German researchers say they have found solid evidence that a past "solar minimum" period of prolonged low solar activity – of the sort which some hefty physicists believe will commence within a few years – significantly cooled the climate. The research flies counter to theories offered by climate scientists, who contend that a solar minimum this century would have little effect.
According to a statement announcing the research, issued by the German GeoForschungsZentrums (GFZ) at Potsdam:
An abrupt cooling in Europe together with an increase in humidity and particularly in windiness coincided with a sustained reduction in solar activity 2800 years ago. Scientists from the German Research Centre for Geosciences GFZ in collaboration with Swedish and Dutch colleagues provide evidence for a direct solar-climate linkage ...
Professor Doktor Achim Brauer of the GFZ and his colleagues measured the temperatures of the remote past by analysis lake sediments desposited in those times in Lake Meerfelder Maar in the Eifel region in Germany. These sediments "allow a precise dating even of short-term climate changes", according to Brauer and his colleagues. They show a definite 200-year-long cool period which corresponds with the onset of the "Homeric Minimum", a quiet spell of low solar activity which occurred at roughly the time the great poet of the ancient world was producing his epic tales of Troy and the Odyssey.
"The Homeric Minimum caused a distinct climatic change in less than a decade in Western Europe," assert the GFZ boffins.
Solar physicists from several major US sun-watching labs announced last year that they believe – based on decades-long trends of declining sunspot activity and other indicators – that the current sunspot cycle may be the last one for a long time, and that a "Maunder Minimum" like that seen in the 17th and 18th centuries is on the cards. The sunspot-less Maunder Minimum was accompanied by a so-called "Little Ice Age" of cold winters and ice-skating on the Thames.
However the idea that variations in the Sun have any serious effect on the climate is a controversial one. In particular the idea that the planet can expect a lengthy cool period until 2100 or beyond would tend to undermine the War On Carbon, and any suggestion that solar variability is as big a factor in climate as carbon emissions leads to intense hostility from many career climate scientists and large sections of the media (as we know well here at the Reg).
The solar-physics announcements had long been expected, and climate scientists had previously hastened to assure the world in blog posts, published papers and press releases that any 21st-century solar minimum "would hardly slow global warming", because "the effect of low solar activity on the climate is very small" (this according to Georg Feulner and Stefan Rahmstorf, among the climate scientists who write at the Real Climate blog).
Brauer and his colleagues beg to differ, however. They say that they have come up with a mechanism that can explain the relation between a weak sun and climate change. They write:
We conclude that changes in atmospheric circulation amplified the solar signal and caused abrupt climate change about 2,800 years ago, coincident with a grand solar minimum.
"The change and strengthening of the tropospheric wind systems likely is related to stratospheric processes which in turn are affected by the ultraviolet radiation," says Brauer. "This complex chain of processes thus acts as a positive feedback mechanism that could explain why [comparatively] small variations in solar activity have caused regional climate changes."
According to Brauer and Co, what's needed now is more research into the climatic consequences of changes in different wavelengths of the solar spectrum, which will give a better picture than we now have of what can be expected during a future solar minimum combined with human-caused pressures on the climate.
Their paper is published in Nature Geoscience. ®