It can probably be traced back to the climate change we have caused.
Researchers come to this conclusion in the magazine Nature Geoscience. Their study deals with the Atlantic Meridional Overturning Circulation – AMOC for short. This is a system of ocean currents to which the well-known Warm Gulf Stream also belongs. These currents carry warm water from the equator to the northern part of the Atlantic Ocean. There, the water releases that heat into the atmosphere and thus ensures that Western Europe is heated. Due to the release of that heat, you are left with cold water. It descends to greater depth and then travels all the way to Antarctica, to end up in the tropics via the Gulf of Mexico, where the story starts all over again. The system thus transports nearly 20 million cubic meters of water per second and has a significant influence on the Western European climate, among other things.
1000 years
Earlier, however, researchers showed that the mighty system of ocean currents has not been so stable anymore, at least in recent decades. For example, the water that the AMOC pumps around is said to have flowed about 15 percent slower since the 1950s. However, it was unclear whether this weakening was very remarkable or not.
The new research leaves no doubt: the weakening is special. In the past 1000 years, the AMOC has never been as weak as it is now. The researchers draw this conclusion on the basis of so-called ‘proxy data’ (see box).
Proxy data
When direct measurements are missing, researchers can replace them with so-called proxy data. For example, tens of thousands of years ago there were no weather stations that took measurements every hour. Fortunately, researchers can use other sources to get a picture of the temperatures or precipitation in earlier times. For example ice cores or growth rings of trees. The ice cores not only reveal how much snow fell in a certain period, but often also harbor air bubbles from which one can even deduce the composition of the atmosphere. And the annual rings tell not only how old a tree is, but also whether it has grown from year to year under favorable conditions or not. For example, a tree that thrives in warm temperatures will have wider annual rings in a year when it is nice and warm, but narrow annual rings in a cold year. In a similar way, trees can also provide more insight into how much precipitation fell a long time ago. Since continuous measurements have only been carried out in the AMOC since 2004, there are also no direct measurements that can tell us what the currents looked like 500 or 1000 years ago. To get a picture of the AMOC in earlier times, researchers therefore looked at alternative information sources, or proxy data. For example, they looked at the grain size of deep-sea sediments: the larger the sand grains deposited, the stronger the currents. By studying multiple sediment layers, researchers can therefore say more about how powerful the currents have been over time. In addition, the researchers also looked at other proxy data, such as the temperatures in the Atlantic Ocean.
Although the aforementioned proxy data alone cannot provide such a reliable picture of the AMOC in earlier times, they can together, according to the scientists. “The research results suggest that the AMOC was fairly stable until the end of the nineteenth century,” says researcher Stefan Rahmstorf. “Towards the end of the Little Ice Age, around 1850, the ocean currents began to weaken and a second, much more drastic weakening followed in the mid-twentieth century.”
Previous studies
The research results do not come as a complete surprise. Scientists have been predicting for some time that the AMOC will weaken due to climate change. And in a special report, the IPCC already wrote in 2019 that there are indications that the AMOC is already weaker than it was in the period between 1850 and 1900. “This new study provides more evidence for this and places the decrease in a broader context,” said Rahmstorf. Moreover, the findings are in line with previous studies that also showed that the current weakening of ocean currents is also historically quite remarkable.
AMOC in klimaatverandering
How could global warming result in a weaker AMOC? For that we have to look again at how the AMOC works. At the surface, warm water is brought north from the equator. That water cools down on the way. In addition, more water evaporates along the way than fresh water is added, which increases the salt concentration. The further north you come, the colder and saltier the water is. But the water also becomes heavier due to the salt and the cooling. This causes it to descend in the north, after which it is pumped back to the equator. There it heats up again and the story starts again. However, due to global warming, we are seeing major changes in the Arctic, especially Greenland. The thick ice cap that rests on the land here is melting and this releases a large amount of (sweet!) Melt water. The fresh water mixes with the salt water, making it less salty and therefore less heavy. This water sinks less easily to the depth and therefore weakens the AMOC.
Many researchers are now convinced that global warming is behind the weakening of the AMOC. The weakening has also been linked to a substantial cooling of the North Atlantic. This so-called ‘cold blob’ has already been predicted by climate models and is said to be the result of a weakened AMOC that pushes less heat to this area.
Future
What the future of the AMOC looks like is unclear. In any case, according to Rahmstorf, it is closely related to our own actions. “If the warming continues, the Warm Gulf Stream will weaken further.” Climate models predict that in that scenario it can flow up to 45 percent slower by the year 2100. It brings us dangerously close to the point where the AMOC becomes unstable and possibly even stops, according to the researchers. This doomsday scenario – which would have profound consequences for large parts of the world – is still a long way off, the researchers emphasize. But if we don’t act, it will come closer and closer.
It is clear that a further weakening of the AMOC can have major consequences. But exactly what those consequences look like is less clear. Some studies suggest it would result in more extreme winter storms for our continent. Others keep the weather more violent heat waves or less rainfall in the summer. Follow-up research should provide more clarity about this. In the future, researchers also hope to clarify which components of these extensive ocean currents have changed and which processes underlie them. Hopefully, it will also be possible to predict more accurately what the AMOC will do in the future.
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