While Hollywood's vision was exaggerated, the 2004 film posed a serious query: If global warming stops the Atlantic meridional overturning circulation, which is crucial for transferring heat from the tropics to northern latitudes, how abrupt and severe will climate change be?
Twenty years after the film's premiere, we all know way more concerning the circulation of the Atlantic Ocean. Instruments deployed within the ocean since 2004 show that circulation within the Atlantic Ocean has slowed noticeably over the past 20 years, more likely to its weakest state in almost a millennium.
The research also suggests that circulation has reached a dangerous tipping point prior to now, resulting in a rapid, unstoppable decline, and that it could reach that tipping point again because the planet warms and glaciers and ice sheets melt.
In a brand new study using the most recent generation of Earth's climate models, we simulated the flow of freshwater until ocean circulation reached a tipping point.
The results showed that the loop might be completely closed inside a century of reaching the tipping point, and that it was heading in that direction. If this happened, average temperatures would drop by several degrees in North America, parts of Asia and Europe, and other people around the globe would feel severe and cascading consequences.
We also discovered a physics-based early warning signal that might alert the world as circulation within the Atlantic Ocean approaches a tipping point.
Ocean conveyor belt
Ocean currents are driven by winds, tides and differences in water density.
In the circulation of the Atlantic Ocean, relatively warm and salty surface water near the equator flows towards Greenland. During its journey, it passes through the Caribbean Sea, loops within the Gulf of Mexico, after which flows along the eastern coast of the United States before crossing the Atlantic.
This current, also called the Gulf Stream, supplies heat to Europe. As it flows north and cools, the water mass becomes heavier. Before it reaches Greenland, it begins to sink and float south. The sinking of water near Greenland draws water from the remaining 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, stopping it from sinking, and weaken the ocean's transmission belt.
The weaker conveyor belt transports less heat north and likewise allows less heavy water to succeed in Greenland, further weakening the strength of the conveyor belt. When it reaches a critical point, it quickly shuts down.
What will occur to the climate on the tipping point?
The existence of a tipping point was first noticed in an oversimplified Atlantic Ocean circulation model within the early Nineteen Sixties. Today's more detailed climate models indicate that conveyor belt strength continues to say no within the face of climate change. However, in these climate models, there appeared to be no sudden stop within the circulation of the Atlantic Ocean.
This is where our study is available in. We conducted an experiment with an in depth climate model to search out the tipping point of a sudden shutdown by slowly increasing the freshwater supply.
We found that when the tipping point is reached, the conveyor belt shuts down inside 100 years. Heat transport northwards is strongly limited, which results in rapid climate changes.
The result: dangerous cold within the north
Regions influenced by the Gulf Stream receive much less heat when circulation stops. This will cool the North American and European continents by several degrees.
European climate is way more influenced by the Gulf Stream than other regions. In our experiment, this meant that some parts of the continent saw temperature changes of greater than 3 degrees Celsius per decade – much faster than today's global warming of about 0.2°C per decade.
We have determined that temperatures will drop by greater than 20°C in some parts of Norway. On the opposite hand, regions within the southern hemisphere would warm by several degrees.
These temperature changes develop over roughly 100 years. This may seem to be an extended time, but on typical climatic timescales it’s sudden.
Turning off the conveyor belt would also affect sea levels and rainfall patterns, which could push other ecosystems closer to tipping points. For example, the Amazon rainforest is vulnerable to declining rainfall.
If a forest ecosystem were to show into grassland, this transition would release carbon dioxide into the atmosphere, leading to the lack of a precious carbon sink, further accelerating climate change.
Atlantic circulation has slowed significantly within the distant past. During glacial periods, when the ice sheets that covered large parts of the planet melted, the influx of freshwater slowed the circulation of the Atlantic, causing huge climate fluctuations.
So when will we see this tipping point?
The most vital query – when the Atlantic circulation will reach a critical point – stays unanswered. Observations don’t return far enough to offer a transparent result.
Although a recent study suggested that the conveyor belt is rapidly approaching a tipping point, likely inside just a few years, several assumptions are made in these statistical analyzes that create uncertainty.
Instead, we were capable of develop a physically based and observable early warning signal covering salinity transport on the southern boundary of the Atlantic Ocean. Once the brink is reached, the tipping point will likely occur inside one to 4 a long time.
The climate effects presented in our study highlight the seriousness of such a sudden conveyor belt collapse. Changes in temperature, sea level and precipitation may have major impacts on society, and climate change can’t be stopped on human timescales.
It could seem counterintuitive to fret about extreme cold because the planet warms, but when the principal circulation of the Atlantic Ocean is shut down by an excessive amount of meltwater flowing in, that may be a risk for the longer term.
RRené van Westen, postdoctoral researcher in climate physics, Utrecht University; Henk A. Dijkstra, Professor of Physics, Utrecht University and Michael Kliphuis, Climate Modeller, Utrecht University
This article has been republished from Conversation under Creative Commons license. Read original article.
Image Source: Pixabay.com
