Massive storms, sudden climate changes and New York City frozen in ice. That’s how the popular Hollywood movie “The Day After Tomorrow” showed the sudden shutdown of the Atlantic Ocean and the disastrous consequences.
While Hollywood’s vision was over the top, the 2004 film raised a serious question: If global warming shuts down the Atlantic Parasitic Circulation, which is critical for transporting heat from the tropics to the northern latitudes, how suddenly and severely so would the climate changes. ?
Twenty years after the release of the film, we know much more about the circulation of the Atlantic Ocean. Instruments used in the ocean beginning in 2004 show that the circulation of the Atlantic Ocean has slowed down significantly in the last twenty years, perhaps to the weakest state in almost a thousand years. Studies also suggest that the circulation has reached a dangerous tipping point in the past that sent it into sharp, unstoppable decline, and that it may hit that tipping point again as the planet warms and glaciers and melting ice sheets.
In a new study using the latest generation of Earth climate models, we simulated the flow of fresh water until the ocean circulation reached that tipping point.
The results showed that the circulation could close completely within a century after hitting the tipping point, and that it is heading in that direction. If that were to happen, the average temperature would drop several degrees in North America, parts of Asia and Europe, and people would see serious cascading consequences around the world.
We also found a physics-based early warning signal that can alert the world when the Atlantic Ocean’s circulation is approaching its tipping point.
Transport zone of the sea
Ocean currents are driven by winds, tides and differences in water density.
In the Atlantic Ocean circulation, the relatively warm and salty surface water near the equator flows towards Greenland. During its journey it crosses the Caribbean Sea, bends up to the Gulf of Mexico, and then flows along the US East Coast 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. By the time it reaches Greenland, it begins to submerge and flow south. Sinking water near Greenland draws water from other parts of the Atlantic Ocean and the cycle changes, like a conveyor belt.
Excess fresh water from melting glaciers and the Greenland ice sheet can dilute the salinity of the water, preventing it from submerging, and weakening this ocean conveyor belt. A weaker conveyor belt transports less heat north and allows less heavy water to reach Greenland, further weakening the strength of the conveyor belt. When it reaches the tipping point, it stops quickly.
What happens to the climate at the tipping point?
A tipping point was first noted in an oversimplified model of the Atlantic Ocean circulation in the early 1960s. Today’s more detailed climate models show that the strength of the conveyor belt is continuing to slow down under climate change. However, an abrupt shutdown of the Atlantic Ocean circulation did not appear to be present in these climate models.
This is where our study comes in. We experimented with a miniature climate model to find the tipping point for a sudden shutdown by slowly increasing freshwater input.
We found that when it reaches the tipping point, the conveyor belt stops down within 100 years. The transport of heat towards the north is greatly reduced, which leads to sudden climate changes.
The result: dangerously cold in the North
Regions affected 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 Gulf Stream has a far greater influence on Europe’s climate than other regions. In our experiment, that meant parts of the continent were warming by more than 5 degrees Fahrenheit (3 degrees Celsius) per decade—much faster than today’s global warming of about 0.36 F (0.2 C ) per decade. We found that parts of Norway would experience temperature drops of more than 36 F (20 C). On the other hand, regions in the Southern Hemisphere warmed a few degrees.
These temperature changes develop over about 100 years. That may seem like a long time, but on typical climate timescales, it’s sudden.
Shutting down the conveyor belt would also affect sea level and precipitation patterns, potentially pushing other ecosystems closer to their tipping points. For example, the Amazon rainforest is in danger of declining in precipitation. If the forest ecosystem were converted to grassland, the transfer would release carbon to the atmosphere and lose a valuable carbon sink, accelerating climate change.
The Atlantic circulation has slowed significantly in the past. During glacial periods when ice sheets covering large parts of the planet were melting, the influx of fresh water slowed the spread of the Atlantic, causing huge climate fluctuations.
So when will we see this tipping point?
The big question – when will the Atlantic circulation reach a tipping point – remains unanswered. Comments do not go back far enough to provide a clear result. Although a recent study indicated that the conveyor belt is rapidly approaching its tipping point, possibly within a few years, these statistical analyzes made several assumptions that led to uncertainty.
Instead, we were able to develop a physically based and observable early warning signal of salinity transport at the southern boundary of the Atlantic Ocean. Once the threshold is reached, the tipping point is likely to come in decades.
The climate consequences from our study highlight the severity and sudden collapse of the conveyor belt. The changes in temperature, sea level and precipitation will affect society greatly, and the climate changes cannot be stopped on human time scales.
It may be unsettling to worry about extreme cold as the planet warms, but if the main circulation of the Atlantic Ocean stops pouring in too much meltwater, that’s the danger we face.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: René van Westen, University of Utrecht; Henk A. Dijkstra, University of Utrechtand Michael Kliphuis, University of Utrecht
René van Westen receives funding from the European Research Council (ERC-AdG project 101055096, TAOC).
Henk A. Dijkstra receives funding from the European Research Council (ERC-AdG project 101055096, TAOC, PI: Dijkstra).
Michael Kliphuis does not work for, consult with, or own shares in or receive funding from any company or organization that would benefit from this article this, and has not disclosed any relevant affiliations beyond his academic appointment.