A dramatized catastrophic consequences of an abrupt shutdown of the Atlantic Ocean’s circulation, causing superstorms and extreme climate shifts, including freezing New York City was displayed in the blockbuster Hollywood film, “The Day After Tomorrow”
The movie raised an important question, even though it was exaggerated, about the potential impact of global warming on the Atlantic Meridional Overturning Circulation (AMOC) which is crucial for heat distribution. Twenty years post-release, advancements in ocean monitoring reveal a notable slowdown in the AMOC, possibly reaching its weakest state in nearly a thousand years.
Recent studies indicate that the Atlantic Ocean’s circulation has previously reached a critical tipping point, leading to a rapid and irreversible decline. As the planet continues to warm and glaciers and ice sheets melt, there’s concern it could reach this tipping point again.
In a new study utilizing advanced Earth climate models, researchers simulated the flow of freshwater until the circulation approached this critical threshold.
The findings suggest that once the circulation reaches this tipping point, it could cease entirely within a century, with current trends indicating it’s moving in that direction. Such a scenario would result in a significant drop in average temperatures across North America, parts of Asia, and Europe, triggering severe and widespread consequences worldwide.
The study also identified a physics-based early warning signal capable of alerting the global community when the Atlantic Ocean circulation is nearing its tipping point.
Ocean’s conveyor belt
Ocean currents, influenced by winds, tides, and differences in water density, play a crucial role in global climate regulation. In the Atlantic Ocean, a significant circulation pattern involves warm, salty surface water moving from the equator towards Greenland. This flow, known as the Gulf Stream, traverses the Caribbean Sea, Gulf of Mexico, and the U.S. East Coast before crossing the Atlantic, bringing warmth to Europe.
As this current travels north and cools, it becomes denser, eventually sinking near Greenland and initiating a southward flow. This sinking motion, akin to a conveyor belt, draws water from other parts of the Atlantic, perpetuating the cycle.
However, excessive freshwater input from melting glaciers and the Greenland ice sheet can disrupt this balance by diluting the water’s saltiness, preventing it from sinking effectively. This weakens the conveyor belt, reducing the transport of heat northward and limiting the heavy water reaching Greenland, thus further weakening the circulation. Once this system reaches a critical tipping point, it can rapidly shut down.
What happens to the climate at the tipping point?
The identification of a tipping point in the Atlantic Ocean circulation was initially observed in a simplified model during the early 1960s. Contemporary climate models, with increased detail, continue to demonstrate a gradual weakening of the conveyor belt’s strength due to climate change. However, these models did not show evidence of an abrupt shutdown of the Atlantic Ocean circulation.
To address this gap, researchers conducted a study using a comprehensive climate model to pinpoint the tipping point for an abrupt shutdown by incrementally increasing freshwater input. The findings revealed that once the tipping point is reached, the conveyor belt ceases operation within a century. Consequently, the transport of heat toward the northern regions experiences a significant reduction, triggering sudden climate shifts.
The result: Dangerous cold in the North
When the Gulf Stream ceases, regions influenced by it experience a notable decrease in heat, leading to a cooling effect on North American and European continents by a few degrees. Among these, Europe is particularly impacted due to its strong dependence on the Gulf Stream. In simulations, parts of Europe warmed at rates exceeding 5 degrees Fahrenheit (3 degrees Celsius) per decade, significantly faster than the current global warming trend of approximately 0.36 F (0.2 C) per decade. Notably, regions such as Norway could face temperature drops exceeding 36 F (20 C), while some Southern Hemisphere areas may experience slight warming.
These temperature shifts unfold over roughly a century, considered abrupt on typical climate time scales. Furthermore, the cessation of the conveyor belt would influence sea levels and precipitation patterns, potentially pushing various ecosystems towards critical tipping points. For instance, the Amazon rainforest, vulnerable to decreased precipitation, could transition to grassland, releasing carbon into the atmosphere and diminishing a crucial carbon sink, thus exacerbating climate change.
Historically, the Atlantic circulation has significantly slowed during glacial periods when melting ice sheets introduced freshwater, disrupting the circulation and triggering significant climate fluctuations.
Witnessing the tipping point
The timing of the Atlantic circulation reaching a tipping point remains uncertain due to limited observational data. While some recent studies suggest it could be imminent, statistical analyses are subject to assumptions, leading to uncertainty.
In contrast, a physics-based early warning signal based on salinity transport at the southern boundary of the Atlantic Ocean offers a more tangible indicator. Once a certain threshold is reached, the tipping point is expected to occur within one to four decades.
The climate impacts outlined in the study underscore the gravity of an abrupt conveyor belt collapse. The resulting temperature, sea level, and precipitation changes will have profound societal effects, with climate shifts unstoppable on human time scales.
While it may seem paradoxical to anticipate extreme cold amid global warming, the risk of the main Atlantic Ocean circulation shutting down due to excessive meltwater intake highlights a potential future concern.