ABCD: Always Be Connecting the Dots

Ocean currents are influenced by winds, tides, and variations in water density. The AMOC plays a crucial role in redistributing warmer and colder waters between the poles through a complex system of deep and surface currents. It transports warm, salty water from tropical regions, moving through the Caribbean, circulating in the Gulf of Mexico, and then along the U.S. East Coast before heading towards the colder waters of Greenland and Iceland.

This Gulf Stream current is vital for moderating the climate in Europe, ensuring cities like Berlin, London, and Paris enjoy milder conditions while absorbing a significant portion of carbon emissions from fossil fuel combustion. Researchers estimate that the energy it carries is equivalent to that generated by one million average-sized nuclear reactors. As it travels north and cools, the water becomes denser, sinks near Greenland, and then flows back south along the eastern U.S. coast, creating a continuous cycle.

Back in 2004, this was widely understood. Fast forward two decades since “The Day After Tomorrow,” and research indicates that the Atlantic's circulation now carries approximately 15% less water than it did in the mid-20th century, reflecting a clear decline. Another study concluded that the current is currently at its weakest in over a millennium.

The Tipping Course

The notion of a tipping point in ocean circulation was first identified in a simplified model during the early 1960s. Subsequent research has highlighted a dangerous tipping point that was reached during the last glacial period, leading to an irreversible decline. As the planet warms and ice sheets melt, the influx of freshwater makes the ocean lighter, weakening this flow and indicating a potential collapse.

Excess freshwater from melting glaciers and the Greenland ice sheet can reduce the salinity of the ocean, hindering the sinking of water and disrupting the conveyor belt effect. This results in less heavy water reaching Greenland, further destabilizing the AMOC and leading to abrupt climate changes.

Recent findings suggest that Greenland's ice cap is losing 20% more ice than previously estimated. Using satellite imagery analyzed through artificial intelligence, researchers have monitored over 235,000 glacier endpoints from 1985 to 2022, revealing a significant retreat of approximately 5,000 square kilometers over 38 years, equating to an average loss of about 30 million tonnes of ice per hour.

Dr. Chad Greene from NASA's Jet Propulsion Laboratory, who led the research, remarked, “The changes around Greenland are immense, occurring across almost every glacier in recent decades.” He warned that introducing freshwater into the North Atlantic could significantly weaken the AMOC, although the exact degree of weakening remains uncertain.

The latest study by Van Westen et al. has made strides by examining salinity levels at the southern reaches of the Atlantic between Cape Town and Buenos Aires. Through simulations over 2,000 years using global climate models, they determined that increasing freshwater input could lead to an abrupt shutdown of circulation, resulting in dramatic climate shifts.

Stefan Rahmstorf, a prominent researcher in the field, described the findings as a significant advancement in understanding AMOC stability and expressed concern that this research dispels hopes that feedback mechanisms might avert a collapse.

The Cascading Chaos Ahead

The last significant collapse of the AMOC occurred approximately 12,000 years ago, likely instigating the Younger Dryas cold event, which caused rapid temperature drops in Greenland and widespread aridity in the Northern Hemisphere. Van Westen et al.'s recent study, utilizing advanced climate models, indicates that within a century of reaching a tipping point, the circulation could cease entirely.

This isn't a distant scenario.

René van Westen, a climate researcher from Utrecht University and co-author of the study, stated, “It’s a global shift.” The potential ramifications of an AMOC collapse are dire:

• The oceans are essential carbon sinks, and the AMOC plays a crucial role in carbon distribution and storage. A shutdown would jeopardize marine ecosystems and food security.
• Rising sea levels and altered precipitation would threaten additional ecosystems.
• Areas in the North Atlantic could see sea levels rise by a meter, inundating numerous coastal cities.
• The Amazon's wet and dry seasons could shift, potentially pushing the rainforest beyond its tipping point, leading to a release of stored carbon and worsening climate change.
• Global temperatures would experience unprecedented variability.
• Agricultural patterns would shift, affecting crop yields and landscapes.
• The southern hemisphere would heat up significantly, while regions influenced by the Gulf Stream would receive much less heat. Europe could experience drastic cooling—over 5 degrees Fahrenheit (3 degrees Celsius) per decade—while rainfall diminishes. Parts of Norway might see temperature drops exceeding 36°F (20°C).

Though the prospect of cooler temperatures might seem attractive amidst current warming, these changes would occur ten times faster than present trends, making adaptation nearly impossible.

Van Westen remarked, “What surprised us was the rate at which tipping occurs. It will be devastating.”

The Big Question

When will the Atlantic circulation hit its tipping point? The timeline remains uncertain, as historical observations do not extend far enough to provide clarity. Consensus on the timing is lacking.

Recent studies utilizing various data suggest we are nearing the tipping point, with some indicating it could occur in the coming decades. However, the UK Met Office has stated that significant, rapid changes in the AMOC are “very unlikely” within this century.

Estimates suggest that the current could cease operating with an increase in global temperatures of between 3 and 5.5ºC (5.4 and 9.9ºF). We are currently on track for a rise of 2.7 to 3.1ºC (4.9 to 5.6ºF) by century's end.

The new research by Van Westen introduces a “physics-based and observable type of early warning signal,” but it does not predict the exact timing of a potential shutdown. Once a threshold is crossed, the tipping point could follow in one to four decades. Van Westen noted that the 2050 tipping point identified in previous research might be a plausible projection.

It may seem paradoxical to worry about extreme cold amidst global warming, yet if the primary Atlantic Ocean circulation collapses due to excessive meltwater, that is the looming danger.

Key Takeaways of an Overturning Future

• AMOC Tipping Point Confirmed: The study by Van Westen et al. validates a tipping point for the AMOC, triggered by freshwater influx, as historically evidenced during events like the Younger Dryas.
• Observational Evidence of Tipping Course but No Concrete Timeline.
• Development of Early Warning Signal: A new physics-based early warning signal based on freshwater transport by the AMOC has been proposed to foresee approaching tipping points.
• AMOC’s Instability Confirmed: Climate models have historically overestimated the AMOC's stability, particularly regarding freshwater transport, leading to underestimations of collapse risks. This bias is a key reason the IPCC has underestimated the risk of AMOC collapse.
• Impact Simulation: Detailed simulations indicate devastating consequences for northern Europe and significant shifts in tropical rainfall patterns following an AMOC collapse.
• Rising Concerns Acknowledged: The pressing issue isn't whether this will happen, but rather ensuring we can rule it out with 99.9% certainty. Once a definitive warning signal emerges, it will likely be too late to take action, given the system's inertia.

Some may misinterpret this as speculative projection, but it is far from that. This research is about tracing the stability curve rather than making predictions.

In essence, observational data from the South Atlantic strongly suggests that the AMOC is on a perilous path. The model simulations serve as tools to identify effective early warning signals and their significance.

“It’s not science fiction,” van Westen emphasized. Alarmist or not, “We need to convey that this isn’t just a Hollywood blockbuster like The Day After Tomorrow. This is real, it can happen, and it’s crucial that we address our emissions urgently.”

Be vocal.