THE SPORER¨ AND MAUNDER MINIMA OF SOLAR ACTIVITY AND THE CLIMATE IN CENTRAL EUROPE MARTIN STANGL1, ULRICH FOELSCHE1,2 1Institute for Geophysics, Astrophysics, and Meteorology, University of Graz, Austria. E-mail: [email protected]. 2Institute of Physics (IGAM/IP), NAWI Graz, University of Graz and Wegener Center for Climate and Global Change (WEGC), Austria Abstract. In the project presented here, we focus on possible solar terrestrial rela- tionship within the so called Spoerer and Maunder Minima of low solar activity, but confronting astronomical and climatological data over a much wider range of two mil- lenia, i.e. between the years 1 and 2000 A.D. A database currently under construction, cross-checks historical data about climatic conditions with astronomical data (solar ac- tivity, aurorae, eclipses) for each year. While the solar physical data, of course, use sources from all over the globe, climatological data concentrate on Central Europe with a special emphasis on Austria Germany, Switzerland and Romania. Direct climatolog- ical clues from the provinces o Transylvania, Moldova and Wallachia will be presented for the first time in an exhaustiv and systematic manner. The sources we work with include verbal historical sources, namely from chronicles, as well as early temperature measurements. Sunspot numbers and solar activity reconstructions based on isotope measurements are compared with climate data in the database in progress. Key words: Solar activity – Sporer¨ and Maunder Minima – Climate in Central Europe. 1. STATUS OF RESEARCH The Sun’s activity is regulated, as we know today, by the solar dynamo which modulates the magnetic field of the Sun and thereby causes a cycle of approximately 11 years. Evidence for this is given by the occurrence of sunspots. Their appearance is the most obvious, recognizable manifestation of complex and not yet fully under- stood processes inside the Sun. Astronomers already believed in earlier times, that solar activity was marked during longer periods by even stronger fluctuations. Gustav Sporer¨ in Germany and Edward Maunder in England had already emphasized in the 19th century, that the second half of the 17th and the beginning of the 18th century were marked by a remarkable absence of larger sunspot groups, at least according to quite sporadic observations. This discovery later fell a bit into oblivion and was regarded quite sceptically until 1976, when John Eddy published an article in Science (Eddy, 1976). It contained the discoveries of Maunder, which were brought to light again and also made accessible to a larger scientific community in general: “I have re-examined the contemporary reports and new evidence which has come to light Romanian Astron. J. , Vol. 30, No. 1, p. 9–23, Bucharest, 2020 10 Martin STANGL, Ulrich FOELSCHE 2 since Maunder’s time and conclude that this 70-year period was indeed a time when the solar activity all but stopped.” Throughout 1672–1699 Sporer¨ found less than 50 sunspot observations, whereas nowadays within the same period one can see about thousandfold that number. Start- ing with 1715, sunspot activity seems to have increased. A spontaneous formation of sunspots which reoccurred on the Sun’s surface in a number we nowadays consider as being normal, has been observed by La Hire in France and Derham in England. Within one year the number of sunspots turned to “normal” again. The absence of reports about northern lights during that period, and also of any naked-eye sunspot observations in China, the apparent absence of the corona before 1715 and finally the connection with C-14 accumulation in tree rings and the concentration of Be-10 in polar ice were considered by Eddy as being clear evidence for his theory. Eddy asked himself if such nearly vanishing sunspot activity should not come along with serious consequences for the Earth’s climate. Based on this idea he postulated a connection between the period named by himself “Maunder Minimum” and the apparently cold- est stage of the so-called “Little Ice Age” between 1675 and 1715. From that moment on, in countless books and papers it was continuously stated that the Thames often froze in winter throughout the 17th and 18th century and that the Dutch master’s paintings of the time were dominated by icy winter-landscapes. Dendroclimatology is an established tool of paleoclimatology today. With the help of measurements of C-14 enrichments in tree rings, it has been shown that before the Maunder Minimum, which was determined by spot counts, further Minima ex- isted in different historical periods. The so-called “Sporer¨ Minimum” between 1460 and 1550, a “Wolf Minimum” 1280–1350, a “Medivial Maximum” 1100–1250, an “Oort Minimum” between 1040 and 1080 and an “Homeric Minimum” between 950 and 800 BC have been identified. Based on C-14 and Be-10 analysis in wood and sediments respectively, during the last years and decades many researchers have tried to retrace solar activity further than Eddy to 10,000 years and more (Usokin, 2008). The Sporer¨ Minimum seems to be reflected in fewer sunspots that could be seen with the naked eye during the Ming-Dynasty between the years 1400 and 1600. The Maunder Minimum forms a sharp contrast with the Modern Maximum, which began around the year 1920 and continued at least until the beginning of the 21st century. Such high activity derived from proxy data for the whole period of the Holocene was obviously unique. This seemingly improbable circumstance must give induce- ment to a certain scepticism towards the interpretation of the proxy data and the conjecture according to which we find ourselves in some really abnormal phase of high solar activity and also if this could be a reason for global warming needs detailed verification. The aspect that solar activity affects Earth’s climate is still controversial. It is still unclear, if the current 11-year fluctuation between high and low sunspot activ- 3 The Sporer¨ and Maunder minima of solar activity and the climate in Central Europe 11 ity is reflected in climatic data. Shaw (1965) investigated the average temperatures in England, New York and The Netherlands for the last 150 years. He came to the conclusion that there is no connection between the 11-year cycle and terrestrial cli- mate. When, however, the so-called solar constant fluctuates over centuries by more than 0.1%, in the same way as C-14 fluctuations suggested, the climatic consequence would be real. Hibler and Johnsen (1979) thought that they could prove that the Sun’s luminosity during the Minima in the 15th/16th and 17 th/18th centuries had been no lower than nowadays in the years of the sunspot minima. From that research they reached the conclusion that the impact on the Earth’s climate should have been minor and they criticized Eddy because he had used local climatic conditions only and had adjusted his hypothesis to them. Besides, they explained that the Little Ice Age, as climatologists know it, had lasted much longer (they give 1430–1850). This affirmation was a contradiction to Eddy’s statement that the solar activity between the Sporer¨ and Maunder Minimum had been a normal one. Eddy and his adherents relied mainly on the only geographical locations which were suitable for their assumptions. The winter temperatures in Tokyo were in fact suitable for their severe temperatures between 1489 and 1528 but only for the Sporer¨ Minimum and not for the Maunder Minimum (Herman and Goldberg, 1978). On the other hand, Eddy shows a persuasive curve of the winter temperatures from Paris and London in connection with his solar activity curve based on the C-14 data. A connection between the Little Ice Age and the Maunder Minimum has be- come so fixed in people’s minds that far-reaching conclusions have been made. US American researchers even claimed that the famous instruments of violin builder Stradivari, who had lived 1644–1737, just in the time of the Maunder Minimum, owed their unique sound to the slower growth of wood from that period, which was caused by reduced solar activity (Burckle and Grissino-Mayer, 2003). By examining Eddy’s alleged link between the Maunder Minimum and the Little Ice Age, based on historical sources from continental or Central Europe, we consider the exemplary studies of the historian Christian Pfister for Switzerland (Pfister, 1996). Weather de- scriptions in historical sources are not absolutely quantifiable, but on the other hand, this disadvantage is largely compensated by high data density. For Switzerland this is, according to Pfister: descriptions of anomalies from chronicles are available for the period 1496–1549; for the interval 1550–1658 there is almost always for every month at least one weather report, in the period 1684–1754 several mentions of tem- perature are available and by 1755 they are available daily for Western Switzerland and Basel, (since 1817 also on the Great St. Bernhard). According to climatologists, the Little Ice Age is marked by the following characteristics: the glaciers of the Alps extended much further than they do now, conspicuously numerous cool summers occurred (possibly as a consequence of ma- jor volcanic eruptions in the tropics) and the winters themselves (in addition to the 12 Martin STANGL, Ulrich FOELSCHE 4 late autumns and the early springs) were colder and drier than today. Lakes and rivers froze frequently and the winter snow lasted longer. But what Pfister clearly stressed is: “The Little Ice Age should not be understood as a unitary cold period. The summers were in some sections as hot as in the 20th century” (Pfister, 1996). At the advent of the 18th century winters became mild again. The end of the Little Ice Age is marked, according to Pfister, by the year 1895, when a sudden rise of temperature was observed.