Utrecht (Netherlands), February 11 (The Conversation) Superstorms, abrupt climate changes and New York City frozen in ice. This is how the successful Hollywood movie “The Day After Tomorrow” described the abrupt closure of the circulation of the Atlantic Ocean and its catastrophic consequences.
While Hollywood’s vision was exaggerated, the 2004 film raised a serious question: If global warming disrupts the Atlantic meridional circulation, which is crucial for transporting heat from the tropics to northern latitudes, how abrupt and severe Would it be climate changes? ?
Twenty years after the film’s release, we know much more about the circulation of the Atlantic Ocean. Instruments deployed in the ocean since 2004 show that the circulation of the Atlantic Ocean has slowed markedly over the past two decades, possibly to its weakest state in nearly a millennium.
Studies also suggest that the circulation has reached a dangerous tipping point in the past that led to a precipitous, unstoppable decline, and that it could reach that tipping point again as the planet warms and glaciers and ice caps ice melt.
In a new study using the latest generation of Earth climate models, we simulated the flow of freshwater until the ocean circulation reached that tipping point.
The results showed that the circulation could stop completely a century after reaching the tipping point, and that it is heading in that direction. If that happened, average temperatures would drop several degrees in North America, parts of Asia and Europe, and people would see serious, cascading consequences around the world.
We also discovered a physics-based early warning signal that can alert the world when the Atlantic Ocean circulation is approaching its tipping point.
The ocean conveyor belt
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, turns into 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, 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 weakening this ocean conveyor belt. A weaker conveyor belt transports less heat north and also allows less heavy water to reach Greenland, further weakening the strength of the conveyor belt. Once it reaches the tipping point, it quickly shuts down.
What happens to the climate at the tipping point?
The existence of a tipping point was first observed in an oversimplified model of Atlantic Ocean circulation in the early 1960s. Today’s most detailed climate models indicate a continued slowdown in conveyor belt strength due to climate change. However, an abrupt closure of the Atlantic Ocean circulation seemed to be absent in these climate models.
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 closure by slowly increasing freshwater input.
We found that once it reaches the tipping point, the conveyor belt stops after 100 years. Northward heat transport is greatly reduced, causing abrupt climate changes.
The result: Dangerous cold in the north
Regions influenced by the Gulf Stream receive substantially less heat when 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 other regions. In our experiment, that meant parts of the continent warmed 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 some parts of Norway would experience temperature drops of more than 36 F (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, it is abrupt.
Closing 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 result in 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 much of the planet were melting, the influx of fresh water slowed down the Atlantic circulation, causing enormous climate fluctuations.
So when will we see this turning point?
The big question: when will the Atlantic circulation reach a tipping point? still no response. The observations do not go back far enough to provide a clear result. While 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 involving salinity transport at the southern edge of the Atlantic Ocean. Once a threshold is reached, the tipping point is likely to be reached in one to four decades.
The climate impacts from 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 Atlantic Ocean’s main circulation shuts down due to excess meltwater, that’s the risk looming. (The conversation) NSA NSA