Circulation in the Atlantic Ocean lowest in at least 1,600 years, study finds – here’s what that means for the climate

An influential current system in the Atlantic Ocean, which plays a critical role in redistributing heat throughout our planet’s climate system, is now moving more slowly than it has for at least 1,600 years. That’s the conclusion of a new study published in the journal Nature Geoscience by some of the world’s leading experts in this field.

Scientists believe that part of this slowdown is directly related to global warming, because melting ice changes the balance in northern waters. Its impact can be seen in storms, heat waves, and sea level rise. And this reinforces fears that if humans are not able to contain the warming, the system could eventually reach a point of. change, plunging global climate models into disarray.

The Gulf Stream along the east coast of the United States is an integral part of this system, known as the Atlantic Reversal Meridional Circulation, or AMOC. It was made famous in the 2004 film “The Day After Tomorrow”, in which the ocean current suddenly stops, causing huge deadly storms around the world, such as a super charged tornado in Los Angeles and a wall of water. water crashing New York City.

As is the case with many science fiction films, the plot is based on a real concept but the impacts are taken to a dramatic extreme. Fortunately, a sudden power failure is not expected anytime soon – if ever. Even if the current ended up stopping – and this is hotly debated – the result would not instantly be larger than life storms, but over the years and decades the impacts would certainly be devastating to our planet.

Recent research has shown that circulation has slowed by at least 15% since 1950. Scientists in the new study say the weakening of the current is “unprecedented in the past millennium.”

Because everything is connected, the slowdown is undoubtedly already having an impact on Earth’s systems, and by the end of the century, it is estimated that traffic could slow by 34% to 45% if we continue to heat the planet. Scientists fear that such a slowdown could put us dangerously close to tipping points.

Importance of the Global Ocean treadmill

Because the equator receives much more direct sunlight than the cooler poles, heat builds up in the tropics. In an effort to achieve equilibrium, the Earth sends this heat north from the tropics and sends cold south from the poles. This is what causes the wind to blow and storms to form.

Most of this heat is redistributed by the atmosphere. But the rest are moved more slowly by the oceans in what is known as the Global Ocean Conveyor Belt – a global system of currents connecting the world’s oceans, moving in all different directions horizontally and vertically.

Through years of scientific research, it has become clear that the Atlantic part of the conveyor belt – the AMOC – is the engine that drives its operation. It displaces water at 100 times the flow of the Amazon River. Here is how it works.

A narrow strip of warm, salty water in the tropics near Florida called the Gulf Stream is carried north near the surface into the North Atlantic. When it reaches the Greenland region, it cools down enough to become denser and heavier than the surrounding waters, at which point it sinks. This cold water is then carried south by deep water currents.

Through indirect recordings like ocean sediment cores, which allow scientists to reconstruct a distant past stretching back millions of years, scientists know that this current has the ability to slow and stop, and when it does. in fact, the climate in the northern hemisphere can change rapidly.

An important mechanism through the ages, which acts as a lever for controlling the speed of AMOC, is the melting of glacial ice and the resulting influx of fresh water into the North Atlantic. This is because fresh water is less salty, and therefore less dense, than sea water, and it does not flow as easily. Too much fresh water means that the conveyor belt loses the flowing part of its motor and thus loses momentum.

This is what scientists think is happening now as ice in the Arctic, in places like Greenland, melts at an accelerated pace due to man-made climate change.

Recently, scientists noticed a cold drop, also known as the North Atlantic warming hole, in a patch of the North Atlantic around southern Greenland – one of the only places that actually chills on the planet.

The fact that climate models predicted this gives more evidence that it points to excessive melting of Greenland’s ice, more precipitation, and a consequent slowing of heat transport northward from the tropics.

In order to determine how unprecedented the recent downturn in AMOC is, the research team compiled proxy data drawn primarily from nature records like ocean sediments and ice cores, dating back over 1,000 years. . This helped them reconstruct the history of AMOC flows.

The team used a combination of three different types of data to gain insight into the history of ocean currents: temperature patterns in the Atlantic Ocean, groundwater mass properties, and grain size. sediments of the deep sea, dating back 1600 years.

While each individual piece of proxy data is not a perfect representation of the evolution of AMOC, their combination has revealed a solid picture of the traffic reversal, says lead author Dr Levke Caesar , climate physicist at the University of Maynooth in Ireland. .

“The results of the study suggest that it was relatively stable until the end of the 19th century,” explains Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany.

The first significant change in their ocean circulation records occurred in the mid-1800s, after a well-known regional cooling period called the Little Ice Age, which lasted from the 1400s to the 1800s. During this time, the Colder temperatures frequently froze rivers across Europe and destroyed crops.

“With the end of the Little Ice Age around 1850, ocean currents began to decline, with a second more drastic decline since the mid-20th century,” Rahmstorf said. This second decline in recent decades was likely due to global warming from combustion and emissions of pollution from fossil fuels.

Nine of the 11 datasets used in the study showed that the weakening of AMOC in the 20th century is statistically significant, proving that the slowdown is unprecedented in the modern era.

Impact on storms, heat waves and sea level rise

Caesar says this is already rippling through the climate system on both sides of the Atlantic. “As the current slows, more water can collect on the east coast of the United States, causing sea level to rise [in places like New York and Boston], “she explained.

Across the Atlantic, in Europe, evidence shows that there are impacts on weather conditions, such as the trajectory of storms coming from the Atlantic as well as heat waves.

“Specifically, the European heat wave of the summer of 2015 was linked to the record cold North Atlantic in that year – this seemingly paradoxical effect occurs because a cold North Atlantic favors an atmospheric pressure pattern that routes warm air from the south to Europe, “she said.

These impacts are likely to continue to worsen as the Earth continues to warm and AMOC slows even more, with more extreme weather events such as a change in the course of winter storms off the Atlantic and potentially more intense storms.

CBS News asked Caesar the million dollar question: if or when AMOC could reach a tipping point leading to a complete shutdown? She replied, “Well, the problem is, we don’t yet know how many degrees of global warming to reach the AMOC tipping point. But the more it slows down, the more likely we are to do it.

Further, she explained, “Failover does not mean it happens instantly, but rather that due to the feedback mechanisms, the continuous slowdown cannot be stopped once the tipping point is crossed, even. if we have succeeded in reducing global temperatures again ”.

Caesar thinks that if we stay below 2 degrees Celsius of global warming it seems unlikely that AMOC will tip over, but if we get 3 or 4 degrees of warming the chances of a tip change increase. Staying below 2 degrees Celsius (3.6 degrees Fahrenheit) is a goal of the Paris Agreement, which the United States has just joined.

If the tipping point is crossed and the AMOC stops, it is likely that the northern hemisphere will cool down due to a significant decrease in tropical heat pushed north. But beyond that, Caesar says science doesn’t yet know exactly what would happen. “It’s part of the risk.”

But humans have some power in all of this, and the decisions we make now in terms of how quickly we move away from fossil fuels will determine the outcome.

“Whether or not to cross the tipping point by the end of this century depends on the amount of warming, that is to say the amount of greenhouse gases emitted into the atmosphere,” explains Caesar.