The weather patterns in northern Europe and North America are significantly influenced by the behaviour of the northern polar jet stream. These powerful high-altitude winds, spanning several miles above the Earth’s surface, play a crucial role in directing weather systems. They can bring cold air from the Arctic to regions like the American Midwest or propel waves of Atlantic storms towards areas such as Ireland and Scandinavia.
Scientists suspect that climate change is impacting the behaviour of these jet streams. Over the past century and a half, data suggests that the northern jet stream has been strengthening. However, in terms of climate, a century is a relatively short period, and it remains uncertain whether this strengthening is a natural occurrence or a result of human-induced climate change.
A recent study published in Geology, conducted by Miaofa Li at Fujian Normal University and Slobodan Markovic at the University of Novi Sad in Serbia, sheds new light on this issue. While climate scientists often analyze ancient air trapped in polar ice to understand past climates, Li and Markovic propose a similar approach using the chemical composition of rock formations in Serbian caves.
In these caves, stalagmites, thin rock formations formed by water dripping from the ceiling, can provide valuable insights. Each droplet carries dissolved minerals, gradually building the stalagmite over time. By analyzing the minerals within the stalagmite, researchers can gather information about the water source.
Serbia’s specific location makes it a pertinent case for this study. The type of precipitation the country receives is closely tied to the strength of the northern polar jet stream. When it is robust, it tends to bring clouds from the Atlantic, while during weaker periods, clouds often come from the Mediterranean.
The water from these two sources has distinct chemical compositions, particularly in the oxygen isotope ratios. By studying these ratios in the stalagmite layers, the researchers aimed to determine when Serbia experienced more Atlantic-driven storms versus those from the Mediterranean, thereby providing insights into the behavior of the jet stream.
The team examined two stalagmites, one from Cerjanska and another from Prekonoska Cave, dating them using uranium and thorium traces. The stalagmite from Cerjanska formed between 434 BC and 1913, while the one from Prekonoska Cave formed between 798 BC and 404. They then analyzed 581 samples of calcium carbonate, an oxygen-containing mineral.
The study’s findings suggest that the North Atlantic jet stream is currently stronger than it has been in the past 2,500 years. Interestingly, the data also indicate that the jet stream was stronger during cooler periods and weaker during warmer ones, which contrasts with the trend observed today.
The reason for this reversal in the modern jet stream’s behavior remains unclear. The researchers speculate that the North Atlantic Warming Hole, a persistent area of cool water thought to be linked to Greenland’s ice melt and changes in oceanic currents, could be a contributing factor. More data from stalagmites in various caves may offer further insights into this intriguing phenomenon.