When the Meridional Overturning Circulation Shuts Down

Atlantic Meridional Overturning Circulation (sketch by Nina Munteanu)

March 6, 2055

Before leaving for the university this morning, I watched a news report on the storm that devastated the northwest coast of Britain last week. Over a thousand people were affected by the sudden deluge, severe winds, and flooding. Scientists are blaming another major AR (Atmospheric River). That’s the tenth so far this year for both Britain and Western Europe. Not surprising either. Due to the global temperature increase, the air holds more moisture, so these atmospheric rivers are growing in frequency and intensity. They are consequently wreaking havoc on the Atlantic west coast and the European coasts. I can hear Daniel’s ghost hissing in my ear: between the relentless sea level rise and these storms, we’re fracked. The ARs that roar about like angry banshees have picked up the slack left by the stagnating great ocean conveyor.

The conveyor or Atlantic Overturning Circulation (AMOC) circulates ocean water very much like a lake, with dense cold water sinking beneath warmer, less salty water. Sunken water flows south along the ocean floor toward the equator; then warm surface water from the tropics flows north to replace the water that sank, keeping AMOC moving and preventing stagnation. As the Arctic turns into the Atlantic, dumping in more and more freshwater, the sinking is beginning to stop and the machine is slowing down. Freshwater is taking over the world. Like a giant wrench in an anarchist’s hand, it’s jamming the conveyor. Scientists underestimated how climate forcing would accelerate Arcic sea ice melt and increase precipitation.

The Atlantic Meridional Overturning Circulation–the great ocean conveyor–is in the process of stalling. It accounts for at least a quarter of the planet’s heat flux. We in the Northern Hemisphere are already seeing its effects: the rivers in Europe are drying up, forcing farmers to try to grow crops in the snow; the angriest storms in history are battering our maritime coast. In the meantime, the entire Southern Hemisphere is growing steadily hotter as the Indian and Asian monsoons dry up. Imagine the dynamic sea turning into a stagnant pond. No one really knows what this all means. It is likely that the oceanic plankton–our last food source–will crash or go toxic. It will probably be both

Lynna Dresden (scientist) in “A Diary in the Age of Water”

“Global ocean circulation will not change abruptly, but it will change significantly, in this century,” writes Cecilie Mauritzen, scientist with the Climate Department of the Norwegian Meteorogical Institute in Chapter 2 of “Arctic Climate Feedbacks: Global Implications”. Yet other researchers who study ancient climate change point to evidence that the AMOC can turn off abruptly. I suppose this depends on one’s definition of “abruptly.”

Mauritzen adds that “the potential for a significant change in global ocean circulation is considered one of the greatest threats to Earth’s climate: it presents a possibility of large and rapid change, even more rapid than the warming resulting directly from the build-up of human-induced greenhouse gases in the atmosphere.” An AMOC collapse would promote major cooling in most of the northern hemisphere, but also strengthen storm tracks in the North Atlantic and lead to further warming in regions of the southern hemisphere.

Climate models of an AMOC shutdown suggest a severe cooling in the whole northern hemisphere, particularly the regions closest to the zone of North Atlantic heat loss (the “radiator” of the North Atlantic central heating system). A shut down of the AMOC circulation would bring extreme cold to Europe and parts of North America, raise sea levels on these coastlines and disrupt seasonal monsoons that provide water to much of the world. It would also further endanger the Amazon rainforest and Antarctic ice sheets.

What climatologists like Mauritzen don’t discuss is the profound effects on the Earth’s biological community supported by this global circulation. An AMOC stall may result in a massive ecological collapse and our existential end due to creatures so small only a microscope can see them.

In the above quote from my eco-fiction novel A Diary in the Age of Water the scientist Lynna Dresden highlights one of the most discernable effects of an AMOC shutdown: extreme weather, a cold snap with more angry and wetter storms in the north, particularly Europe, that could last hundreds of years. Scientists report that when AMOC stopped near the end of the last Ice Age, the cold spell lasted a thousand years. 

But Lynna also talks about our primary producers, the phytoplankton (and their cousins the zooplankton). The phytoplankton—which is made up mostly of single-celled diatoms—drift on the ocean currents and sustain all life from producing the first source of a massive food chain to sequestering carbon, creating clouds and rain, and helping to create fifty percent of the oxygen we breathe. 

Freshwater plankton, similar to marine environments, showing copepods, diatoms, dinoflagellates and protista (illustration by Nina Munteanu)

According to Velasco et al. in Nature, “An AMOC shutdown could lead to the collapse of North Atlantic plankton stocks.” 

When plankton populations crash, recovery is slow. Plankton ecosystems in Earth’s oceans took 3 million years to fully recover after the mass extinction event 65 million years ago, according to scientists at the University of California—Santa Cruz. In their 2006 paper in the journal Geology, the researchers concluded “that the time required to repair food chains and reestablish an integrated ecosystem is extremely long.”

Perhaps even more likely is that the plankton will only partially crash; likely shifting their distribution and characteristics with many going extinct and some even exploding in numbers. This is called a regime shift—a widespread and prolonged change of biological systems due to climate change—something that is occurring throughout the world right now.

Summer phytoplankton bloom in the Berents Sea, 2016 (photo by NASA)

For instance, a study in NRDC reported a massive surge in plankton in the Arctic Barents Sea in 2020. Researcher Brian Palmer shared that “phytoplankton blooms are growing faster and thicker than ever seen before.” Summer blooms of Coccolithophores (unicellular Protista with calcium carbonate plates) generally occur from July through September in the Barents Sea when this shallow northern sea is ice free. The 2020 study showed that these blooms are thicker and more extensive as nutrients influx from other oceans. A recent Stanford study indicated that the growth rate of phytoplankton in the Arctic Ocean has increased 57 percent in the last twenty years. 

SEM Microscopic view of Coccolithophore (image by NASA)

While higher productivity may naively seem a good thing, these blooms are problematic: to begin, their growth is often not synchronous with what might feed on them, creating waste and detrimental trophic cascades (see below); although the algal blooms absorb more carbon, this higher carbon also contributes to the acidification of the ocean, which, in turn, impacts the phytoplankton: their growth, behaviour, and succession. The dying blooms may also liberate the excess carbon under certain circumstances. This becomes a positive feedback cycle with ever more impact.

Stephanie Dutkiewicz, principal research scientist in MIT’s Center for Global Change Science, says that while scientists have suspected ocean acidification might affect marine populations, the group’s results suggest a much larger upheavel of phytoplantion—and the species that feed on them. “The fact that there are so many different possible changes, that different phytoplankton respond differently, means there might be some quite traumatic changes in the communities over the course of the 21st century. A whole rearrangement of the communities means something to both the food web further up, but also for things like cycling of carbon.” Dutkeiwicz’s team also found that the interactive behaviour, including competition, among phytoplankton species might change. What this might mean can only spark my science fiction writer’s mind.

Regime shifts also cause trophic cascades.

The guillemot seabird is an example of one casualty. The guillemot, which typically nests on the Isle of Shetland off the coast of Scotland, is starving and few are nesting. This is because the guillemot feed on sandeel fish that have all but disappeared because the cold-water plankton the fish eat have moved north. The historically icy waters between England and Scandinavia have become too warm for the plankton to survive. Of course, if the AMOC stalls, these warming waters may cool substantially.

References:

Dybas, Cheryl Lyn. 2006. “On a Collision Course: Ocean Plankton and Climate Change.” BioScience56(8): 642-646.

Mauritzen, Celilie. 2009. “Ocean Circulation Feedbacks”,  Chapter 2 of “Arctic Climate Feedbacks: Global Implications,” Martin Sommerkorn and Susan Joy Hassol, editors. World Wildlife International Arctic Programme. 97pp.

Munteanu, Nina. 2020. “A Diary in the Age of Water.” Inanna Publications, Toronto. 300pp.

Palmer, Brian. 2020. “A Massive Surge in Plankton Has Researchers Pondering the Future of the Arctic.” NRDCSeptember 09, 2020.

Schmittner, Andreas. 2005. “Decline of the marine ecosystem caused by a reduction in the Atlantic overturning circulation.” Nature434: 628-633.

Velasco, Julian A. et. al. 2021. “Synergistic impacts of global warming and thermocline circulation collapse on amphibians” Nature, Communications Biology4(141)

Peggy’s Cove on Atlantic Ocean, Nova Scotia (photo by Nina Munteanu)

Nina Munteanu is a Canadian ecologist / limnologist and novelist. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit www.ninamunteanu.ca for the latest on her books. Nina’s bilingual “La natura dell’acqua / The Way of Water” was published by Mincione Edizioni in Rome. Her non-fiction book “Water Is…” by Pixl Press(Vancouver) was selected by Margaret Atwood in the New York Times‘Year in Reading’ and was chosen as the 2017 Summer Read by Water Canada. Her novel “A Diary in the Age of Water” was released by Inanna Publications (Toronto) in June 2020.

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