Changes in the sun’s energy output may have caused climate change in Europe over the past millennium, according to a study published Sunday in the journal Nature Geoscience.
The researchers, from Cardiff University in the U.K. and the University of Bern in Switzerland, found that changes in the sun’s activity can considerably impact ocean-atmospheric dynamics in the North Atlantic, with potential effects on the region’s climate.
Warm surface currents flowing across the North Atlantic and warm westerly winds cause Europe’s relatively mild climate, particularly in the winter. A slight change in how these ocean-atmospheric systems transport heat varies the region’s climate.
"We used seafloor sediments taken from south of Iceland to study changes in the warm surface ocean current,” Paola Moffa-Sanchez, the lead author from Cardiff, said in a statement. “This was done by analyzing the chemical composition of fossilized microorganisms that had once lived in the surface of the ocean.”
The measurements were used to reconstruct the ocean current’s temperature and salinity over the past 1,000 years. The team’s temperature determinations coincided with historic accounts of weather changes, including harsh winters in the 1500s and 1700s before global industrialization. Recorded wheat prices, high due to crop failure and causing famines, and many paintings, including the famous London Frost Fairs on the Thames River, point to the severity of the periods’ winters.
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Cold ocean conditions matched periods of low solar energy output and low sunspot activity observed on the surface of the sun. The researchers then used a physics-based model to test the ocean’s response to solar output.
“By using the climate model it was also possible to explore how the changes in solar output affected the surface circulation of the Atlantic Ocean," Ian Hall, a co-author of the study, said. "Analysis of the atmosphere component in the climate model revealed that during periods of solar minima there was a high-pressure system located west of the British Isles.”
When that system emerges it blocks the warm westerly winds, allowing cold Arctic air to blow south. This phenomenon, called atmospheric blocking, caused Europe’s harsher winters in 2010 and 2013.
Previous studies have linked solar variability to atmospheric wind blocking over the last 50 years, but the nature of the relationship is not yet clear.
"In this study we show that this relationship is also at play on longer time-scales and the large ocean changes, recorded in the microfossils, may have helped sustain this atmospheric pattern,” Moffa-Sanchez said.
The researchers predict low sun activity over the next few decades, and say any temperature changes associated with the sun’s output will be smaller than those created by carbon dioxide emissions. But they conclude the study saying that regional climate variability associated with natural causes should be taken into account when forecasting climate change.