The Atlantic Meridional Overturning Circulation (AMOC) ocean current, which includes the Gulf Stream, was just confirmed to have been slowing down and destabilizing for decades, thanks to the climate crisis. When and if it does stall completely, the impact on the planet would be catastrophic.
Computer models provided by NASA and NOAA are now providing a far more accurate model of how the Atlantic Meriodional Overturning Circulation current moves. Photo: NOAA's Atlantic Oceanographic and Meteorological Laboratory
Ocean currents flow in directions dictated by the rotation of the Earth and simple thermodynamics.
The Atlantic Meridional Overturning Circulation (AMOC) current is one of the most important of those. For all human history, the portion of the Atlantic Ocean which lies closer to the equator has always been significantly hotter than in the northern latitudes. Just as heat rises naturally to move to cooler areas of the atmosphere or even within our homes, the warm waters of the tropics travel upwards to colder waters in the North American eastern coast, then northeasterly over to Europe and England.
At the end of that journey, in times before the climate crisis, the salty seawater surface along the top of the ocean current as it approaches Arctic regions has in the past flipped down and sunk deep underneath.
Global heating has made the Atlantic Ocean hotter everywhere. It has also caused changes in the stratification of the thermal profile of the waters. Both are creating havoc with the AMOC.
In the past, oceans went through a regular churning process, triggered by current motion tied in part to the rotation of the earth. That resulted in cold waters from deep underneath the water’s surface being regularly pushed up to the top, and the hotter surface liquid pulled down below.
In a paper published in September 2020, researchers revealed that much of that churning has ceased just in the last six decades. It has particularly affected the top 650 feet of the ocean. The cause of this is directly tied to more solar energy trapped close to the planet as higher concentrations of carbon dioxide are being dumped into the atmosphere. Since that top layer is no longer changing out much, any cooling effect such churning would have had on the average ocean surface temperature is going the wrong way. The ocean surface is much hotter than ever.
Among the impacts a hotter ocean surface has already produced are more powerful hurricanes and cyclones on the planet, as well as faster intensification from tropical storm level gusts to Category 4 and higher winds. The hotter ocean surface throughout also increases melting of ice sheets in the Arctic and Antarctic.
Higher ocean temperatures in the North Atlantic would also cause the single most important ocean current in the world in terms of its impact on weather patterns and ecosystems, the AMOC, to slow. With less of a transition in temperature for waters in the tropics to those in the North Atlantic, the thermodynamic effects pulling that water upwards will ease in time.
If the water current speed were to drop significantly, multiple ecological effects would follow quickly. Among them are that Western Europe and the United Kingdom would at least initially experience cooler waters nearby than before. The United Kingdom might even see major blizzards previously unheard of as temperatures dropped father than normal in the winter.
Another consequence of the lack of movement of the waters is that marine life would likely change habitats from their norm. Life that requires cooler waters than are no longer available up north might become extinct as well. The food chain that relies on all would also suffer extinctions for the same reason.
Coastal habitats would be significantly affected for much the same reason.
A schematic of the Meridional Overturning Circulation (MOC) by all major Earth oceans. Photo: NOAA\'s Atlantic Oceanographic and Meteorological Laboratory
If the ocean currents do not move northwards to redistribute their mass, another frightening side effect would occur. As the Earth rotates, the ocean mass would be draw further away from the center of the Earth because of the Earth’s centrifugal force. That would produce even more rapid sea level rise across the entire planet, with North America bearing the brunt of the effect. That on top of sea level rise as the glaciers at the poles, in Greenland, and in the Himalayas melt, could cause rapid sea level changes in just a decade or so.
In news released just this week, scientists attempting to understand precisely how serious all this has been on the AMOC have discovered proof that this current is in far more trouble than previously understood.
On top of past research which has shown the Atlantic Meridional Overturning Circulation current is at the slowest it has been for 1,600 years or longer, the new research found hard proof that the currents now have “an almost complete loss of stability.” That translates into a lack of consistent momentum of the AMOC.
Such lack of consistent momentum is dangerous in two ways. With the currents alternating between moving at one rate of speed to one that is much slower, the current’s momentum is now not stable. With less momentum at one point, it now takes more energy to cause the currents to restore movement at their previous rate. That translate it a lower likelihood that the current will ever regain its previous speed.
The analysis in this study, led by Niklas Boers from the Potsdam Institute for Climate Impact Research in Germany, discovered one cause of this loss of ocean current stability is from the rapid melt of ice from Greenland as the current headed across its eastern boundary and to points further north. That ice converts into freshwater at the surface of the ocean. The freshwater is interfering with the saltwater from the AMOC sinking deep into the Arctic, causing the current to slow because the saltwater represents a barrier to the continued current movement.
By looking at past data from ice core analysis and other information, Dr. Boers’ team determined the Atlantic Meridional Overturning Circulation current has existed in two different states during the time humans have walked the earth. There have been slow, unsteady, and weak versions of the current in the past. In the last few millennia, the AMOC has been characterized as steady, fast-moving, and powerful.
The researchers determined we have already moved into the unsteady and weak form of that current, though it is nowhere near as slow as at its lowest in history. If it stops almost completely, something which is looking more likely every year, the side effects could tear landforms and ecosystems apart at the edges.
The researchers laid out the difficult truth of their conclusion succinctly at the end of the paper.
“This decline [of the AMOC in recent decades] may be associated with an almost complete loss of stability over the course of the last century, and the AMOC could be close to a critical transition to its weak circulation mode.”
“The signs of the destabilization being visible already is something that I wouldn’t have expected and that I find scary,” said lead author Boers in an interview about his paper. “It is something you just can’t allow to happen.”
The paper by Boers and his team, “Observation-based early-warning signals for a collapse of the AMOC,” was published in the journal Nature Climate Change.