AMOC The global heat distributor, Atlantic current.

AMOC The global heat distributor, Atlantic current.

The Worrying Weakening of the AMOC

AMOC

Ask a climate scientist about possible “tipping points,” and they’re likely to mention the Atlantic Meridional Overturning Circulation (AMOC). This ocean current, as it flows from the southern to the northern part of the Atlantic, grows cooler and saltier. Eventually, it sinks to the ocean floor 3km down and flows back across the abyssal plain. Recent evidence suggests that this crucial global heat distribution system is weakening, leading scientists to express deep concerns.

The Power and Influence of AMOC

AMOC is often referred to as a “poster child” for tipping points. Its significance lies in its immense power and global impact. The rate at which it transfers heat towards the pole accounts for about a quarter of all the northward flow of heat from the tropics. In fact, it transfers heat at a rate roughly 60 times greater than the rate at which humans produce energy through burning fossil fuels. Moreover, at least half of the water that reaches the ocean depths does so in the North Atlantic.

The Tipping-Point Nature of AMOC

What makes AMOC even more concerning is its tipping-point nature. Unlike gradual changes, AMOC can suddenly switch from an “on” state to an “off” state if pushed too far. It is a clear instability in the climate system, demonstrated by both theory and modeling. There is strong historical evidence suggesting that global warming may be pushing AMOC towards this tipping point, with some studies indicating it may occur by the middle of this century.

The Crucial Difference Between AMOC and the Gulf Stream

AMOC is often confused with the Gulf Stream, which also runs across the Atlantic. However, there is a crucial difference. The Gulf Stream is driven by persistent winds, regardless of climate conditions. On the other hand, AMOC is driven by the sinking of water ahead of it, creating a conveyor-belt-like circulation. If the near-surface part of AMOC stops getting salty enough to sink, the circulation halts, and warm water remains stagnant. This would lead to dire consequences, including colder and drier conditions in Northern Europe, exacerbating storms, droughts, and negatively impacting agriculture.

A Cautionary Tale for Climate Change

Wallace Broecker, a renowned American oceanographer, once described climate as an “angry beast” and humanity as “poking it with a sharp stick.” The metaphorical sharp stick in this case is the freshening of water in the North Atlantic due to increased rainfall and melting ice caused by greenhouse gases. Climate models, such as those used by the Intergovernmental Panel on Climate Change (IPCC), predict a weakening of AMOC over this century. While a shutdown of AMOC is deemed “very likely,” the models do not show it crossing the tipping point and shutting down completely.

Uncertainty and Early Warnings

Recent assessments by the IPCC acknowledge the limitations of climate models in predicting potential AMOC shutdowns. They recognize the need for further study and the effects of melting Greenland ice. In light of this uncertainty, scientists are seeking early warning signs of a shutdown. A July 2021 paper published in Nature Communications by Peter and Susanne Ditlevsen of the University of Copenhagen offers statistical analysis suggesting a 95% chance of a shutdown occurring between 2025 and 2095. However, it’s important to note that assumptions made in this study could still prove inaccurate, and further research is required to fully understand AMOC’s behavior.

AMOC

The Urgent Need for Continued Research

AMOC is not alone among systems with tipping points. Similar uncertainties and the need for further study exist in other critical areas, including the Amazon rainforest and the West Antarctic ice sheet. It is crucial to better characterize their behavior and understand their tipping points. However, conclusive studies are unlikely to provide definitive answers. Until humanity reduces the factors pushing these systems to their limits, there can be no guarantee of calm. Climate change is an “angry beast,” and until we stop poking it with sharp sticks, it will continue to pose significant risks.