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.

In a new study using the latest generation of Earth’s climate models, researchers simulated the flow of fresh water until the ocean circulation reached that tipping point.

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.

Photo: Aaron Ulsh

Breakdown would plunge Northern Hemisphere into ice age

Known as the Atlantic Meridional Overturning Circulation (Amoc), the network of deep and surface currents help keep temperatures milder in Western Europe compared to other regions at similar latitudes – such as parts of eastern Canada as well as Siberia.

Scientists say a breakdown of this system could potentially plunge the UK and large parts of the Northern Hemisphere into a new ice age. It would also disrupt rainfall that billions rely on for agriculture, cause sea levels to rise in many parts of the world and lead to changes in weather patterns with significant impacts on ecosystems and human societies.

Dr Rene van Westen, a postdoctoral researcher in climate physics at Utrecht University in the Netherlands, said in ITV News: “Cooler temperatures over Europe may seem positive, but the repercussions are far-reaching, with other regions experiencing accelerated warming and altered precipitation patterns.

“Additionally, a 100cm rise in European sea levels is projected due to the abrupt ocean circulation collapse.”

Model run for 2,500 years

Unlike in previous simulations, the team added fresh water gradually, rather than in one go, New Scientist reported. This produced a positive feedback that amplified the effect: as less water sank because of the reduced salinity, less salty water flowed north, reducing salinity still further.

This eventually shut down the overturning circulation, causing temperatures to rise in the southern hemisphere, but plummet in Europe. For instance, in the model, London cools by 10°C (18°F) on average and Bergen in Norway by 15°C (27°F). Other consequences include local sea level rises in places such the US East Coast.

What’s more, some of the changes seen in the model ahead of the collapse correspond with changes being seen in the real Atlantic in recent decades.

However, to produce this collapse, the researchers had to run the model for 2500 years. And they had to add a huge amount of freshwater – less than in previous simulations, but still around 80 times more than is currently entering the ocean as Greenland’s ice sheet melts. “So that is absurd and not very realistic,” says van Westen.

Read the Science Advances article here and a piece in The Conversation here.