Gulf Stream current at its weakest in 1,600 years, studies show
Such a collapse would see western Europe suffer far more extreme winters, sea levels rise fast on the eastern seaboard of the US and would disrupt vital tropical rains. The new research shows the current is now 15% weaker than around 400 AD, an exceptionally large deviation, and that human-caused global warming is responsible for at least a significant part of the weakening.
The current, known as the Atlantic Meridional Overturning Circulation (AMOC), carries warm water northwards towards the north pole. There it cools, becomes denser and sinks, and then flows back southwards. But global warming hampers the cooling of the water, while melting ice in the Arctic, particularly from Greenland, floods the area with less dense freshwater, weakening the Amoc current.
Scientists know that AMOC has slowed since 2004, when instruments were deployed at sea to measure it. But now two new studies have provided comprehensive ocean-based evidence that the weakening is unprecedented in at least 1,600 years, which is as far back as the new research stretches.
“AMOC is a really important part of the Earth’s climate system and it has played an important part in abrupt climate change in the past,” said Dr David Thornalley, from University College London who led one of the new studies. He said current climate models do not replicate the observed slowdown, suggesting that AMOC is less stable than thought.
During the last ice age, some big changes in AMOC led to winter temperatures changing by 5-10C in as short a time as one to three years, with major consequences for the weather over the land masses bordering the Atlantic. “The [current] climate models don’t predict [an AMOC shutdown] is going to happen in the future – the problem is how certain are we it is not going to happen? It is one of these tipping points that is relatively low probability, but high impact.”
The study by Thornalley and colleagues, published in Nature, used cores of sediments from a key site off Cape Hatteras in North Carolina to examine AMOC over the last 1600 years. Larger grains of sediment reflect faster AMOC currents and vice versa.
They also used the shells of tiny marine creatures from sites across the Atlantic to measure a characteristic pattern of temperatures that indicate the strength of AMOC. When it weakens, a large area of ocean around Iceland cools, as less warm water is brought north, and the waters off the east coast of the US get warmer.
The second study, also published in Nature, also used the characteristic pattern of temperatures, but assessed this using thermometer data collected over the last 120 years or so.
Both studies found that Amoc today is about 15% weaker than 1,600 years ago, but there were also differences in their conclusions. The first study found significant Amoc weakening after the end of the little ice age in about 1850, the result of natural climate variability, with further weakening caused later by global warming.
The second study suggests most of the weakening came later, and can be squarely blamed on the burning of fossil fuels. Further research is now being undertaken to understand the reasons for the differences.
However, it is already clear that human-caused climate change will continue to slow Amoc, with potentially severe consequences. “If we do not rapidly stop global warming, we must expect a further long-term slowdown of the Atlantic overturning,” said Alexander Robinson, at the University of Madrid, and one of the team that conducted the second study. He warned: “We are only beginning to understand the consequences of this unprecedented process – but they might be disruptive.”
A 2004 disaster movie, The Day After Tomorrow, envisaged a rapid shutdown of Amoc and a devastating freeze. The basics of the science were portrayed correctly, said Thornalley: “Obviously it was exaggerated – the changes happened in a few days or weeks and were much more extreme. But it is true that in the past this weakening of Amoc happened very rapidly and caused big changes.”
Serious disruption to the Gulf Stream ocean currents that are crucial in controlling global climate must be avoided “at all costs”, senior scientists have warned. The alert follows the revelation this week that the system is at its weakest ever recorded.
Past collapses of the giant network have seen some of the most extreme impacts in climate history, with western Europe particularly vulnerable to a descent into freezing winters. A significantly weakened system is also likely to cause more severe storms in Europe, faster sea level rise on the east coast of the US and increasing drought in the Sahel in Africa.
The new research worries scientists because of the huge impact global warming has already had on the currents and the unpredictability of a future “tipping point”.
The currents that bring warm Atlantic water northwards towards the pole, where they cool, sink and return southwards, is the most significant control on northern hemisphere climate outside the atmosphere. But the system, formally called the Atlantic Meridional Overturning Circulation (Amoc), has weakened by 15% since 1950, thanks to melting Greenland ice and ocean warming making sea water less dense and more buoyant.
This represents a massive slowdown – equivalent to halting all the world’s rivers three times over, or stopping the greatest river, the Amazon, 15 times. Such weakening has not been seen in at least the last 1,600 years, which is as far back as researchers have analysed so far. Furthermore, the new analyses show the weakening is accelerating.
“From the study of past climate, we know changes in the Amoc have been some of the most abrupt and impactful events in the history of climate,” said Prof Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research in Germany and one of the world’s leading oceanographers, who led some of the new research. During the last Ice Age, winter temperatures changed by up to 10C within three years in some places.
“We are dealing with a system that in some aspects is highly non-linear, so fiddling with it is very dangerous, because you may well trigger some surprises,” he said. “I wish I knew where this critical tipping point is, but that is unfortunately just what we don’t know. We should avoid disrupting the Amoc at all costs. It is one more reason why we should stop global warming as soon as possible.”
Oceanographer Peter Spooner, at University College London, shares the concern: “The extent of the changes we have discovered comes as a surprise to many, including myself, and points to significant changes in the future.”
A collapse in the Amoc would mean far less heat reaching western Europe and plunge the region into very severe winters, the kind of scenario depicted in an extreme fashion in the movie The Day After Tomorrow. A widespread collapse of deep-sea ecosystems has also been seen in the past.
But as the Amoc weakens, it might actually increase summer heatwaves. That is because it takes time for the cooling of the northern waters to also cause cooling over the adjacent lands. However, the cooler waters affect the atmosphere in a way that helps warm air to flood into Europe from the south, a situation already seen in 2015.
Other new research this week showed that Greenland’s massive ice cap is melting at the fastest rate for at least 450 years. This influx will continue to weaken the Amoc into the future until human-caused climate change is halted, but scientists do not not know how fast the weakening will be or when it reaches the point of collapse.
“Many people have tried to check that with computer models,” said Rahmstorf. “But they differ a lot because it depends on a very subtle balance of density – that is temperature and salinity distribution in the ocean. We are not able to model this with any confidence right now.”
“We are hoping to somehow make some headway, but I have been in this area for more than 20 years now and we still don’t understand why the models differ so much in the sensitivity of the Amoc,” he said.
However, Rahmstorf said the international climate deal agreed in 2015 offers some hope if its ambition is increased and achieved: “If we can keep the temperature rise to well below 2C as agreed in the Paris agreement, I think we run a small risk of crossing this collapse tipping point.”