The following essay is reprinted with the permission of The conversationan online publication covering the latest research.
Superstorms, abrupt climate changes, and New York City frozen in ice. “This is how the blockbuster Hollywood movie unfolds.”Day after tomorrow” described an 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? ?
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Twenty years after the film’s release, we know much more about the circulation of the Atlantic Ocean. Instruments deployed in the ocean as of 2004 show that the circulation of the Atlantic Ocean has noticeably slowed down in the last two decades, possibly at Weakest state in almost a millennium.. Studies also suggest that circulation has reached a dangerous turning point in last which sent him into a precipitous and unstoppable decline, and which could reach that tipping point again as the planet warms and glaciers and ice sheets 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 completely closed within a century reaching the tipping point and 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 water. density differences.
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 in water from other parts of the Atlantic Ocean and the cycle repeats itself, like a conveyer belt.
Too much fresh water Melting glaciers and the Greenland ice sheet can dilute the salinity of the water, preventing it from sinking, and weakening it. ocean conveyor belt. TO weaker conveyor belt transports less heat towards the north and also allows lighter water to reach Greenland, which weakens further the strength of the conveyor belt. Once it reaches inflection pointit turns off quickly.
What happens to the climate at the tipping point?
The existence of a tipping point was first observed in an oversimplified model of the circulation of the Atlantic Ocean in the early 1960s. today is more detailed climate models indicate a continuation Decrease in the strength of the conveyor belt. under 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 changed at 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 bring other ecosystems closer to their tipping points. For example, the Amazon rainforest is vulnerable to decreasing precipitation. If your forest ecosystem were converted to grasslands, the transition would be 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 much of the planet were melting, the influx of fresh water slowed the circulation of the Atlantic, causing huge 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 turning pointPossibly 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 seriously 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.