Solar minima triggers climate event in pre-Roman Iron Age
A European team of researchers has discovered that a Grand Minima of solar activity can affect climate conditions. Scientists from the German Research Centre for Geosciences GFZ, in collaboration with Swedish and Dutch colleagues, have provided evidence for a direct solar-climate linkage on centennial timescales. What the scientists found was that an abrupt cooling in Europe, together with an increase in humidity and in particular wind, coincided with a sustained reduction in solar activity 2 800 years ago. The study was presented in the journal Nature Geoscience.
As the basis for their study, the researchers examined sediment from Lake Meerfelder Maar, a maar lake in Eifel, western Germany, to determine annual variations in climate proxies and solar activity, utilising the most modern methodological approach. A maar is a volcanic crater that is created when a groundwater comes into contact with hot lava or magma. The maar usually fills up with water to form a shallow crater lake. The Eifel region was a site of intense volcanic activity and it is worth noting that maars were first described in this region.
Their study revealed that the so-called Grand Minima of solar activity can affect climate conditions in western Europe through changes in regional atmospheric circulation pattern. These Grand Minima are periods of low solar activity. Around 2 800 years ago, around the time Homer was writing his epic poems of the Iliad and Odysseus, one of these Grand Solar Minima, the Homeric Minimum, caused a distinct climatic change in less than a decade in western Europe.
The maar lake provided exceptional seasonally laminated sediments, allowing for a precise dating even of short-term climate changes. As a consequence of this, the results indicated that Europe experienced a 200-year-long period of strongly increased springtime winds during a period of cool and wet climate. In combination with model studies, they suggested a mechanism that can explain the relation between a weak Sun and climate change.
'The change and strengthening of the tropospheric wind systems likely is related to stratospheric processes which in turn are affected by the ultraviolet radiation,' explained Achim Brauer of GFZ, the initiator of the study. 'This complex chain of processes thus acts as a positive feedback mechanism that could explain why assumingly too small variations in solar activity have caused regional climate changes.'
Achim Brauer stressed that these findings cannot be directly transferred to future projections because the current climate is additionally affected by human interference. However, they provided clear evidence for an aspect of the climate system which continues to be poorly understood.
Further research is required but scientists should focus on the climatic consequences of changes in different wavelengths of the solar spectrum. Only when the mechanisms of solar-climate linkages are better understood will it be possible to provide a reliable assessment of the potential impact of the next Grand solar minimum in a world of anthropogenic climate change, according to them.