AMOC, or the Atlantic Meridional Overturning Circulation, consists of surface and deep currents that help keep Northern Europe's climate habitable. Warm water moves from the equator along the coast of North America toward higher latitudes, cooling along the way and becoming saltier due to evaporation.

"Saltier and colder water is heavier. When it reaches the area between Iceland and Greenland, it sinks and helps drive the current. But as the climate warms, the water is also warmer and if glaciers melt at the same time, the water becomes fresher — meaning that at some point it will no longer sink," explained Siim Veski, a geology professor at TalTech and one of the study's co-authors, describing the process behind AMOC's slowdown.

Scientists took a closer look at a period known as the Younger Dryas at the end of the last ice age, around 12,000 years ago, when AMOC suddenly weakened. To their surprise, they found that while winter temperatures dropped by up to 10 degrees compared with today — and in some places even by 22 degrees — summer temperatures remained relatively stable, ranging between 12 and 16 degrees Celsius. Some warmth-loving plants, such as black alder and cattail, continued to grow in Estonia even during the harshest period.

According to Veski, the difference was driven by stronger summer sunlight, which helped keep temperatures stable. "Summer insolation (the amount of solar radiation reaching the surface — ed.) was much higher back then than it is today. In summer, the Northern Hemisphere is tilted toward the sun, which warms us more effectively. In winter, when we are tilted away from the sun, we are more influenced by AMOC's heat — without it, winter temperatures are clearly colder," Veski said.

He added that it has long been known that the climate cools when AMOC weakens. The question, however, was whether the drop affects the annual average temperature or specifically summer or winter temperatures. "A few decades ago, we were not able to reconstruct winter temperatures using microfossil data. Today we can — meaning we can determine seasonality. Using new models, we can say that winters cool more, while summers may not cool at all," Veski said.

To analyze the Younger Dryas climate, researchers also collected samples from lake sediments in Estonia. "We take samples, determine their age and look at which microfossils are present — for example pollen, plant macrofossils or aquatic organisms," Veski explained. These species and communities are then linked to the environmental conditions in which they can grow.

Previous studies often produced misleading results regarding summer temperatures because they did not account for a more continental climate. The new dynamic method used in this study allowed researchers to consider the entire region. In the absence of ocean currents, high-pressure systems spreading over land were able to block cooler air coming from the ocean. As a result, summers could remain warm despite harsher winters.

Veski noted that the Intergovernmental Panel on Climate Change (IPCC) has estimated that a collapse of AMOC is unlikely before 2100. However, he said some studies suggest it could happen significantly earlier — possibly in as little as 35 years. "The question now is whether, if overall climate warming is strong, this cooling effect would outweigh the general warming," Veski explained.

According to the geologist, a slowdown, and especially a shutdown, of AMOC could mean a return to a Younger Dryas-like climate in Northern Europe, with warm summers but very cold winters. At the same time, he cautioned that past changes cannot be directly applied to the present. "Today, humans are also involved, having intervened in climate processes," he said.

The scientists describe their findings in more detail in the journal Quaternary Science Reviews.

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