The salt in the sea in the Caribbean affects the climate in Europe. German and Canadian scientists discovered this after research into ocean currents, which carried less salt north than normal during cold times.
It has long been known that during cold periods in the past, such as the Little Ice Age, the North Atlantic Current was less powerful and surface salinity increased in the Caribbean. This was accompanied by a disrupted salt supply to the north, which in turn led to longer cold periods in the Northern Hemisphere. Researchers from Canada’s Dalhousie University and the University of Bremen, among others, have now proven this. The study supports the hypothesis that salt transport by ocean currents is a crucial regulator of global climate.
The North Atlantic Current
The North Atlantic Current is a strong and warm ocean current that begins approximately in the Caribbean and flows towards the North Pole. It is the continuation of the warm Gulf Stream. Sometimes they are considered together as one whole. The current has an important warming effect.
Previously, researchers suggested that increasing sea ice and desalination of the North Atlantic Ocean south of the Arctic were possible triggers for colder periods. But processes in the tropical Atlantic region seem just as important. “In fact, unlike at higher latitudes, little is known about these recent climate events in the subtropical and tropical Atlantic and their impact on Northern Hemisphere regions,” says paleoclimatologist Henning Bauch.
The Little Ice Age
To find out more, the researchers studied what happened in the ocean during the Little Ice Age, which lasted from the 15th to the mid-19th century. It was then 1 to 2 degrees colder in Western Europe than now and the period led to poor harvests, famine and disease outbreaks. Although that time has been extensively studied, the climate mechanism underlying it remains controversial. While that is very interesting, because it can also tell something about the current climate crisis. “Research on recent, natural climate anomalies helps us understand the processes that may be driving human-induced global warming,” said lead researcher Anastasia Zhuravleva.
Colder water
So what happened in the tropical Atlantic Ocean during that remarkably cold period? To find out, the researchers studied sediments from the southern Caribbean and reconstructed the salinity and temperature of surface water over the last 1,700 years. They also looked at the isotopic composition of the calcareous shells of plankton.
The results indicate a cooling of 1 degree Celsius in the Little Ice Age. “That is a significant temperature change for this region,” explains researcher Mahyar Mohtadi. “Also notable is another distinct cool period in the 8th and 9th centuries. Colder temperatures in a normally warm tropical ocean have led to less precipitation in the region, coinciding with extreme drought in the Yucatan Peninsula and the decline of the Classical Maja culture.”
Warm Gulf Stream
But this cooling had even more consequences. For example, it was accompanied by a weaker ocean current and increased salinization in the Caribbean. “Advection, or the movement of tropical salt towards higher latitudes, is essential to maintain high surface density in the sub-polar North Atlantic Ocean. This is a prerequisite for the overall stability of the large-scale ocean circulation, including the warm Gulf Stream, which is responsible for the mild temperatures in Europe,” says Bauch.
Direct evidence
The cooling was probably initially caused by, for example, volcanic eruptions, low solar activity and an interaction between the sea ice and the ocean in the north. But the new study provides evidence that the decrease in salt transport to higher latitudes caused the cooling to worsen and last longer. Conversely, if more salt flows north from the tropics, the density of surface water near the North Pole will increase. This will cause more warm water to flow north, resulting in higher temperatures in Europe and North America. “This process is known from models and was assumed to apply to the Little Ice Age as well, but due to a lack of tropical ocean data, these assumptions were based on much less direct precipitation data,” Zhuravleva explains.
There is evidence that the Gulf Stream is weakening and that global warming is the cause. What is certain is that the consequences of this change are being felt worldwide. However, the extent to which the different climate mechanisms affect each other is not yet completely clear. But this study certainly confirms that the salt carried from the south to the north is an important factor in these processes.
