THE SPORER¨ AND MAUNDER MINIMA OF SOLAR ACTIVITY AND THE CLIMATE IN CENTRAL

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 , 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 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 “” 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 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 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. The fact that the Little Ice Age as a phenomenon cannot be clearly detected in summer temperature curves, is remarkable. Between 1530 and 1895 the summers were not colder than they were in the period of 1901–1960; also the autumns cannot represent the Little Ice Age. Let us now turn to the overall picture, which results from Pfister’s researches. First we consider the winter, because this time of year has usually served as an ar- gument for Eddy’s theory (as it is the only one of the four seasons, which makes it credible at all). During the Little Ice Age there was an average of 64 days with snow coverage, but in the next 90 years (1898–1987) only of 46 days. In the past half- millennium, cold anomalies clearly dominated, especially in the period from 1566 to 1895. If counting by calendar years we get the following distribution: In some decades only 2 or 4 temperature anomalies occurred (1546–55, 1656–65), in others up to 27 (1676–85; 11 warm and 16 cold). In some decades there were only negative anomalies (e.g.1506–15), in others almost only positive ones (1526–35, 1986–95). In any case, the oversimplification of representation in the popular scientific litera- ture, according to which the Maunder Minimum is identical with the Little Ice Age, is erroneous. While the simple correlation between reduced solar constant during a minimum period, as Eddy had originally assumed, today is rarely mentioned based on new ideas and insights, a cooling effect is still considered, but in an indirect way, which is explained by an altered flow of cosmic rays in the atmosphere. The opinion is widespread that flux increases when the solar magnetic field decreases and that it is recorded in climate data, so that Eddy’s postulate ”inactive Sun = cold climate” is maintained. At least for the time of the Maunder Minimum such an effect is believed as being confirmed (Yamaguchi et al., 2010). Other authors assumed more or less locally limited climatic effects. Steinhilber et al.(2012) investigated potential impacts on the climate of Asia and deduced that climate impacts are evident for some minimum periods over the past 10 millennia, but not for other ones, a fact they attribute to increased volcanic activity. The works of Lockwood and his collaborators (Lockwood et al., 2010) have even proposed that the ordinary 11-year cycle of the Sun brings significant changes in the winter climate of the British Isles . While the total irradiance of the Sun varies in an interval of 11 years only by 0.1% and thus can hardly explain climatic differences, Lockwood and others tried to establish a connection via complex mechanisms, i.e. the 5 The Sporer¨ and Maunder minima of solar activity and the climate in Central Europe 13 impact of reduced solar irradiation on tropospheric jet streams. Lockwood assumes that a change of the North Atlantic Oscillation caused by solar activity influences the winter climate in Europe. As a conclusion, based on his collected material, he states: ”...the coldest UK winters tend to have occurred at low solar activity.” (Lockwood et al., 2010)

2. AIMS OF THE PROJECT

The ”Austrian Assessment Report on Climate Change 2014” stated in Volume 1, Chapter 3 ”Past climate change in Austria”, p.237: “Until now, for Austria, com- pared to neighbouring countries (Switzerland, The ), there have been few comparative studies with this paleoclimatological approach (e.g.Rohr, 2006a,b; Maurer et al., 2009), although the point of departure, meaning the richness of histor- ical sources, was similar” We plan to fill this gap, and with help of our cooperation partners Ioan Albu and Joachim Wittstock from the University of Sibiu, Romania, we identified another unexploited, rich field of historical climate information in the region of Transylvania, formerly politically connected to Austria. Preparatory work revealed promising sources (see also examples in the bibliography), waiting to get explored in detail. Main aims of the project are: • Approaching the “Little Ice Age” issue through the means of the latest research in astrophysics (modern reconstructions of solar activity in earlier centuries). There is a terminological confusion concerning the term “Maunder Minimum” in the field of historical climate research, as it does not indicate the period of low solar activity itself, like in astrophysics, but a roughly simultaneous cold phase of terrestrial climate. This is problematic because it presupposes a causal relationship, which should first be proven, and because there is no exact times- pan equivalence. In our project we confront indicators of solar activity (based on cosmogenic isotopes, sunspot numbers and auroral frequencies) with cli- matological data (measurements, historical descriptions, proxy data) and large volcanic eruptions as possible causes for climate cooling. • Systematic comparison of the information collected by the partner projects TAMB- ORA (Freiburg/Breisgau; Rudiger¨ Glaser) and Euro-Climhist (Bern; Christian Pfister, Christian Rohr) from Germany and Switzerland and their evaluation in terms of the Maunder Minimum. • Source analysis of historical climate data from Austria and Romania. This ge- ographical area has not been taken into consideration in the historical climato- logical discourse on the Maunder Minimum yet. 14 Martin STANGL, Ulrich FOELSCHE 6

• Direct comparison of climate data from Central Europe with different solar ac- tivity indicators (relative sunspot numbers, naked-eye sunspot observations, au- rora sightings, isotope studies, as well as registered geomagnetic storms and remarkably strong eruptions in the photosphere, chromosphere and corona for more recent times) on an annual basis.

• Processing and analysis of daily (!) observations for the whole period of the Maunder Minimum from Paris, and – with some lacunae – from Zurich, and Berlin, which have been assessed only partly by earlier research. The most impressive series of daily weather observations has been preserved from Paris, where Louis Morin, personal physician of King Louis XIV of France, made entries three times a day for the period between 1665 and 1715 (!). His obser- vations include the frequency and type of precipitation, the wind direction and wind force, the origin of the clouds, temperature und air pressure. The Morin observation series will be provided by the cooperation partner Euro-Climhist (Pfister, Rohr), as well as the Zurich data (observation series for days with pre- cipitation). Additional data from Switzerland (monastery of Einsiedeln, 1670– 1705) are currently under evaluation within an ongoing research project at the University of Bern.

• Search for historical records mentioning the presence or absence of the solar corona during total solar eclipses prior to the year 1715.

A major goal of our project is to collect data material, similar to the ones gath- ered by Pfister for Switzerland and Glaser for Germany, for the Eastern Alps of Aus- tria and the Carpathian region of the former Principality of Transylvania, to examine and to interpret it with reference to the periods of the Sporer¨ and Maunder Minimum, respectively. Furthermore, we want to research whether specific tendencies during the Sporer¨ and Maunder Minimum result from the data put together by Pfister and Glaser. An interpretation-compatible basis will be created in form of a database that compares the historical climatic information in the German-speaking space to one another, and contrasts it with astronomical data (sunspots, borealis) for the period in question. As a first step, we want to verify the existence of the Maunder Minimum and re-evaluate other assumed solar activity minima and maxima and then analyse if a relation with the Little Ice Age is noticeable, which started long before the postulated solar activity minimum and which outlasted it by two centuries. We also want to investigate Eddy’s probably flawed assumption that solar activity came to such a standstill, that the solar corona disappeared completely. Not only after Pfister’s researches we must conclude that for the Little Ice Age (which, as mentioned before, is not identic to the restricted timespan, which John 7 The Sporer¨ and Maunder minima of solar activity and the climate in Central Europe 15

Eddy adopted in order to support his theory) between 1560 and 1895 the temperature deviations clearly prevail, but with 70% cold and 30% warm anomalies, the whole period was by no means uniformly too cold. It is important to verify, in how far the connection between the Maunder Minimum and the Little Ice Age can be made credible with the help of proxy data and historical material. There are only few in- terdisciplinary experts, thus the climatologist has to rely on the astronomer and the astronomer on the historian, and so on, and the mistakes potentially increase because of uncritical acceptance of half-truths and rather unproven theories. A way to break the cliche´ of the idea of constant extreme negative temperatures in the mentioned period, a turn from the persistent reuse of the same, regional point-like facts, like the funfairs on the frozen Thames, instead of searching for similar evidence in other geographic regions, is also part of the task we want to pursue. The geographic fo- cus on South England on which the first climatic studies for the Maunder Minimum focused, is undoubtedly conditioned by the measurement series from 1659, which is known as the CET (Central England Temperature) series (Manley, 1974) and which until now represents the most homogenous source of that time. We will now use completely new, more detailed measured data for three different regions in Central Europe provided by the cooperation partners from the University of Bern. The ongoing discussions regarding the significance of the weak current activity cycle no. 24 and the possibility of a relapse in a calm period similar to the Maun- der Minimum, in correlation with Lockwood’s forecast that this would lead to colder winters at least in the European space, render new historical investigations regard- ing the latest large minima (Sporer¨ and Maunder Minima) and their climatic effects urgently necessary. The determination of the question whether these periods could, as well in Central Europe as in the Alpine and Carpathian space, be distinguished through exceptionally cold winters, should either confirm or challenge Lockwood’s spectacular theory (“we stress that this phenomenon is largely restricted to Europe and not global in extent”; Lockwood et al., 2010). Despite the local limitations, the effects should be subject to being determined at least in Central Europe, too. Lock- wood establishes for the British area an interesting accumulation in the incidence of Eastern winds during the Maunder Minimum (see also Slonosky, 2001). The ultimate aim is an interdisciplinary synopsis of long-term fluctuations in solar activity and their possible implications on terrestrial climate. This overall view should form, under a critical consideration, an image that is equally fair to the status of individual scientific disciplines. The discourses of three different disciplines (his- tory, astrophysics, climate research) are put in relation and confronted to one another within the framework of this project. The eminent importance to project a clear, high-definition image of the weather behaviour of Europe during the periods of the Sporer¨ and Maunder Minima, is undis- puted, and any further contribution to this target takes us a step closer to understand- 16 Martin STANGL, Ulrich FOELSCHE 8 ing the effects of solar activity fluctuations on terrestrial weather. Were the 1430’s years effectively the coldest decade in Europe in the entire previous millennium, as it is assumed nowadays in historical climate research? And is there perhaps any causal connection between this fact and the Sporer¨ Minimum, which was at this stage perhaps looming or even already setting in very abruptly? We think that the precon- ceived opinions presented here bring together things that are incompatible, because the Sporer¨ Minimum began, according to today’s knowledge, only around 1460, while, as already mentioned, the coldest periods had previously occurred, which must seem a purely coincidental time proximity. Similarly, the assumed complex- ity of the relationship between solar and climatic fluctuations (cf. Lockwood et al. 2010) makes it particularly complicated to prove or to disprove them. The complex- ity brings also about the fact that even the weather phenomena that had not been regarded within the context of this correlation until then, like regional windstorms, atmospheric perturbations and floods, are of interest and should be included within our project.

3. SOURCES AND METHODOLOGY

Concerning the climate history of the latest centuries there is, especially from the recent past, a large number of studies based on proxy data, however few historical descriptive sources have been consulted. These sources suffer in fact from the prob- lem of deficient quantifiability, but they have the advantage over proxy data to avail from direct, immediate information and the sources of error emerging in the case of proxy data do not occur here. Descriptive representations of climatic conditions of the elapsed entire, respectively half millennium were performed in an exemplary manner by Glaser for Germany and Pfister for Switzerland. We intend to interpret the data material gathered by them in order to establish any possible causal connections with the Maunder Minimum. This rich informative material has to be examined in detail, considering that mainly weather data from North-Western Europe, Northern America and Asia have been consulted by the adepts of the connection between the Maunder Minimum and the Little Ice Age. The purpose of our work is to investigate the situation of continental Europe focusing on Central and Eastern Europe. In order to integrate the material assembled by Pfister and Glaser, we aim to gather data from the area of the former Danube Monarchy, namely from the Austrian Alpine countries and from the Carpathian region belonging to the former Principality of Transylvania. Thereby several more important pieces can be added to the mosaic in order to be able to determine in the future the situation throughout the entire Europe in higher density and higher spatial and temporal resolution in a way as comprehensive as possible. For the time being, an enlargement of the gathered material to the Carpathian Arch 9 The Sporer¨ and Maunder minima of solar activity and the climate in Central Europe 17 means an important extension in relation to the previous state of knowledge. The area of Eastern Europe has been step-motherly treated until now and therefore insuf- ficiently investigated. Many things slumber deeply hidden within the less accessible old literature, or they can be found in various archives, in hand-written documents that have never been printed. The most important cities in Transylvania which have to be subject to exhaustive research are: Sibiu, Brasov, Cluj-Napoca and Sighis¸oara. Historical sources such as chronicles, letters, descriptions of battles, etc. are very valuable and necessary in order to complete and, above all, to calibrate the proxy data. The importance of the historical approach is underlined by the fact that some exceptional years like the extremely hot summer of 1540 do not show up well in proxy data (Wetter, 2014). The field of art history should be investigated as well. As a piece of evidence for a relation between reduced solar activity and colder weather circumstances in the 17th century, Eddy has quoted the fact that on the canvases of the Dutch masters of that time snow and ice-abundant landscapes prevail among winter motifs. To what extent does such a painting depict representative views of the time, and to what extent does the selective perception and choice play any role in this? Pictorial representa- tions should be searched for in the Austrian and Transylvanian area also in order to perform a comparison to the representation from the Netherlands. A database will be created which is supposed to put in contrast and compare the qualitative historical descriptions from the various geographical areas to one another (namely Switzerland after Pfister and Germany after Glaser, as well as the data which are to be collected from Austria and Transylvania). A “plausibility index” and an annotation regarding the “signal strength” (relevance) should make the data easier to compare. Such a data comparison is of high interest for depicting and revising regional fluctuations. Pfister’s investigations, for instance, have revealed several such cases for the Swiss Alps and we intend to extend these investigations towards Eastern Europe and to make regional differences recognizable. In order to perform a comparison with the periods of ”undisturbed” solar activ- ity, this database will include information starting from the year 1 until 2000, namely interpolated sunspot numbers by means of isotopic analyses and information about actually observed sunspot numbers including also naked-eye observations, as well as auroral phenomena. The main focus and data density however must be restricted to the period of the Sporer¨ and Maunder Minima. A crucial point has been completely neglected in the research of the last years and decades, namely the question if during the Maunder Minimum and perhaps also before it, the solar corona has not been visible at all or at least not in today’s form, as Eddy has stated in his theory of the Maunder Minimum. At this point, the descrip- tions of solar eclipses from the relevant time and even before it should be taken into account. Original first hand text paragraphs must be searched for and consulted. We 18 Martin STANGL, Ulrich FOELSCHE 10 suspect that in the past information was mainly taken from the secondary literature based on Eddy’s work without any further examination. A decision on this issue would be of vital importance, because according to current knowledge, there is not a single (!) certain description of the solar corona from any (!) eclipse before the year 1715. The main difficulty in the whole complex of problems that we aim to inves- tigate lies in the lack of multidisciplinary collaboration between astronomers, cli- matologists and historians. One discipline depends on the other only too often and adopts its theories uncritically as a basis for further conclusions and research. His- torical climate research is nowadays supplied with rich data (, sed- iment analyses, wind data from ship’s logs, etc.) and yet, the collaboration with astronomers and historians has been rather sporadic and superficial. An impermissible simplification of the complex problems has spread in pop- ular literature and unfortunately spans continually into the field of science too. The connection between solar activity and climate is often considered as a proven fact, but it is not. Also several interests, not least of economic policy origin, are con- nected to the problem, in order to blame increased solar activity since about 1920 on a possibly anthropogenic induced climate change. A better classification of the solar activity as a parameter of historical climatology must represent a priority in the collaboration between the two scientific disciplines, namely astrophysics and cli- matology. In any case, the simplification “Maunder Minimum = Little Ice Age” is inadmissible, because the latter started long before the Maunder Minimum and ended a long time afterwards. The image that prevails in the opinion of the general public and in the science world, too, is characterized by the Central England Temperature (CET) series. If all the conclusions are based on it, there is a risk that random local circumstances lead to misinterpretations. Descriptive details about the climate of past times include basic information, which is worth being standardized to such an extent that the included information of a standardization must be based on the method of source criticism, in order to be comparable with each other. The objectivity and quantification of data represent a crucial point within the selection of text sources. This aim of comparability can be fulfilled only partially, but one shall try to assign indices to the created database, which allow a certain scale of calibration, so that in the end, a comparison with proxy data can be made. The scope is the mutual verification of data that have been recovered from proxy data and from historical sources. When interpreting historical-descriptive sources, there arise several problems which need to be taken into consideration. For example, stereotypical statements that are often characteristic of their times and used as cliches,´ are particularly strik- ing. The information according to which a concerning event would be out of the ordinary, should also be treated with care, as it is known that human memory can 11 The Sporer¨ and Maunder minima of solar activity and the climate in Central Europe 19 be deceptive. Obvious exaggerations, reflected in unrealistic overstatements of the event, must be eliminated as such in the creation of databases; for example, when referring to earthquake descriptions from the late Middle Ages in the Austrian region there can be found depictions of effects and damages, which are clearly dispropor- tionate. In order to evaluate the data material from a critical point of view, there is a need for more tested calibration procedures, such as those used for about half a century in the field of historical climatology (Glaser, 2008). In addition to the purely descriptive information regarding the weather, there are also indirect data that can be collected in order to attain further evidence con- cerning the climate. This includes written records of crop incomes, especially those about grain and the quality of the wine. Additional information about the social consequences (price increases, famines, diminishing of trade) can be extracted from historical material. Time and again, weather disasters were reflected in the calls for more piety and penance, as well as in superstition, which lead to accusations, dis- crimination and persecutions. Similarly, one can, to some degree, infer about the weather conditions based on the occurrence of plagues (e.g. locusts) and diseases. Eligible sources range from handwritten documents and records (in calendars) to chronicles, the so-called cosmographies and second-hand compilations, for which the underlying original sources are often no longer available today. In any case, one cannot always rely on printed sources, but must also consult unedited material from archives. Related to crop incomes, there are, among others, issued accounts, proto- cols and other files, having the advantage of being quantifiable data. Unfortunately, in chronicles and annals, only sporadic and unsystematic information can be found on the topic of weather, but for our present purpose these references have the ad- vantage that the extreme weather conditions (for example, abnormally warm or cold summers, respectively winters) are in most cases precisely described, being valuable for our considerations on the possible impact of the Sporer¨ or Maunder Minimum. Weather journals kept mainly in monasteries can be found more frequently starting with the 16th century and are of particular importance. Furthermore, travel diaries are essential as they usually contain comments on the weather. Especially for the Transylvanian space, such accounts do constitute a welcome contribution. Even his- torical pictorial source materials can provide certain information, if their origin is known and if they are available in sufficient numbers so that statistical methods can be applied. It must always be sought to quantify descriptive sources. Contrary to proxy data, which, due to their high spatial and temporal resolution, have become the main focus of climate research in the last years, the collection of historical descriptive data is a hard patchwork. The achievement of homogeneous, continuous and quantifiable data cannot always be ensured. However, we have to be content with the situation that the historical descriptive method is the only one that gives us immediately pro- 20 Martin STANGL, Ulrich FOELSCHE 12 vided information about the actual weather in a particular region, while proxy data always represent indirect indicators only. To increase the spatial resolution, regional considerations such as Pfister’s (1996) or Glaser’s (2008) are very worthy and should be extended by our project towards Austria and Transylvania. The first step of the further processing of the historical descriptive raw data is a critical examination of the sources. All available biographical notes about the author and the purpose of its publication should be collected. Concerning the latter the determination of the source type is the crucial starting point. Furthermore, the text is classified in its contemporary historical context and any appearing stereotypes and exaggerations typical for the time in question must be considered. Superstitions and fixed ideas, which have generated a lot of commentaries, have to be filtered. Uncommon, imprecise or circumscribed time data require being recognized and re- constructed. Data corrections incurred at least until the late 16 th century, when there started a gradual switch from the previous Julian calendar to the new Gregorian one, beginning with the year 1582, but this varied among regions, with sometimes serious delays. Naturally we must also take into consideration if the writer of the concerning lines describes a weather phenomenon first-hand or relies on informants. The latter is to be considered as problematic in each case, as they come close to the well-known children’s game “Chinese whispers”. The definitive transfer of the spatially and temporally sorted material in a data- base, which can be later drawn to interpret the impact of solar activity on the climate of the Alps and the Carpathians, is made via index values, which involve text seman- tics and other discussed factors as well as objective parameters (such as inherent ef- fects on crop yields, damage to crops, effects on human physique and psyche, known relationships between wine parameters and climate, temperature development and bloom, moisture and vegetation, between water temperature, flow rate and freezing processes) to get to reality as close as possible. The distillation of quantified data is carried out by comparison with proxy data and finally parallelization and calibration with similar conditions from recent times, where absolute values can be estimated with the help of instruments measurements. In evaluating proxy data, filtering out the non-climatic component plays a central role and forms a permanent source of er- ror, which can be avoided through the use of the historical method. This immediacy of the data is the strength of the historical approach compared to the absolute one given by natural science, which seeks quantifiable absolute values. Especially for the investigated period in our project, historical data are essential, as direct objective climate-specific values are almost never available in the targeted regions. 13 The Sporer¨ and Maunder minima of solar activity and the climate in Central Europe 21

4. DISCUSSIONS • The project itself already includes 3 different disciplines: astrophysics, clima- tology, history. • In addition, it sheds new light on the relationship between Sun and climate on almost all scientific fields, including economy and technology. • A better understanding of the causes of global warming (as a result of cosmic factors versus terrestrial-natural and human-induced ones) is of paramount im- portance for the future. Long-term abnormal solar activity, as it is considered to be at lowest during the Maunder Minimum and at its maximum at the end of the 20th century, has in each case far-reaching repercussion over an unforeseeable number of scientific fields. At the moment it is being discussed whether the so-called Modern Maximum has al- ready come to an end and could possibly pass into a new Maunder Minimum (Miya- hara et al., 2010). This could have far-reaching repercussion, as via influence on Earth’s climate, which we analyse in our project, various human sciences like econ- omy, technology and various branches of nature study would be affected. In the course of centuries, sooner or later, seen on the long run, the Sun will return again to the same state it had during the second half of the 17th century. The consequences cannot be determined at that moment, as mankind did not interfere as extensively with the environment as today and in consequence nowadays we cannot know either, taking also into consideration the changes made to the life conditions, what it would mean if the climate adjusted to the same natural state specific to former times. And we still do not know for certain to what extent the climate was cooler or regionally different then. Above all, we are trying to determine here, whether the cooler phase of the climate was triggered by the state of the Sun at all or at least favoured by it. C-14 measurements suggest that the Sun in the 13th century was almost as active as nowadays, and a concurrence of warm phases of the Earth’s climate in the Mid- dle Ages gave way to speculations that the warm climate then was generated by an intense solar activity. It can be concluded that the relationship between variations in solar activity and those of Earth’s climate, notwithstanding any other opinions, is not clear in any way. The earlier assumption that cooler climate on Earth during activity minima of the Sun was directly caused by lower total , is now classified as being rather unrealistic due to the small irradiance fluctuations during the 11-year , though irradiance fluctuations on longer timescales are less well known (Froehlich, 2006). Today, however, indirect mechanisms are favoured, such as ion- ization effects in the atmosphere through more intense cosmic rays in phases with 22 Martin STANGL, Ulrich FOELSCHE 14 a weak solar magnetic field, yielding more condensation nuclei. It is therefore gen- erally assumed that the cooling of local or global climate is the result of increased cloudiness and not of reduced sunlight. New insights into the relationship between changes in solar activity and the Earth’s climate, especially at a time when anthropogenic influences were still small, will help to quantify the solar contribution to global climate change, which may have non-negligible in the first half of the 20th century. Current knowledge is too patchy due to the lack of temporal and spatial resolution and our project would like to add new pieces to it from the Eastern Alps and Carpathian regions, in order to work towards the future goal of organizing the global research collective, so that reliable statements as regarding this matter could be met on a global level. We would like to close with a quote from Usoskin: “Therefore, more precise knowledge of past solar activity, especially since it is accompanied by continuous efforts of the paleo-climatic community on improving climatic data sets, is crucial for improved understanding of the natural (including solar) variability of the terrestrial environment.” (Usoskin, 2008) The climate history’s model for the climatic evolution of the present and the future makes research along this line undoubtedly a very important and highly topical subject.

Acknowledgements. We thank Arnold Hanslmeier for his intensive participation in the planning and execution of the present project. This paper is part of the project “Climate History of Central Europe during the Little Ice Age”, funded by the Austrian science fund (FWF), project number P 31088.

REFERENCES

Austrian Assessment Report on Climate Change 2014, http://hw.oeaw.ac.at/?arp=0x0031449b. Burckle, L. and Grissino-Mayer, H.D.: 2003, Stradivari, violins, tree rings, and the Maunder Mini- mum: a hypothesis. Dendrochronologia 21(1), 41-45. Eddy, J. A.: 1983, The Maunder Minimum - A reappraisal, Solar Phys. 89 , 195-207. Eddy, J. A.: 1976, The Maunder Minimum. Science 192, 1189-1202. Fritz, H.: 1873, Verzeichniß beobachteter Polarlichter. Wien. Froehlich, C.: 2006, Solar Irradiance Variability since 1978. Space Sci. Rev. 125 , 53-65. Glaser, R.: 2008, Klimageschichte Mitteleuropas. 1200 Jahre Wetter, Klima, Katastrophen, Darmstadt. Herman, J.R. and Goldberg, R.A.: 1978, Sun, weather, and climate (Vol. 426). Scientific and Technical Information Office, NASA. Hibler, W.D. and Johnsen, S.J.: 1979, The 20-yr cycle in Greenland ice core records. Nature, 280(5722), 481-483. Hoyt, D. V., Schatten, K .H.: 1996, How Well Was the Sun Observed during the Maunder Minimum? Solar Physics 165, 181-192. Hoyt, D. V., Schatten, K. H.: 1998, Group Sunspot Numbers: A New Solar Activity Reconstruction, Solar Phys. 179, 189-219. Lockwood, M., Harrison, R. G., Woollings, T., Solanki, S. K.: 2010, Are cold winters in Europe 15 The Sporer¨ and Maunder minima of solar activity and the climate in Central Europe 23

associated with low solar activity? Env. Res. Lett. 5, 24001. Manley, G.: 1974, Central England Temperatures: monthly means 1659 to 1973. Q. J. R. Meteorol. Soc. 100, 389-405. Miyahara, H., Kitazawa,K., Nagaya, K., Yokoyama, Y., Matsuzaki, H., Masuda, K., Nakamura, T., Muraki, Y.: 2010, Is the Sun Heading for Another Maunder Minimum? Precursors of the Grand Solar Minima. Journal of Cosmology 8, 1970-1982. Mossman, J. E.: 1989, A comprehensive search for sunspots without the aid of a telescope, 1981–1982, Quart. J. R. Astron. Soc. 30, 59-64. Pfister, C.: 1996, Wetternachhersage. 500 Jahre Klimavariationen und Naturkatastrophen (1496– 1995), Bern. Rethly, A., Berkes, Z.: 1963, Nordlichtbeobachtungen in Ungarn. Budapest. Ribes, J.C., Nesme-Ribes, E.: 1993, The solar sunspot cycle in the Maunder minimum AD 1645 to AD 1715. Astron. Astrophys. 276, 549-563. Schroeder, W.: 1992, On the existence of the 11-year cycle in solar and auroral activity before and after the so-called Maunder minimum, J. Geomag. Geoelectr. 44, 119-128. Shaw, D.: 1965, Sunspots and Temperatures. J. Geophys. Res., 70, 4997. Siscoe, G.L.: 1980, Evidence in the auroral record for secular solar variability, Rev. Geophys. Space Phys. 18, 647-658. Solanki, S. K., Usoskin, I. G., Kromer, B., Schussler,¨ M., Beer, J.: 2004, Unusual activity of the Sun during recent decades compared to the previous 11,000 years. Nature 431, 1084-1087. Steinhilber, F., Abreu, J. A., Beer, J., Brunner, I., Christl, M., Fischer, H., Heikkila, U., Kubik, W., Mann, M., McCracken, K. G., Miller, H., Miyahara, H., Oerter, H., Wilhelms, F.: 2012, 9,400 years of cosmic radiation and solar activity from ice cores and tree rings. PNAS 109 (16), 5967-5971. Usoskin, I. G.: 2008, A History of Solar Activity over Millennia. Living Reviews in Solar Physics, 5, 3. Usoskin, I. G., Kovaltsov, G.A.: 2004, Long-Term Solar Activity: Direct and Indirect Study, Solar Phys. 224, 37-47. Usoskin, I. G., Mursula, K., Kovaltsov, G.A.: 2000, Cyclic behaviour of sunspot activity during the Maunder minimum, Astron. Astrophys. 354, L33-L36. Usoskin, I.G., Solanki, S.K., Kovaltsov, G.A.: 2007, Grand minima and maxima of solar activity: new observational constraints, Astron. Astrophys. 471, 301-309. Wetter, O. et al.: 2014, The year-long unprecedented European heat and drought of 1540 - a worst case) Climatic Change 125, 349-363. Yamaguchi, Y. T., Yokoyama, J., Miyahara, H., Sho, K., Nakatsuka, T.: 2010, Synchronized Northern Hemisphere climate change and solar magnetic cycles during the Maunder Minimum. PNAS, 107 (48), 20697-20702.

Received on 29 November 2019