An influential current system in the Atlantic Ocean, which plays a critical role in the redistribution of heat through our planet’s climate system, is now moving more slowly than it has in 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 foremost experts in the field.
Scientists believe some of this slowdown is directly related to our warming climate, such as melting ice changes the equilibrium in the northern waters. Its impact can be seen in storms, heat waves and sea level rise. And it reinforces the concern that if humans cannot contain global warming, the system could eventually hit a tipping point, confusing global climate patterns.
The Gulf Stream along the US East Coast is an integral part of this system known as the Atlantic Meridional Overturning Circulation or AMOC. It became famous in the 2004 film ‘The Day After Tomorrow’ in which the ocean current abruptly stops, sending immense deadly storms flying around the world, like a supercharged tornado in Los Angeles and a wall of water crashing into New York City.
As with many sci-fi movies, the plot is based on a real concept, but the impact is taken to a dramatic extreme. Fortunately, an abrupt shutdown of the flow is not soon – if ever – expected. Even if the currents eventually stopped – and this is hotly debated – the result would not immediately be life-sized storms, but after years and decades, the consequences 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 flow is “unprecedented in the last millennium.”
Since everything is interconnected, the slowdown will no doubt already have an impact on Earth’s systems, and by the end of the century, it is estimated that circulation would slow by 34% to 45% if we continue to warm the planet. Scientists fear such a slowdown would bring us dangerously close to tipping points.
Importance of the Global Ocean Conveyor Belt
Because the equator receives much more direct sunlight than the colder poles, heat builds up in the tropics. In an effort to balance, the Earth sends this heat from the tropics to the north and from the poles to the south. This is what causes the wind to blow and create storms.
Most of that heat is redistributed by the atmosphere. But the rest is moved more slowly through the oceans in what is called the Global Ocean Conveyor Belt – a global system of currents that connect the world’s oceans and move horizontally and vertically in all different directions.
NOAA
Years of scientific research has shown that the Atlantic portion of the conveyor belt – the AMOC – is the motor that drives its operation. It moves water at 100 times the flow of the Amazon River. This is how it works.
A narrow band of warm, salty water in the Florida tropics called the Gulf Stream is carried north to the surface in the North Atlantic Ocean. When it reaches the Greenland region, it cools enough to become denser and heavier than the surrounding waters, after which it sinks. That cold water is then carried to the south in deep streams of water.
Through proxy records such as ocean sediment cores, which allow scientists to reconstruct millions of years’ distant past, scientists know that this flow has the ability to slow and stop, and when it does, the Northern Hemisphere’s climate can quickly change.
An important mechanism over the centuries, acting as a kind of lever that controls the speed of the AMOC, is the melting of glacial ice and the resulting influx of fresh water into the North Atlantic. That’s because fresh water is less salty and therefore less dense than seawater, and it doesn’t sink that easily. Too much fresh water means that the conveyor belt loses the sinking part of its engine and thus loses momentum.
That’s what scientists think is happening right now as ice in the Arctic, in places like Greenland, melts at an accelerated rate due to man-made climate change.
Climate Central
Recently, scientists have noticed a cold blob, also known as the North Atlantic warming hole, in part of the North Atlantic Ocean around southern Greenland – one of the few places on Earth that is actually cooling.
The fact that climate models predicted this provides more evidence that it is indicative of excessive melting of Greenland’s ice, increased rainfall and a consequent slowdown in heat transport north from the tropics.
NASA
To determine the unprecedented level of the AMOC’s recent slowdown, the research team collected proxy data, mostly from wildlife archives such as ocean sediments and ice cores, dating back more than 1,000 years. This helped them reconstruct the AMOC’s flow history.
The team used a combination of three different types of data to obtain information about the history of the ocean currents: temperature patterns in the Atlantic Ocean, subsurface water mass properties, and grain sizes of deep-sea sediment, dating back 1,600 years.
While every single piece of proxy data does not perfectly reflect AMOC evolution, its combination revealed a robust picture of the reversing circulation, said lead author of the paper, Dr. Levke Caesar, climate physicist at Maynooth University in Ireland. .
“The research results 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 data occurred in the mid-19th century, after a well-known regional cooling off period called the Little Ice Age, which spanned from the 14th century to the 19th century. During this time, colder temperatures often froze rivers in 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. That second drop in recent decades was likely due to global warming from the combustion and emissions of fossil fuel pollution.
Nine of the 11 data sets used in the study showed that the 20th century AMOC attenuation is statistically significant, proving that the slowdown in modern times is unprecedented.
Impact on storms, heat waves and sea level rise
Caesar says this is already echoed in the climate system on both sides of the Atlantic. “As the current decreases, more water can accumulate on the east coast of the US, leading to increased sea level rise [in places like New York and Boston], ”she explained.
Across the Atlantic, in Europe, evidence shows that weather patterns are affected, such as the trail of storms coming from the Atlantic and heat waves.
Specifically, the European heat wave of the summer of 2015 has been associated with the record cold in the North Atlantic that year – this seemingly paradoxical effect occurs because a cold North Atlantic Ocean promotes an air pressure pattern that transports warm air from the south to Europe. leads, ”she said.
According to Caesar, these effects are likely to worsen as the Earth continues to warm and the AMOC slows even further, with more extreme weather events such as a change from the winter storm coming from the Atlantic and possibly more intense storms.
CBS News posed the million dollar question to Caesar: if and when the AMOC reaches 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 will hit the tipping point of the AMOC. But the more it slows down, the more likely we are to do that.”
In addition, she explained, “Tipping does not mean that this is instantaneous, but rather that due to feedback mechanisms, the ongoing slowdown cannot be stopped once the tipping point is crossed, even if we have managed to lower global temperatures again.”
Caesar believes that if we stay below 2 degrees Celsius of global warming, it seems unlikely that the AMOC would flip, but if we hit 3 or 4 degrees of warming, the likelihood of an increase increases. Staying below 2 degrees Celsius (3.6 degrees Fahrenheit) is a goal of the Paris Agreement the US has just come back
If the tipping point is crossed and the AMOC stops, it is likely that the Northern Hemisphere would cool due to a significant decrease in the tropical heat being pushed north. But further, Caesar says that science is not yet sure what would happen. “That’s part of the risk.”
But people do have some freedom of choice here, and the decisions we make now in terms of how quickly we switch from fossil fuels will determine the outcome.
“Whether or not we pass the tipping point by the end of this century depends on the degree of warming, the amount of greenhouse gases being emitted into the atmosphere,” Caesar explains.