Superstorms, sudden climate changes and New York completely frozen. This is how the Hollywood blockbuster film The Day After Tomorrow described a sudden cut in the circulation of the Atlantic Ocean and its catastrophic consequences.
Although Hollywood's vision was exaggerated, the 2004 film raised a serious question: whether global warming disrupts the Atlantic meridional overturning circulation (AMOC), which is crucial for transporting heat from the tropics to northern latitudes, how abrupt and severe would the climate changes be?
Twenty years after the film's release, we know much more about the circulation of the Atlantic Ocean. Instruments installed in the ocean since 2004 show that its circulation has slowed observably over the past two decades, possibly to its weakest state in nearly a millennium. Studies also suggest that the circulation reached a dangerous tipping point in the past that sent it into a precipitous, unstoppable decline, and that it could reach that tipping point again as the planet warms and glaciers and ice sheets warm. ice melt.
In a new study using the latest generation of Earth climate models, freshwater flow was simulated until ocean circulation reached that tipping point.
The results showed that circulation could stop completely within a century of reaching the tipping point and that it is heading in that direction. If that were to happen, average temperatures would drop several degrees in North America, parts of Asia and Europe, and the consequences would be severe and cascading around the world.
We also discovered a physics-based early warning signal that can alert the world when the Atlantic Ocean circulation approaches its tipping point.
Ocean currents are driven by winds, tides, and differences in water density.
In the Atlantic Ocean circulation, relatively warm, salty surface water near the equator flows toward Greenland. During its journey it crosses the Caribbean Sea, loops in the Gulf of Mexico and then flows along the east coast of the United States before crossing the Atlantic.
This current, also known as the Gulf Stream, brings heat to Europe. As it flows north and cools, the mass of water becomes heavier. When it reaches Greenland, it begins to sink and flow south. Sinking water near Greenland draws water from other parts of the Atlantic Ocean and the cycle repeats itself, like a conveyor belt.
Too much freshwater from melting glaciers and the Greenland Ice Sheet can dilute the water's salinity, preventing it from sinking, and weaken this ocean conveyor belt. A weaker conveyor belt transports less heat north and also allows less heavy water to reach Greenland, further weakening its strength. Once it reaches the tipping point, it stops quickly.
The existence of a tipping point was first observed in an oversimplified model of Atlantic Ocean circulation in the early 1960s. Today, more detailed climate models indicate a continued slowdown in the strength of the conveyor belt. due to climate change. However, in these climate models there does not appear to be a sudden interruption of the circulation of the Atlantic Ocean.
This is where our study comes into play. We conducted an experiment with a detailed climate model to find the tipping point for an abrupt collapse by slowly increasing freshwater input.
Once the tipping point is reached, the conveyor belt stops in 100 years. The transport of heat towards the north is strongly reduced, causing abrupt changes in climate.
Regions influenced by the Gulf Stream receive much less heat when the circulation stops. This cools the North American and European continents by a few degrees.
The European climate is much more influenced by the Gulf Stream than that of other regions. In our experiment, that meant some parts of the continent were changing by more than 3°C per decade, much faster than today's global warming of about 0.2°C per decade. We found that some areas of Norway would experience temperature drops of more than 20°C. On the other hand, regions in the southern hemisphere would warm by a few degrees.
These temperature changes develop over about 100 years. It may seem like a long time, but on typical climate time scales, these are abrupt changes.
The collapse of the conveyor belt would also affect sea level and precipitation patterns, which may push other ecosystems closer to their tipping points. For example, the Amazon rainforest is vulnerable to decreased rainfall. If its forest ecosystem were converted to grasslands, the transition would release carbon into the atmosphere and cause the loss of a valuable carbon sink, further accelerating climate change.
The Atlantic circulation has slowed down significantly in the distant past. During glacial periods, when the ice sheets that covered large parts of the planet melted, the influx of fresh water slowed down the Atlantic circulation, triggering enormous climate fluctuations.
The big question – when will the Atlantic circulation reach its tipping point – remains unanswered. The observations do not go back far enough to provide a clear result. Although a recent study suggested that the conveyor belt is rapidly approaching its tipping point, possibly within a few years, these statistical analyzes made several assumptions that give rise to uncertainty.
Instead, we were able to develop an observable, physics-based early warning signal related to salinity transport at the southern edge of the Atlantic Ocean. Once the threshold is reached, the tipping point is likely to occur one to four decades later.
The climate impacts revealed by our study underscore the severity of such an abrupt conveyor belt collapse. Changes in temperature, sea level and precipitation will severely affect society, and climate changes are unstoppable on human time scales.
It may seem counterintuitive to worry about extreme cold as the planet warms, but if the main circulation of the Atlantic Ocean is stopped by excess meltwater, that is the risk looming.
This article was originally published in English and translated and edited by The Conversation in Spain.
René van Westen is a postdoctoral researcher in Climate Physics at Utrecht University. Henk A. Dijkstra is Professor of Physics at Utrecht University. Michael Kliphuis is a specialist in Climate Models at the University of Utrecht.
