Climate change threatens the North Atlantic’s currents, ecosystems and stability (commentary)

The ocean’s thermohaline circulation, a system of various ocean currents and water-mass conveyors, is crucial for distributing heat, salinity, minerals, dissolved gases, and nutrients around the globe, sustaining a habitable planet. In the Atlantic, this circulation, a continuous conveyor-belt of interconnected currents, is known as the Atlantic Meridional Overturning Circulation (AMOC).

The AMOC is a vital component of the Earth’s global ocean circulation, and it encompasses a network of surface and deep currents within the Atlantic Ocean. It is characterized by the northward movement of warm, saline surface waters and the southward flow of colder, deep waters. These two distinct limbs constituting the circulation are connected by overturning processes occurring in the Nordic and Labrador Seas as well as the Southern Ocean. The warm, salty, and dense northbound water from the tropics cools as it slowly loses heat to the atmosphere and gradually descends as it approaches the northern reaches of the Atlantic, moving back south past the equator into the southern torrid zone where it starts to heat up again, thus completing the cycle.

This is made possible by an intricate interplay of temperature and salinity which have counteracting effects on density, leading to layering. The AMOC thus transports heat northward from the tropics, preventing overheating in those latter regions, thus maintaining climate balance. This circulation system has been instrumental in maintaining Europe’s climate stability over millennia. The AMOC is a crucial element in the Earth’s climate system and is influenced by both atmospheric and thermohaline factors. Like other oceanographic systems, it exhibits variations on annual, decadal, and centennial timescales.

Increased ocean heat content and greater cold freshwater inputs from melting Arctic ice sheets caused by climate change could threaten the AMOC. Global warming is itself uneven along various dimensions. For one, the North Atlantic ocean is warming faster than other oceanic regions, while the Arctic region is warming roughly four times faster than the rest of the world. Since ice cover reflects sunlight away from the Earth, its melting leads to a cycle of doom, where the more the ice over a region melts, the faster the region heats up, accelerating the meltdown.

Overall ocean warming, localized sharp warming in the Arctic and Atlantic, and the resultant accelerated meltwater flux from Greenland and other ice-sheets together enfeeble the thermohaline differentials that drive the North Atlantic gyre. The ice-melt dilutes the ocean water, lowering its salinity, making it less dense, and thus preventing its natural downwelling.

The spread of the cold meltwater also smudges the well-defined temperature-density gradients running these belts. As the subducting flow weakens, a growing patch of cold, relatively still water materializes. This ‘cold blob’ is surrounded by warm water which is further warming as a result of climate change. Thus, the North Atlantic gains a new differential with an extraordinarily cold zone at the center and extraordinarily warm waters around it. This differential runs counter to the gradients which drive the AMOC, and thus attenuate it.

Reconstructions based on oceanographic data generally indicate that the AMOC is currently weaker than it was in the pre-Industrial era. However, there is vigorous debate concerning the contributions of climate change versus natural variabilities and periodicities over various timescales. Climate models consistently forecast a further weakening of the AMOC over the 21st century, with potential consequences including altered temperatures in regions like Scandinavia and Britain, which are influenced by the North Atlantic drift. Additionally, such a weakening could accelerate sea-level rise along the North American coast and reduce primary production in the North Atlantic.

The AMOC has two distinct stable modes that it switches between and persists in, over large timescales – a strong, fast circulation which is its current state and a much weaker, slower circulation. During the most recent ice age, the AMOC underwent a significant mode shift, causing temperatures near Greenland to surge by 10 to 15 degrees Celsius (18 to 27 degrees Fahrenheit) in just a decade. This was its most recent mode switch. If the AMOC were to cease functioning, both Europe and North America could experience a rapid temperature decline of up to 5 degrees Celsius (9 degrees Fahrenheit) within a similar timeframe.

A significant and sustained weakening of the AMOC has the potential to lead to its outright collapse, which would have far-reaching and mostly irreversible effects on marine and terrestrial ecosystems. This collapse could therefore represent a critical tipping point in the climate system. The impacts of a shutdown would be more severe than a slowdown and could include lowered average temperatures and reduced precipitation in Europe, resulting in a significant drop in agricultural output. Moreover, it might substantially increase the frequency of incidences of extreme weather events.

Complex earth system models in the Coupled Model Intercomparison Project suggest that a shutdown is unlikely until well after 2100. While much of paleo-oceanographic research indicates that the AMOC may be more stable than what most contemporary models predict, lower-complexity studies contend that a collapse could occur much sooner, with one recently published projection even suggesting a possible collapse around 2057. This research, released in July 2023 in Nature Communications by a pair of Danish academics, physicist Peter Ditlevsen and his sister, statistician Susanne Ditlevsen, indicates that the AMOC has been showing signs of alarming decline, with a potential shutdown looming in the middle of this century. Their findings also posit a tiny likelihood of the collapse happening as early as 2025.

Whenever it happens, such an event would carry profound implications, including rising sea levels, exacerbated global warming, catastrophic disruptions to marine ecosystems, biogeochemical cycles going awry, and a significant threat to food security. It would also have a severe impact on the highly productive upwelling marine ecosystem along the western coast of South Africa and Namibia, which is the most productive system of its kind in the Atlantic. According to the Ditlevsens’ projections, Europe might face a substantial cooling of approximately 5°C to 10°C, while tropical regions could experience intensified heat. Different parts of the world would grapple with severe droughts and increased flooding. Furthermore, the world’s oceans would become more acidic.

Direct data on the AMOC’s strength has only been recorded since 2004, so to analyze changes to the current over longer timescales, the duo of researchers turned to surface temperature readings of the subpolar gyre between the years of 1870 and 2020, a system which they argue provides a ‘fingerprint’ for the strength of AMOC’s circulation. By feeding this information into a statistical model, the researchers gauged the diminishing strength and resilience of the ocean current by its growing year-on-year fluctuations. The window for the system’s collapse could begin as early as 2025, and it grows more likely as the 21st century continues.

The sensational findings have been met with skepticism from a large chunk of the scientific community. While most experts are satisfied with the mathematical rigor and internal consistency of the model, many question its confidence given its low complexity, considering that it links surface temperatures with the likelihood of collapse of the circulation without the underlying mechanics of the same being clear.

The AMOC branches into – and contributes to – driving the Gulf Stream, a powerful warm ocean current that flows from the Gulf of Mexico up the East Coast of the United States and across the North Atlantic to western Europe. The AMOC and the Gulf Stream are interconnected components of the North Atlantic ocean circulation system. The Gulf Stream has significant effects, including regulating regional climate by transporting warm water and heat from the tropics to northern latitudes, which moderates temperatures in Europe and eastern North America. It also influences weather patterns, supports diverse marine ecosystems, and aids in shipping and navigation.

Moreover, the warm current plays a crucial role in shaping the distribution of marine life and impacting coastal economies by influencing fish migration patterns. The Gulf Stream and the AMOC share a connection primarily through the North Atlantic Drift, a minor offshoot of the Gulf Stream that travels northward and combines with the AMOC, subsequently continuing southward. Together constituting the North Atlantic ocean circulation system, these currents keep the west coast of Northern Europe, including the British Isles and Norway, several degrees warmer than places in other parts of the world lying at the same latitude. The Gulf Stream is a more superficial wind-propelled current while the AMOC is a convoluted conveyor with superficial and deep components.

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The Gulf Stream is responsible for a greater share of heat and water mass transport in North Europe than AMOC and is much more resilient given that it is majorly driven by broad wind patterns and Earth’s rotation, unlike the latter which is driven majorly by thermohaline gradients which in turn lead to density gradients.

If the AMOC goes amok, the overall flow of the Gulf Stream would remain largely unaffected. Nonetheless, the disruption of the AMOC would create a sizable ‘cold blob’ in the North Atlantic, destabilizing the climate in North America, northern Europe, and possibly other parts of the world. It could jeopardize food security by drastically affecting fishing and agriculture by usurping delicate natural cycles through primary, secondary, and tertiary effects. Western Europe’s climate dynamics are intricate, convoluted, and determined by inter-competing trends, a number of major wind and oceanic circulations, and overlapping cycles of varying frequencies.

How pronounced an effect a particular geographical phenomenon or cycle has on the climate of the region is often swayed by high altitude jet streams which guide the path of major air masses. However, such is the predicted effect of AMOC weakening that it is expected to affect the course and vigor of the Atlantic jet stream itself.

The northern parts of Europe encountered two successive severe winters from 2009 to 2011, which were subsequently linked to a brief slowdown of the AMOC. During this time, there was an accumulation of heat in the tropics, contributing to an unusually active hurricane season from June to November in 2010. In 2013, a cold blob began to emerge in the North Atlantic, its effect peaking in the summer of 2015, coinciding with heatwaves in central Europe, making it one of the few regions globally cooler than its long-term average.

Although the cold blob bore a resemblance to the signature of a weakened AMOC, any significant causal connection with AMOC weakening was later dismissed, as the transient episode was attributed to more localized atmospheric influences. Nonetheless, the event hinted at the effects that such an anomalous temperature pool, which could also result from AMOC weakening albeit at a greater scale, could precipitate on the region, even if this particular instance itself did not stem from it.

Greater ocean temperature extremes have the potential to reshape weather systems fueled by oceanic heat and moisture. One such effect is the intensification of Atlantic storms in areas observing unprecedented temperature spikes and dips. Moreover, extreme ocean temperature patterns might exert additional influences on tropical hurricane paths and the jet stream, redirecting storms to increasingly unexpected areas.

In the event of an AMOC collapse, more pronounced extreme weather events including heat waves, cold waves, drought, and flooding are anticipated which would create a vicious cycle of climate crisis. The compounding unpredictability involved would make their mitigation and management even more difficult. These potential climate ramifications should heighten the urgency of our decision-making processes.

The Atlantic Ocean currents operate as a continuous global conveyor belt, facilitating the movement of oxygen, nutrients, carbon, and heat around the world. For all of our civilization’s history, this giant oceanic machinery has run more or less steadily – warmer, saltier, and denser southerly waters travel northward, cooling and sinking in higher latitudes after which these sunken waters gradually flow southward, warm up, and the cycle repeats. However, our collective actions in the last thousandth of our species’ span of existence are threatening to usurp this grand order that naturally takes millennia to alter. Regardless of when the collapse happens, it is high time for us to take preventive and corrective action.

Pitamber Kaushik is a writer, columnist, and independent researcher whose words have appeared in over 300 publications across 60 countries, including Asia Times, New Humanist Magazine, Brussels Times, Helsinki Times, The New Delhi Times, Gulf News, and many others.

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