the record cold in the northern Atlantic [caused by the inflow of cold Greenland meltwater] in that year—this seemingly paradoxical effect occurs because a cold northern Atlantic promotes an air pressure pattern that funnels warm air from the south into Europe.”
While the scientists are not being alarmist Jeremiahs and warning of an imminent climapocalypse as depicted in the Hollywood movie “The Day After Tomorrow,” they nevertheless make it clear that if this Global Warming caused (fossil-fuel-burning human caused) slowing of the thermohaline cycle continues to the point of a dead stop, then this would likely be a tipping point of the entire Earth System of climate leading to “a relatively rapid and hard-to-reverse change” — not for the better.