Ocean currents are driven by winds, tides and water density differences.
In the Atlantic Ocean circulation, the relatively warm and salty surface water near the equator flows toward Greenland. During its journey it crosses the Caribbean Sea, loops up into the Gulf of Mexico, and then flows along the U.S. East Coast before crossing the Atlantic.
This current, also known as the Gulf Stream, brings heat to Europe. As it flows northward and cools, the water mass becomes heavier. By the time it reaches Greenland, it starts to sink and flow southward. The sinking of water near Greenland pulls water from elsewhere in the Atlantic Ocean and the cycle repeats, like a conveyor belt.
Too much fresh water from melting glaciers and the Greenland ice sheet can dilute the saltiness of the water, preventing it from sinking, and weaken this ocean conveyor belt. A weaker conveyor belt transports less heat northward and also enables less heavy water to reach Greenland, which further weakens the conveyor belt’s strength. Once it reaches the tipping point, it shuts down quickly.
Instruments deployed in the ocean starting in 2004 show that the Atlantic Ocean circulation has observably slowed over the past two decades, possibly to its weakest state in almost a millennium. Studies also suggest that the circulation has reached a dangerous tipping point in the past that sent it into a precipitous, unstoppable decline, and that it could hit that tipping point again as the planet warms and glaciers and ice sheets melt.
The results showed that the circulation could fully shut down within a century of hitting the tipping point, and that it’s headed in that direction. If that happened, average temperatures would drop by several degrees in North America, parts of Asia and Europe, and people would see severe and cascading consequences around the world.
Regions that are influenced by the Gulf Stream receive substantially less heat when the 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—far 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 would warm by a few degrees.
The conveyor belt shutting down would also affect sea level and precipitation patterns, which can push other ecosystems closer to their tipping points. For example, the Amazon rainforest is vulnerable to declining precipitation. If its forest ecosystem turned to grassland, the transition would release carbon to the atmosphere and result in the loss of a valuable carbon sink, further accelerating climate change.
We were able to develop a physics-based and observable early warning signal involving the salinity transport at the southern boundary of the Atlantic Ocean. Once a threshold is reached, the tipping point is likely to follow in one to four decades.
The big question—when will the Atlantic circulation reach a tipping point—remains unanswered. Observations don’t 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 analyses made several assumptions that give rise to uncertainty.
It might seem counterintuitive to worry about extreme cold as the planet warms, but if the main Atlantic Ocean circulation shuts down from too much meltwater pouring in, that’s the risk ahead.