While Hollywood's sights were on, the 2004 film raised a serious question: If global warming were to halt the Atlantic meridional overturning cycle, which is critical for transporting heat from the tropics to northern latitudes, how sudden and drastic would climate changes be? ?
Twenty years after the release of the movie, we know a lot about the circulation of the Atlantic Ocean. Instruments deployed at sea since 2004 show that the Atlantic Ocean circulation has slowed over the past two decades, possibly to its weakest level in nearly a millennium.
Studies suggest that the cycle has reached a critical tipping point in the past, sending it into a rapid, unstoppable decline, and that it may hit 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 fresh water until the ocean cycle reaches that tipping point.
The results show that the cycle can stop completely within a century of hitting the tipping point, and is headed in that direction. If that happens, average temperatures in parts of North America, Asia and Europe will drop by several degrees, and people around the world will see severe and cascading effects.
We also discovered a physics-based early warning signal that could alert the world when the Atlantic Ocean circulation is approaching its tip.
Ocean currents are driven by wind, waves, and water density differences.
In the Atlantic Ocean circulation, relatively warm and salty surface water near the equator flows toward Greenland. During its journey it crosses the Caribbean Sea, meanders into the Gulf of Mexico, and then flows along the US East Coast before crossing the Atlantic.
Also known as the Gulf Stream, this current brings warmth to Europe. As it flows north and cools, the water mass becomes heavier. By the time it reaches Greenland, it sinks and flows south. Sinking water near Greenland pulls water from elsewhere in the Atlantic Ocean and the cycle repeats like a conveyor belt.
Excess fresh water from melting glaciers and the Greenland ice sheet dilutes the water's salinity, preventing it from sinking and weakening this ocean conveyor belt. A weaker conveyor belt transports less heat northward and enables less heavy water to reach Greenland, further weakening the strength of the conveyor belt. Once it reaches the tipping point, it shuts down quickly.
What will happen to the climate at the tip?
The existence of a tipping point was first observed in an oversimplified model of the Atlantic Ocean circulation in the early 1960s. Today's detailed climate models indicate that the strength of the conveyor belt continues to decline under climate change. However, the abrupt cessation of Atlantic Ocean circulation appeared to be absent in these climate models.
This is where our research comes in. We conducted an experiment with a detailed climate model to detect the tipping point for abrupt cessation by slowly increasing the input of fresh water.
We found that once the tipping point is reached, the conveyor belt will shut down within 100 years. The northward heat transport is strongly reduced, leading to abrupt climate changes.
The result: dangerously cold in the north
Areas affected by the Gulf Stream receive significantly less heat when the circulation stops. It cools the North American and European continents by a few degrees.
European climate is more influenced by the Gulf Stream than other regions. In our experiment, parts of the continent warmed by 5 degrees Fahrenheit (3 degrees Celsius) per decade—much faster than today's global warming of 0.36 F (0.2 C) per decade. We found that some parts of Norway experience temperature drops of more than 36 F (20 C). On the other hand, areas in the Southern Hemisphere will warm by 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.
Conveyor belt closures will also affect sea levels and precipitation patterns, pushing other ecosystems closer to their tipping points. For example, the Amazon rainforest is vulnerable to reduced rainfall. If its forest ecosystem becomes grassland, the change will release carbon into the atmosphere and lose a valuable carbon sink, further accelerating climate change.
The Atlantic Ocean Circulation has decreased significantly in the past. During the Ice Ages, when ice sheets covering large areas of the planet melted, the influx of fresh water slowed the Atlantic circulation and triggered large climate fluctuations.
So, when will we see this tipping point?
The big question — when the Atlantic circulation will reach a tipping point — remains unanswered. Observations do not go back far enough to provide a clear conclusion. A recent study suggests that the conveyor belt is rapidly approaching its tipping point, perhaps within a few years, and these statistical analyzes make many assumptions that lead to uncertainty.
Instead, it was able to generate a physically based and observable early warning signal involving salinity transport along the southern boundary of the Atlantic Ocean. Once a threshold is reached, the tipping point is likely to continue for one to four decades.
The climate implications of our study underscore the severity of such a sudden conveyor belt collapse. Changes in temperature, sea level, and precipitation will severely affect society, and climate change is irreversible on human timescales.
It may seem counterintuitive to worry about extreme cold as the planet warms, but that's the danger if the main Atlantic Ocean gyre shuts down, dumping more meltwater.
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