The churning seas are slowing down: The Atlantic Ocean circulation at its weakest in millennia
What is AMOC?
In the Atlantic Ocean there is a giant “river” that affects many aspects of life for us terrestrial dwellers, from the regional climates we enjoy to the sea level at our shore. This “river” is the Atlantic Meridional Overturning Circulation (AMOC), one of the planet’s major ocean circulation systems. AMOC functions like a conveyor belt of water and heat, bringing warm waters from the equator up to the Arctic, and cold waters from the Arctic back down to the equator. This ocean circulation helps distribute heat and moderate climate. Without these ocean circulation systems, the equator would be much hotter, and the high latitudes much colder. The Gulf Stream brushing against the eastern coast of North America is an arm of AMOC, bringing warm waters and milder winters to western Europe. Because of this redistribution of heat through water circulation, European cities like Barcelona enjoy moderate temperatures, while North American cities at similar latitudes, like Boston, experience harsh winters.
How do we measure this underwater river?
This circulation is driven by differences in water density. As water warms near the equator, it becomes less dense, and rides northwards over other denser water masses, where it eventually cools and in turn becomes denser. This cooler, denser water sinks and returns back to the equator underneath the warmer waters. A stronger AMOC means more water and heat is transported in this conveyor belt system, and a weaker AMOC means slower water transport. Directly measuring this conveyor belt system and its strength is tricky— do you measure water velocities at the surface? At the ocean bottom or middle of the water column? What about heat transport or salinity? Currently, there are efforts to directly measure AMOC by using an array of sensors throughout the Atlantic Ocean at various water depths. However, these sensor arrays were installed only relatively recently and while it might show short-term fluctuations in AMOC strength, it cannot yet show clear long-term trends.
To peek at AMOC’s trends throughout geologic history, scientists use proxies, which are physical, chemical, or biological materials preserved in the past that help us indirectly look at and measure historical conditions. These proxies help reconstruct past ocean circulation conditions, either directly or indirectly. Proxies include reconstructing water surface or subsurface temperature patterns, subsurface water mass properties and how these different water masses with different densities move over time, and physical changes to currents at the ocean bottom. Common proxies used in this field, a subset of geoscience called paleoceanography, include deep-sea corals, marine sediments, and fossilized foraminifera—microscopic amoeboid protists that commonly make shells out of calcium and live in different parts of the water column. The chemical isotopes and types of elements these organisms contain change depending on their environmental conditions like water temperature or salinity, capturing information on the ocean conditions of when they were growing or alive. These proxy records let us catch glimpses of what the Atlantic Ocean looked like and was doing as far back as AD 400.
Tracking our evolving ocean
A proxy by itself is only a snapshot in a specific time or space, but if you look at multiple proxies over time and at many geographic locations, you can better build a sense of the evolution of AMOC. A team of scientists in Europe, including Dr. Levke Caesar from Maynooth University in Ireland and Dr. Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research, did just that in a recent study. They used a multi-proxy comparison to trace the evolution of AMOC at the centennial scale and longer. Each proxy by itself has its limitations and the data derived from them can include signals other than AMOC, but when brought together, they can provide a bigger picture of AMOC activity over time and space.
Using this approach, the authors of the study found a fairly consistent picture of AMOC evolution: after being fairly stable for centuries, AMOC initially weakened in the nineteenth century, then significantly weakened in the mid-twentieth century. Now in these recent decades, AMOC is at its weakest state yet. This slowdown of AMOC was predicted by climate models and hinted at in several previous studies. However, researchers still need to delineate which parts of AMOC have been changing, and how and why this precipitated.
Climate change is ocean change
The likely cause of this slowdown is climate and environmental change. The Gulf Stream arm of AMOC is being impacted by the melting of Arctic ice; the melting ice sheets in Greenland release significant amounts of freshwater into the ocean, changing the temperature and densities of these waters, and consequently affecting the movement of this conveyor belt system. If warming continues, these ocean currents might become unstable and no longer regulate our climate, weather, and ocean. The consequences of an AMOC slowdown will be felt even if we do not fully resolve the reasons behind it. Since AMOC and our climate, weather, and sea level are intertwined, an AMOC slowdown could mean an increase in storms and heatwaves in Europe, as well as more severe winters. As water moves northwards through this circulation system, it deflects off the US East Coast, keeping water levels on the East Coast relatively low. If AMOC slows down, less water will be transported away from the East Coast, resulting in a pile up of water and increased sea level rise— on top of all the other contributors of sea level rise. The ocean, atmosphere, and land are deeply connected, and if we want to understand and mitigate the consequences of climate change on our environment, we will not only have to look at terrestrial ecosystems but also the vast, deep unknown that comprises our ocean.
References:
L. Caesar, G. D. McCarthy, D. J. R. Thornalley, N. Cahill, S. Rahmstorf (2020): Current Atlantic Meridional Overturning Circulation weakest in last millennium. Nature Geoscience [DOI: 10.1038/s41561-021-00699-z]
