An Underestimated Record Breaking Event – Why Summer 1540 Was Likely Warmer Than 2003

Clim. Past, 9, 41–56, 2013 www.clim-past.net/9/41/2013/ Climate doi:10.5194/cp-9-41-2013 of the Past © Author(s) 2013. CC Attribution 3.0 License.

An underestimated record breaking event – why summer 1540 was likely warmer than 2003

O. Wetter1,2 and C. Pﬁster1 1Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland 2Institute of History, Section of Economic, Social and Environmental History (WSU), University of Bern, Bern, Switzerland

Correspondence to: O. Wetter ([email protected]) and C. Pﬁster (christian.pﬁ[email protected])

Received: 13 June 2012 – Published in Clim. Past Discuss.: 20 July 2012 Revised: 11 December 2012 – Accepted: 12 December 2012 – Published: 14 January 2013

Abstract. The heat of summer 2003 in Western and Cen- and drought events. Such assessments thus need to be com- tral Europe was claimed to be unprecedented since the Mid- plemented with the critical study of contemporary evidence dle Ages on the basis of grape harvest data (GHD) and late from documentary sources which provide coherent and de- wood maximum density (MXD) data from trees in the Alps. tailed data about weather extremes and related impacts on This paper shows that the authors of these studies overlooked human, ecological and social systems. the fact that the heat and drought in Switzerland in 1540 likely exceeded the amplitude of the previous hottest summer of 2003, because the persistent temperature and precipitation anomaly in that year, described in an abundant and 1 Introduction coherent body of documentary evidence, severely affected the reliability of GHD and tree-rings as proxy-indicators for Future climate change will likely enhance the frequency and temperature estimates. Spring–summer (AMJJ) temperature intensity of extreme anomalies (IPCC the Physical Science anomalies of 4.7 ◦C to 6.8 ◦C being significantly higher than Basis, 2007). However, nobody is able to imagine the mag- in 2003 were assessed for 1540 from a new long Swiss GHD nitude and severity of low-probability, high-impact events series (1444 to 2011). During the climax of the heat wave that are expected more frequently in the future as the result in early August the grapes desiccated on the vine, which of continuing global warming (Field et al., 2012). Informa- caused many vine-growers to interrupt or postpone the har- tion on such “climatic surprises” is important, as the impacts vest despite full grape maturity until after the next spell of of these events on human, ecological and physical systems rain. Likewise, the leaves of many trees withered and fell might be very severe (Fuhrer et al., 2006). Studying the past to the ground under extreme drought stress as would usu- is the only way to get an idea about the magnitude and the ally be expected in late autumn. It remains to be determined context of such “ultimate” extremes and their likely impacts. by further research whether and how far this result obtained Extreme weather or climate events are to be understood as from local analyses can be spatially extrapolated. Based on “the occurrence of a value of a weather or climate variable the temperature estimates for Switzerland it is assumed from above (or below) a threshold value near the upper (or lower) a great number of coherent qualitative documentary evidence ends of the range of observed values of the variable” (Field about the outstanding heat drought in 1540 that AMJJ tem- et al., 2012). peratures were likely more extreme in neighbouring regions Regarding heat waves in Western and Central Europe, the of Western and Central Europe than in 2003. Considering summer of 2003 is usually taken as a benchmark for future the significance of soil moisture deficits for record breaking extreme events. It was so far discussed in a hundred scientific heat waves, these results still need to be validated with es- papers (Garc´ıa-Herrera et al., 2010, and references therein; timated seasonal precipitation. It is concluded that biologi- Fischer and Schar,¨ 2010; Barriopedro et al., 2011; Stott et al., cal proxy data may not properly reveal record breaking heat 2011; Weisheimer et al., 2011; Quesada al., 2012; Stefanon et al., 2012; Field et al., 2012). In fact, during the first two

Published by Copernicus Publications on behalf of the European Geosciences Union. 42 O. Wetter and C. Pfister: An underestimated record breaking event weeks of August at least four countries, the UK, Germany, including a focus on reported drought effects on grapes and Switzerland and Portugal, experienced new all-time records trees in 1540. Section three outlines the reconstruction of of measured daily maximum temperatures (Diaz et al., 2006). spring–summer (AMJJ) temperatures from this series using The August heat wave claimed approximately 40 000 extra the calibration-verification approach and presents the results. deaths, mostly elderly people (Garc´ıa-Herrera et al., 2010). Estimated temperatures are compared with the results of The financial loss due to crop failure over Europe alone is other reconstructions in the fourth section taking into account estimated at $ 12.3 billion (Heck et al., 2004), not consider- studies highlighting the significance of early soil desiccation ing other sectors of the economy. for the generation of heat waves. The final section summa- The summer 2003 is claimed to be unprecedented, which rizes the results and the main lessons that can be drawn from is beyond doubt for the instrumental period reaching back comparing the extreme events of 1540 and 2003. 250 to 300 yr. According to the analysis of a series of grape harvest dates (GHD) in Burgundy (France) by Chuine et al. (2004) the heat of spring–summer (AMJJA) 2003 was 2 Data probably “even higher than in any other year since 1370”. Documentary sources being understood as physical units of Luterbacher et al. (2004) concluded from that result that the man-made information on weather and climate provide the summer half-year (AMJJAS) 2003 was the warmest of the backbone of the analysis. They may contain two different last 500 yr in Europe. According to a series of tree-ring maxi- kinds of data (Brazdil´ et al., 2010, and references therein): mum late wood density (MXD) measurements in Lotschental¨ (Canton Valais, Switzerland), the summer 2003 was even a. direct weather descriptions relating to warm and cold claimed to be the warmest since AD 755 (Buntgen¨ et al., spells, sunshine, rain, snow, wind force, etc.; and 2006). This issue, however, still needs additional investigations. b. indirect (bio-) physical data about vegetation advances Schar¨ et al. (2004) do not exclude the possibility “that such or delays in the summer half-year (AMJJAS) and the warm summers might have occurred in the more distant his- presence or absence of frost, ice and snow-cover in torical past, for instance in the Medieval Warm Period or in the winter half-year (ONDJFM) (Pfister, 1984, 1992; 1540”. Indeed, the thousand year-long series of summer tem- Brazdil´ et al., 2005). perature indices for the Low Countries reaches the maximum value of +9 in 1540 (Shabalova and van Engelen, 2003). With regard to source generation, a distinction is made be- Beniston and Diaz (2004) based on documentary evidence tween documents produced by individual amateur observers by Pfister (1984), Glaser et al. (1999) and the climatologi- and those produced by members of institutions (Pfister et al., cal analysis by Jacobeit et al. (1999) argued that their results 2009). “suggest that 2003 is likely to have been the warmest sum- a. Individually generated sources such as chronicles and mer since 1540” (emphasis added by the authors), whereas diaries often contain both direct and indirect data. They this event scores second after 2003 in the index and measure- are laid down on daily to seasonal time-scales putting ment based reconstruction for Germany, the Czech Republic a special focus on extreme anomalies and nature in- and Switzerland since 1500 by Dobrovolny´ et al. (2010). duced disasters that affected human societies. In or- In this paper, which includes additional information and der to allow comparison of outstanding anomalies over uses more abundant daily to seasonal time-scale analysis time, most chroniclers referred to indirect (bio-) phys- compared to previous studies for the year 1540, we show ical proxy data in the natural environment. They pre- that the point made by Chuine et al. (2004) and Buntgen¨ et sented such observations within their meteorological al. (2006) with regard to 2003 cannot be held up. It will be context which allows cross checking narrative meteo- demonstrated from coherent first-hand observations reported rological and (bio-) physical proxy data. Individually by chroniclers in Western and Central Europe that drought generated sources are relatively short, ending with the conditions in 1540 were so extreme that the timing of the death of the observer or before. Scholars need to as- grapevine harvest hinged on sufficient rainfall rather than on sure that they were written by contemporaries, because grape maturity, while trees suffered from the same extreme copies are known to be error-prone (Alexandre, 1987). conditions. To reassess spring–summer (AMJJ) temperatures for this outstanding year a long GHD series for Switzerland b. Institutional sources were produced by officials of or- encompassing the period 1444 to 2011 was composed, which ganizations such as churches or municipalities. These in the case of 1540 was complemented with additional data to officials were in charge of managing resources that of- assess full grape maturity under extreme drought conditions. ten fluctuated according to climate. The resulting docu- The study is organized as follows: The first section reviews ments were laid down in a standardized form regularly different documentary data types that are used in the analy- and thus provide contemporary, continuous, quantitative sis. The steps to merge Swiss partial GHD series into a ho- and quantifiable proxies for climate elements. At the mogenised main series are presented in the second section same time, they are available for long time periods up

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to several centuries. This allows the calibration and ver- Prior to the French revolution, in most areas vine-growers ification with instrumental measurements (Brazdil´ et al., were not free to harvest at their will. They had to wait for a 2010). To avoid biases in climate reconstruction, schol- public order by the seigneur or the municipality. As soon as ars need to assure that metadata relating to the climate the earliest or the most important grape varieties were found proxy should not change over time. to be ripe (Daux et al., 2012), the vineyards were guarded day and night to prevent the common people as well as the vine-growers from entering. The main reasons for the vintage Evidence from individual and institutional sources is com- ban were the prevention of theft or clandestine harvesting be- plementary, in particular with regard to the reconstruction fore the owners of the vineyards and the beneficiates of tithe and interpretation of extreme events. On the one hand, the (i.e. taxes) could monitor the correct delivery of their dues. statistical analysis of institutional sources allows assessing Moreover, the time of the ban was needed to mobilize the pre-instrumental mean temperatures for the temporal resolu- many hands for picking the grapes. In France, the seigneur tion of seasons of several consecutive months. On the other – not being submitted to the ban – had the right to begin the hand, data from individual sources related to the same time harvest in his vineyards one day ahead to benefit from lower span allow verifying the results, often at a time resolution of wages (Lachiver, 1988). individual months, sometimes even days (Pfister, 1992). This The practice of setting the grape harvest ban was described procedure not only refers to documentary evidence (e.g. Wet- in some detail on the example of Besançon by Garnier et ter and Pfister, 2011), but also to the comparison of tree-ring al. (2011). In this town situated in eastern France, scheduling data with documentary data (Buntgen¨ et al., 2011). the grape harvest was one of the prerogatives of the people’s The term GHD as it is used in this paper, includes ev- representatives who met at the town hall almost daily be- idence from both institutional and individual sources used fore the revolution (1789). The procedure was conventionally as proxies for the phenological stage of full grape maturity. based on a meteorological assessment of the previous months Such data might be grouped into (a) grape harvest ban re- taking into account incidences such as military threats and lated data (GHBD), (b) other kinds of evidence from institu- plague outbreaks. The result was dutifully noted and dated tional sources such as the first wage payments for grape har- in the registers in which the debates were recorded. In addi- vest labourers (WPD), (c) observations about the beginning tion to GHD, registers include meteorological information. of grape harvest laid down by chroniclers, so called historic After the revolution vine-growers were theoretically free to phenological data (HPD) and (d) standard phenological ob- begin the harvest at their will, but in practice most munici- servations made by observers in the framework of phenolog- palities maintained a compulsory vintage ban to preserve law ical networks (PNO). Data from institutional and individual and order (Le Roy Ladurie and Daux, 2008). From 1889 the sources may, of course, overlap, as chronicler’s reports of- municipalities were entitled to keep or give up the practice of ten referred to vineyards submitted to the grape harvest ban. the harvest ban which led to its disappearance almost all over With regard to the faithful transmission manuscript sources France. In 1979 the vintage ban was reintroduced in the en- or critically edited sources, they are both considered to be tire country for reasons of quality control (Daux et al., 2012). more reliable than uncritical publications. The practice in Dijon documented since the late 14th cen- The probably first long series of GHD was set up for tury was somewhat different. Two historians, Thomas Labbe´ the vineyards surrounding the town of Dijon (France) by and Damien Gaveau (2011), attempted an in-depth critique the physician and scientist Jules Lavalle (1855) in coop- and reinterpretation of the Dijon GHD series based on the eration with the local archivist. The Swiss M. Louis Du- extraordinarily rich documentation available in the munic- four (1870) was the first scholar to use GHD for investigating ipal archives of this town. At first, the two historians no- climatic change. This proxy became then universally known ticed that the series set up by Lavalle (1855) consists of both through the pioneering work of Le Roy Ladurie (1972) and GHBD and reports about the day/date, when the first grapes Le Roy Ladurie et al. (1967) who drew on a GHD compila- were brought into town for pressing in the municipal vine tion by Angot (1885). To this day, 378 GHD series, mainly presses. More importantly, the two authors discovered that from France, were compiled, critically reviewed, statistically prior to 1535 the vineyards around the town of Dijon were analysed and made available on internet (Daux et al., 2012). divided into a variable number of small local bans in which Phenologists classify crop harvests under the “aprocryphal” grape harvests began at different dates extending over a pe- (i.e. questionable) phases, because they also depend on hu- riod of 13 days. Subsequently, the number of local bans was man decision making in contrast to phases of wild plants reduced (Labbe´ and Gaveau, 2011). Not until 1607 the mu- (DWD, 1991). Working with GHD thus entails investigating nicipal council considered grape maturity to be the most im- the social context in which the evidence was generated. The portant parameter to begin the harvest. These circumstances procedure of the grape harvest ban already practiced in Ro- led the two historians to conclude that GHD prior to 1600 man antiquity (Ruffing, 1999) is subsequently discussed on seem to be “artificially early” with regard to grape maturity. the example of France which is comparable to the situation The authors of the Swiss series set up by Meier et in Switzerland. al. (2007) (including C. Pfister) did not correct their dates www.clim-past.net/9/41/2013/ Clim. Past, 9, 41–56, 2013 44 O. Wetter and C. Pfister: An underestimated record breaking event A Grape-Temp-18-5 12.12.2012 7

305 from Julian to Gregorian calendar by adding 9 to 10 days, Protestant Cantons following Julian Introduction of Gregorian Calendar until 1700, uncorrected by Calendar in all Swiss leading to too warm reconstructed temperatures before 1700 300 Meier et al. (2007) Cantons in 1701 (Fig. 1). In Switzerland the new style was introduced by the Catholic cantons except Valais in 1584, whereas most Protes- 295 tant cantons, not considering individual latecomers, adopted 290 the Gregorian reform after 1700 (Richards, 1998). Minor dif- 285 ferences between Meier et al. (2007) and our reconstruction 280 after 1701 occur, because of the effects of the 11-yr moving average, some potential Protestant latecomers to the new cal- 275 Grape harvest date in Day of Year (DOY) Year of Day in date harvest Grape endar style and the fact that we used a somewhat different set 270 of data, especially in the second half of the 20th century. 265 In order to get a more reliable basis for assessing pre- 1444 1464 1484 1504 1524 1544 1564 1584 1604 1624 1644 1664 1684 1704 1724 1744 1764 1784 1804 1824 1844 1864 1884 1904 1924 1944 1964 1984 2004 instrumental warm season temperatures a new GHD series Year 11-yr mov average Swiss grape harvest compilation; Wetter and Pfister (2012) (1444–2011) being longer and more complete than Meier et 11-yr mov average grape harvest Switzerland; Meier et al. (2007) al. (2007) was being set up for Switzerland. The procedure is Fig. 1. Noncalendar-style corrected GHD by Meier et al. (2007) vs. corrected GHD by Wetter and Pfister (2012) Fig. 1. Non-calendar-style corrected GHD by Meier et al. (2007) vs. discussed below. corrected GHD by the authors of this article. The new Swiss GHD series The new Swiss GHD series Switzerland is located in central Europe and divided up into three major geographic regions, namely a) the Alps, b) the Jura Mountains and their foothills and c) the intermediate region Switzerland is located in central Europe and divided up into called the Swiss Plateau, where most vineyards are located. The country is situated in a zone three major geographic regions, namely (a) the Alps, (b) the of a.transitionalWage climate payment influenced data by bot (WPD)h maritime of and the continental Basel air Hospital masses. (1444– Jura Mountains and their foothills and (c) the intermediate Figure 1705)2 (Fig. 2, (Fig.upper part) 2, summarizes series S1):the spaBrtial azdil´location andof the 17 Kotyza local GHD (2000) together region called the Swiss Plateau, where most vineyards are with thefirst metadata discovered used for merging the the potentiallocal series into of a main account series (Fig. books 2.; box). for The lower cli- located. The country is situated in a zone of transitional cli- part of mateFigure 2 reconstructionshows the composition inof the their main analysisseries over time. of the Czech town mate influenced by both maritime and continental air masses. of Louny. WPD for the Swiss series were drawn from Figure 2 (Fig. 2, upper part) summarizes the spatial loca- the books of expenditure of the hospital of Basel, which tion of the 17 local GHD together with the metadata used for according the above mentioned scheme are a first class merging the local series into a main series (Fig. 2.; box). The source. The hospital of Basel was a profit orientated lower part of Fig. 2 shows the composition of the main series enterprise providing the upper classes of the munici- over time. pal community in return for donations or inheritances The new Swiss GHD series is composed from four differ- with pensions for the elderly and a disability-insurance. ent kinds of GHD, namely (a) (institutional) wage payment In the mid-fifteenth century the hospital was in pos- data (WPD) (b) (Institutional) grape harvest ban related data session of numerous estates in more than 80 villages; (GHBD), (c) (individual) historic phenological data (HPD) the estate accounts were recorded on a daily basis (von (d) (institutional) phenological network observation (PNO). Tscharner-Aue, 1983). Like the seigneurs, the hospital Assessments of full grape maturity in 1540 is drawn from was not subject to the vintage ban and therefore could HPD in combination with narrative information. begin the harvest without regard to other parties. For In order to assess data uncertainties, the quality of the gaining the advantage of hiring day labourers at a lower sources needs to be assessed depending on whether the ob- price, this often meant early harvests (Lachiver, 1988). servations are made by contemporaries and if we have ac- The hospital only hired day labourers to cultivate their cess to the original manuscript or to a high quality publica- own vineyards. These hands were paid at the end of each tion. Non-contemporary sources or uncritical data publica- working day. The first appropriate entry in the account tions, often without metadata, may contain printing or copy- books thus represents the actual beginning of the corre- ing errors. The 17 local GHD series (see Fig. 2) and the sponding agricultural activity. Three different vineyards main Swiss series are published online with the supplemen- cultivated by the hospital are mentioned in the books tary material including the appropriate correlation matrix and of expenditure. One was in Biel-Benken (“Spittelhof” the scheme of quality criteria. First-quality GHD are avail- 352 m a.s.l.), another one called “Spittelschure”¨ in Basel able in the form of contemporary manuscripts (c/m). Con- (275 m a.s.l.) and a third one in Fischingen (Germany) temporary published data (c/p) are of secondary quality. Data (313 m a.s.l.). Each vineyard had a southern exposure from non-contemporary manuscripts (nc/m) or from uncriti- and was located in a perimeter of 10 km from Basel. cal data publications (nc/p) have to be considered with cau- Weather and climate influence in this area may thus tion. be seen as generally coherent. Books of expenditure were kept on a daily basis from 1444 until 1705 (Wet- ter and Pfister, 2011) (Fig. 2, series S1). During these 262 yr cultivation practices and accounting procedures

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A Grape-Temp-18-5 12.12.2012 8 Germany overlapping, so as to make their integration into a long composite main series difﬁcult. S4, S8, S10, S11 are France completely based on HPD, whereas S3 and S13 are based on GHBD (S3) and PNO (S13) as well. Further- more, reliable HPD were used to assess grape maturity Austria in 1540, as will be subsequently shown.

d. Phenological network observation (PNO) (Fig. 2, series S7, S13, S14, S15, S16, S17): Compared to Ger- many, which has a long tradition of phenological network observations (DWD, 1991), continuous PNO in Switzerland started as late as in 1951 (Deﬁla and Clot, Italy 2001). The BBCH standard manual, which is a system 18 1540

) S17 for a uniform coding of phenologically similar growth 16 S16 S15 GHD stages of all mono- and dicotyledonous plant species, ( 14 S14 S13 defines 39 observation categories for vines. “Begin of 12 S12 S11 10 S10 harvest” is undeniably easy to be observed, whereas ob- S9 8 S8 servations about physiological stages such as grape ma- S7 6 S6 turity are more difficult to be clearly identified (Meier S5 4 S4 e harvest data series series data e harvest et al., 2009). All in all, 6 PNO series (S7, S13, S14,

p S3 2 S2

Gra S1 S15, S16, S17), observed and recorded in the frame- 0 1540 1444 1465 1486 1507 1528 1549 1570 1591 1612 1633 1654 1675 1696 1717 1738 1759 1780 1801 1822 1843 1864 1885 1906 1927 1948 1969 1990 2011 work of the Swiss Weather Service Meteo-Swiss, were included in the new Swiss GHD series. Existing his- Year Legend: toric GHD series were, whenever possible, completed GHBD Grape Harvest Ban related Data HPD Historic Phenological Data with corresponding PNO series, from which places of PNO Phenological Network Observation WPD Wage Payment Data observation are known. Series S7 and S13 from Twann Fig. 2. Location-, metadata- and composition of 17 Swiss GHD series. (GHBD + PNO) and Hallau (HPD + PNO) were com- Fig. 2. Location, metadata and composition of 17 Swiss GHD se- bined with other GHD data types. Series S14–S17 are ries.The new Swiss GHD series is composed from four different kinds of GHD, namely a) PNO series only. (institutional) Wage Payment Data (WPD) b) (Institutional) Grape Harvest Ban related Data- (GHBD), c) (individual) Historic Phenological Data- (HPD) d) (institutional) Phenological Vineyards of the PNO series, from which all necessary remained the same as can be concluded from the fact metadata is available, are known to be south facing. that the series is stationary and without a trend. It only The same is true for most vineyards still existing to- contains a few minor gaps. day near the locations of the historic GHD series making it quite plausible that these data relate to the same b. Grape harvest ban related data (GHBD) (Fig. 2, se- places and exposures. Altitude was derived from the lo- ries S2, S3, S5, S6, S7, S9): The generation of GHBD cality where the vineyards were cultivated taking into is related to the lifting of the previously described vin- account mean altitudes of the still existing vineyards tage ban. All 7 series are contemporary. S2, S5 and S9 based on information in Google Earth. This procedure are entirely GHBD based. S3 and S7 are composed of possibly involves some uncertainties. Furthermore, up GHBD and HPD, whereas S6 and S7 consist of GHBD to the late nineteenth or even to the early twentieth cen- and PNO. tury it is not known what varieties of red grapes were grown. This is also a major source of uncertainty. Va- c. Historic Phenological Data (HPD) (Fig. 2, series S3, rieties of Pinot Noir were known both in the German S4, S8, S10, S11, S12, S13): Prior to the establishment of speaking and the French speaking part of Switzerland national phenological observation networks working ac- since the Middle Ages under different local terminology cording to standardised guidelines, historical plant and (Schlegel, 1973), which, however, cannot be resolved animal phenological data (HPD) were laid down by am- any more, today. ateur observers at their discretion (e.g. Rutishauser et al., 2007). In many cases the observations are presented Pearson correlations of all series having a sufficient within their meteorological context. HPD are not nec- overlap of > 15 values are significant at values between essarily of lower quality than phenological network ob- r = 0.63 and r = 0.92 (p = 0.01 and p = 0.03; only servations (PNO) though the lack of metadata (e.g. alone correlation). Weaker correlations are related to dis- titude, plant varieties etc.) is a source of uncertainty. tance, climate and soils as well as uncertainties related As HPD were produced by individuals, the resulting to grape varieties (e.g. north eastern vs. south western series are rather short, spatially scattered and hardly Switzerland). www.clim-past.net/9/41/2013/ Clim. Past, 9, 41–56, 2013 46 O. Wetter and C. Pfister: An underestimated record breaking event

e. Assessment of grape maturity in 1540 from HPD and i. The first draws on 89 systematic observations in the weather narratives: None of the Swiss GHD series pro- open vineyard in Zollikon (473 m a.s.l.) from 1732 to vides a date for 1540. Grape maturity was assessed from 1832 and concerns the phenological stages of verai- 3 HPD laid down in the context of detailed weather nar- son, which refers to the colour change and softening of ratives about the effects of this extreme year motivated berries (Mullins et al., 1992) and the beginning of grape by drought scare. Chroniclers agree that rain only fell harvest (Kohler, 1879). The mean difference between three or four times between early April and early Au- the two stages is 37 days which is consistent with the gust (Table S1, Sc2, in the Supplement). According to values of 35 to 40 days given by Daux et al. (2012). reconstructions of dominant meteorological situations In 1540 veraison was reported on 5 July (Table S1, by Jacobeit et al. (1999), blocking anti-cyclonic situa- Sc3, in the Supplement) in Schaffhausen (403 m a.s.l.) tions were quasi-persistent in summer (JJA) over large and around 10 July (Table S1, Sc1, in the Supplement) parts of Europe not considering the dusty-dry hot spring in Zurich¨ (408 m a.s.l.). As the delay between veraison and the warm autumn. Vine-growers in Schaffhausen and harvest is known to be quite constant (Daux et al., (Switzerland) were “long waiting for rain to begin the 2012), this suggests a maturity related harvest date be- harvest”, as chronicler Oswald Huber relates (Table S1, tween 12 and 17 August. This conclusion is consis- Sc3, in the Supplement). However, he writes “vine- tent with an observation about sweet grapes found in growers finally tackled the work nevertheless, because Schaffhausen on 4 August (Table S1, Sc3, in the Sup- the plants withered” (Table S1, Sc3, in the Supplement). plement). Vine-growers at the shores of Lake Constance and in the Upper Alsace interrupted the vintage after picking the ii. The starting point for the second approach is Hein- juicy grapes (Burmeister, 2008; Stolz, 1979), because rich Bullinger’s narrative that he tasted grape must the remaining ones were quasi dried out. The vintage (“Sauser”) in Zurich¨ on 10 August (Table S1, Sc1, in was then resumed after a two-day spell of rain around the Supplement). According to Werner Siegfried (per- St. Michaels Day (8 October). The GHBD for Dijon sonal communication, March 2012) grape must nor- available according to the supplementary material on mally is obtained between one to two weeks before the internet is 4 October (DOY 278) (Chuine et al., 2004), main grape harvest starts which points to grape matu- whereas the corrected date contained in the Dijon se- rity between 17 and 24 August. This estimate coincides ries is 3 September (DOY 247) (Labbe´ and Gaveau, with a note in the chronicle of Ulm (south Germany) 2011) which agrees with the date given for Besançon (438 m a.s.l.), situated in the comparatively cool climate (Daux et al., 2012). The wrong value given by Chuine et of the Swabian Alb, that new wine was served already al. (2004) might be due to a copying error in the compi- on 20 August (Table S1, Sc5, in the Supplement). lation by Angot (1885) used by Le Roy Ladurie (1972) Considering the result of the two approaches, we as- and Le Roy Ladurie et al. (1967). sess the likely time of full grape maturity in 1540 to have been somewhere between 12 and 24 August. Both At harvest time grapes in many vineyards had with- values were inserted in the temperature-GHD regres- ered (grapes became raisins). They yielded a sweet sion as a maximum and minimum value, which was sherry-like wine (Glaser et al., 1999) which made peo- derived from the calibration verification approach with ple rapidly drunk (Table S1, Sc4, in the Supplement). HISTALP temperature anomalies (Auer et al., 2007), In Wurzburg¨ (Germany) the premium wine of 1540 was to assess AMJJ temperature anomalies in the extreme stored in a nicely decorated barrel and only offered to drought year (see Sect. 3). guests of the court. The wine became so famous that Swedish soldiers after their conquest of the town in 1631 were seeking the precious barrel. However, be- 3 Methodology cause it was hidden behind a wall, they were unable to find it. The last bottle of the 1540 vintage containing We applied the standard palaeo-climatology calibration- the world’s oldest still-drinkable wine is today exposed verification approach using linear regression models between in the Wurzburg¨ citizen’s hospital (Glaser et al., 1999). measured temperatures (dependent variable) and the proxy, which in our case is GHD (independent variable). The overlapping period of the two series is divided up in two sub- These descriptions suggest that due to the record breaking periods. This configuration allows one of them to be defined heat and drought human decision making to begin the harvest as a calibration period, for which the linear regression model in 1540 was related to rainfall rather than to grape maturity. was calculated, and subsequent independent verification of Detailed observations by contemporary alert observers allow the results using data from the second period, then vice versa. assessing the likely time of grape maturity using two com- The usefulness of each model is characterized by (squared) plementary approaches correlation, standard error of estimate (SEE) (Dobrovolny´ et

Clim. Past, 9, 41–56, 2013 www.clim-past.net/9/41/2013/ O. Wetter and C. Pfister: An underestimated record breaking event 47 al., 2008) and the Durbin Watson autocorrelation test. In or- c. Homogenisation for grape variety (series S1): It turned der to investigate the record breaking extreme event of 1540 out that the adjusted Basel WPD series appeared to we relied on coherent records made by contemporary chron- be considerably too early with respect to altitude. This iclers reporting on the advanced development of trees and fact suggests the cultivation of earlier grape varieties vines and on the vine-growers’ reasons for taking particular in the Basel region. According to Dominik Wunderlin decisions in an exceptional situation. (personal communication, September 2011; Wunderlin, A longer-standing discussion refers to the period within 1986) it is assumed that an early variety of Pinot Noir the year to be assessed from GHD, whether it is April to named “Augstler”¨ (early Red Burgundy) was grown in August (Chuine et al., 2004; Meier et al., 2007), April to the Greater Basel region including southern Alsace and September (Daux et al., 2012) or just April to July, as it is south western Germany. In Canton Schaffhausen (north- done in this investigation. In this context, Gladstones (2011) ern Switzerland) early Red Burgundy vines were also refers to the “widely observed phenomenon that tempera- grown until the first decades of the twentieth century, tures of the first two or three growing season months, or al- for which appropriate phenological data are also avail- ternatively the date of flowering, can usually predict quite able. Based on a statistical analysis of this evidence, closely the dates of veraison and maturity to follow [...]. The Pfister (1984) demonstrated that Augstler¨ were on av- later phenological intervals show little response to tempera- erage ripe on 10 September, i.e. at the end of August ture, and tend to be constant from year to year”. His assess- (Julian style). This might also indicate the origin of the ment confirmed by Daux et al. (2012) is in agreement with word “Augstler”¨ – the name being probably linked to the results of stepwise regression analysis by Legrand (1979), the month of August. He established a mean difference Pfister (1984) and Guerreau (1995) showing that tempera- for veraison of 17 days between early Red Burgundy tures in August are not significant for the harvest date. We and the ordinary Pinot Noir grapes as well as a signif- furthermore could replicate this result from a stepwise re- icant correlation of r = 0.87 (N = 37) between the ve- gression of our, and also from the GHD data published by raison date of both varieties. The Basel WPD series was Chuine et al. (2005). homogenized accordingly. In a final step all GHD available for a particular 4 Reconstruction of spring–summer (AMJJ) year were annually averaged. The standard deviation temperatures in Switzerland, 1444–2011 of the averaged Swiss GHD series (average DOY of all available GHD series per year) amounts to 9.54 The 17 local series presented in the previous section were ad- days. The new Swiss GHD series resulting from this justed (homogenised) for (a) dating style, (b) altitude and (c) homogenization-procedure was then calibrated with the variety (only series S1) before being merged into a composite monthly anomalies from the 1901 to 2000 mean of main series. the long HISTALP temperature series going back to 1774. The HISTALP database consists of monthly qual- a. Homogenisation for dating style: As previously men- ity controlled and homogenised instrumental records tioned GHD series need to be adjusted for dating style. of temperature, pressure, precipitation, sunshine and The series from Catholic cantons were adjusted by cloudiness for the “greater Alpine region” comprising adding 9 days in the 15th century and 10 days in the 724 000 km2, covering the whole territory of Switzer- 16th century prior to the Gregorian reform applied in land, Liechtenstein, Austria, Slovenia and Croatia, to- 1584. Those from protestant cantons were adjusted by gether with parts of adjacent countries. Switzerland adding 9 and 10 days, respectively, prior to 1701. north of the Alps and the nearby regions are situated in b. Homogenisation for altitude: The local GHD series the north-western sub-region of the “greater Alpine re- were then adjusted for altitude as follows: the mean alti- gion” for which a particular temperature series is avail- tude of all 17 series is 436 m a.s.l. Two series –S6 (Orbe, able (Auer et al., 2007; Bohm¨ et al., 2010). It is used as Canton Vaud, 438 m a.s.l.) and S17 (Neuhausen, Canton a predictand for the present investigation. Schaffhausen, 437 m a.s.l.) – are situated at almost the same altitude as the overall mean altitude of all series. The dates of 12 and 24 August were used in the regres- Each of them has a mean of DOY 289, which, accord- sion for 1540 as a proxy of full grape maturity. As previ- ing to the homogenisation methodology by Chuine et ously mentioned, they were obtained from two complemen- al. (2004), was taken as the reference, to which the re- tary approaches using detailed observations by contemporary maining 15 series were adjusted. This homogenisation meticulous observers. They represent the likely maximum was done by adding or subtracting to each record the and the minimum GHD derived from the above-mentioned difference between the long-term mean of the particu- approaches for this extreme year (see Sect. 2, (e). In sum- lar local series to the long-term mean of the reference mary, the two dates of 12th and 24th mark the margins of series (i.e. DOY 289). fluctuation within which full grape maturity likely occurred. www.clim-past.net/9/41/2013/ Clim. Past, 9, 41–56, 2013 A Grape-Temp-18-5 12.12.2012 16 Croatia, together with parts of adjacent countries. Switzerland north of the Alps and the nearby regions are situated in the north-western sub-region of the ‘Greater Alpine Region’ for which a particular temperature series is available (Auer et al., 2007; Böhm et al., 2010). It is used as a predictand for the present investigation. The dates of 12th and 24th August were used in the regression for 1540 as a proxy of full grape maturity. As previously mentioned, they were obtained from two complementary approaches using detailed observations by contemporary meticulous observers. They represent the likely maximum and the minimum GHD derived from the above-mentioned approaches for this extreme year (see section 2,e). In summary, the two dates of 12th and 24th mark the margins of fluctuation within which full grape maturity likely occurred. Multiple stepwise linear regression revealed May temperatures to be the most important factor for GHD, followed by June, July and April (not shown). August was not significant in agreement with grape physiology (Mullins et al., 1992) which confirms earlier results by 48 O. Wetter andLegrand C. (1979) Pfister: and Pfister An (1984). underestimated Several independent calibration- record and breaking verification event50-year sub-periods of the 1774 – 2005 HISTALP temperature anomaly series have been tested. It was found that there were overall good calibration and verification results.

Multiple stepwise linear regression revealed May temper- 3 Calibration 1774-1824 3 Calibration 1955-2005 atures to be the most important factor for GHD, followed by 2 2 June, July and April (not shown). August was not signiﬁcant 1 1

0 0 Temperature Temperature Temperature Temperature anomalies (°C) anomalies in agreement with grape physiology (Mullins et al., 1992) anomalies (°C) which confirms earlier results by Legrand (1979) and Pfis- -1 -1 -2 -2 Year 177417841794180418141824Year 1955 1965 1975 1985 1995 2005 ter (1984). Several independent calibration- and verification HISTALP_APR-JUL_temperature anomalies HISTALP_APR-JUL_temperature anomalies Temprec_Calib_1774-1824 Temprec_Calib_1955-2005 50-yr sub-periods of the 1774–2005 HISTALP temperature r = 0.86 r = 0.78 3 Verification 1955-2005 Verification 1774-1824 3 anomaly series have been tested. It was found that there were 2 2

1 overall good calibration and veriﬁcation results. 1

0 0 Temperature anomalies (°C)

The best verification match was found in the 1774–1824 Temperature anomalies (°C) anomalies -1 sub-period where HISTALP spring–summer (AMJJ) mean -1 -2 -2 Year = 1955 1965 1975 1985 1995 2005 1774 1784 1794Year 1804 1814 1824 temperatures significantly correlated with Pearson r 0.86 HISTALP_APR-JUL_temperature anomalies HISTALP_APR-JUL_temperature anomalies VERIFICATION_Temprec_Calib_1774-1824 VERIFICATION_Temprec_Calib_1955-2005 (p = 0.01) (Fig. 3). The standard error of estimate (SEE) r = 0.78 r = 0.86 ◦ Fig. 3. Calibration and verification of homogenised Swiss Grape Harvest series with HISTALP 50-yr sub amounts to 0.5 C. periods; 1774–1824 and 1955–2005. Fig. 3. Calibration and verification of homogenised swiss grape har- Figure 4 displays an 11-yr high-pass filter of recon- vestThe best series verification with HISTALPmatch was found 50-yr in thesub 1774–1824 periods; sub-period 1774–1824 where HISTALP and 1955–spring- structed temperature anomalies. It is noticeable that the 2005.summer (AMJJ) mean temperatures significantly correlated with Pearson r = 0.86 (p=0.01). curve after 1990 does not fully represent the lengthening (Fig. 3). The Standard Error of Estimate (SEE) amounts to 0.5° C of the average growing season usually observed in the context of global warming. This reflects the previously men- interesting to note that the latest GHD (1542) almost imme- tioned fact established by Menzel et al. (2006) that re- diately follows the record breaking value of 1540. sponses of wild plants to global warming are larger than that of phases of crops which are also subject to management practice alterations. Moreover it has to be kept 5 Discussion in mind that the reconstructed temperature variability is likely to be suppressed as a result of the regression method. The discussion first involves a comparison of the new Swiss The underestimation of the low-frequency variability typi- GHD series with those from neighbouring regions as well cally amounts to 20 %–50 %. Nevertheless, low-frequency as with the index-based monthly and seasonal T reconstruc- shapes are generally well reconstructed (Christiansen et al., tion by Dobrovolny´ et al. (2010) and 4 MXD tree ring series. 2009). The GHD based temperature reconstruction indi- Best correlations are shown in Table 1. Subsequently, the es- cates that 1540 April–July mean temperature was between timates for 1540 are compared with those for 2003. 4.7 ◦C and 6.8 ◦C(±0.5 ◦C SEE) higher than the mean Overall correlations between the homogenised Swiss 1901–2000 HISTALP temperature (Auer et al., 2007) de- GHD and other GHD series show good results (Table 1). pending on the assumed date of full grape maturity (12 The Besançon series (Garnier et al., 2011) correlates best vs. 24 August). According to this approach 1540 was by with 0.82, followed by the corrected Dijon series (Labbe´ and far the warmest April to July temperature anomaly in the Gaveau, 2011). Correlations with the series from Western last 566 yr. The estimated record breaking value for 1540 is Hungary and Vienna are somewhat lower, which is related to followed in descending order by that for 1822 (+3.51 ◦C) spatial distance. The Pearson correlation of the uncorrected and for 2003 (+2.86 ◦C). The difference between 1540 and Burgundy GHD series (Chuine et al., 2004) warrants a closer 2003 is quite large considering that measured AMJJ temper- inspection. ature anomalies from the 1901–2000 average in 2003 were 31-yr moving correlations with the uncorrected Dijon 2.9 ◦C in HISTALP (Auer et al., 2007) and 2.7 ◦C in Basel GHD compilation series (green curve) reveal that the low (Begert et al., 2005). Fourth in the ranking is the year 1865 correlation is due to the period 1516 to 1555, where the val- (+2.83 ◦C), followed by 1868 (+2.76 ◦C), 1718 (+2.67 ◦C), ues drop to r = 0.19. This is a consequence of wrong val- 1473 (+2.54 ◦C), 1556 (+2.5 ◦C), 1616 (+2.26 ◦C), 1794 ues in 1522 and 1523 and the questionable value for 1540 (+2.22 ◦C), 1834 (+2.21 ◦C), 1536 and 1945 (+2.17 ◦C) as (Fig. 5). The overall correlation of 0.76 obtained with aver- well as 1846 (+2.1 ◦C) and 1471 (+2.07 ◦C). Coldest mean aged AMJJ Swiss documentary temperature indices (Dobro- March–July temperatures appeared in 1542 (−2.63 ◦C) and volny´ et al., 2010) is relatively low which might be due to 1698 (−2.59 ◦C). They were followed by 1628 (−2.39 ◦C), an accumulation of errors involved with estimating tempera- 1675 (−2.32 ◦C) and 1692 (−2.02 ◦C). Reconstructed tem- tures for individual months. peratures for the well-known year without a summer (1816) Correlations with tree ring (MXD) series are all signifi- surprisingly only are −1.36 ◦C. All significant positive and cant, albeit on very different levels. The series presented by negative temperature anomalies exceeding ±2 ◦C are con- Battipaglia et al. (2010) averages MXD data for Larch from sistent with narrative documentary evidence about warm, Lotschental¨ (Canton Valais) as well as for Pine from Laue- respectively cold seasonal conditions (Pfister, 1999). It is nen (Bernese Oberland) and Tyrol (Austria). Considering the inclusion of Tyrol situated some distance from the Western

Clim. Past, 9, 41–56, 2013 www.clim-past.net/9/41/2013/ O. Wetter and C. Pﬁster: An underestimated record breaking event 49

Table 1. Overall Pearson correlations between Swiss GHD and other climate proxy series.

Type of evidence Origin Correlation (Pearson) Authors GHD Besanc¸on (France) r = 0.82; p = 0.01 Garnier et al. (2011) Documentary based (AMJJ) T Indices Switzerland r = 0.76; p = 0.01 Dobrovolny´ et al. (2010) GHD Dijon (France) corrected r = 0.73; p = 0.01 Labbe´ and Gaveau (2011) GHD Burgundy (France) uncorrected r = 0.67; p = 0.01 Chuine et al. (2004) MXD (Pine, Larch) Swiss Alps, Tyrol (Austria) r = 0.55; p = 0.01 Battipaglia et al. (2010) GHD and grain harvest dates Koszeg¨ (Hungary) r = 0.53; p = 0.01 Kiss et al. (2011) GHD Vienna (Austria) r = 0.51; p = 0.01 Maurer et al. (2009) MXD (Larch) Swiss Alps r = 0.23; p = 0.01 Buntgen¨ et al. (2009) MXD (Pine) Swiss Alps r = 0.15; p = 0.01 Buntgen¨ et al. (2006) MXD (Pine) Italian Alps r = 0.14; p = 0.01 Carrer et al. (2007)

Swiss Alps, the correlation of 0.55 is surprisingly high. On Estimates of the AMJJ temperature anomaly in 1540 ob- the other hand, correlations with the other two series from the tained from the Swiss series are 4.7 ◦C and 6.8 ◦C(±0.5 ◦C Swiss Alps are surprisingly low which would warrant further SEE), respectively, according to the assumed date of full inspection (see Table 1). Correlations with tree ring based grape maturity of 24 and 12 August. These estimates are un- temperature series derived from other regions in Europe far doubtedly higher than the 2003 AMJJ temperature anoma- from Switzerland are very close to zero (e.g. Finland, Swe- lies of 2.5 ◦C from the 1960–1989 average measured in Paris den; Esper et al., 2012; or Albania; Seim et al., 2012, etc.). (Rousseau, 2009). Further uncertainties of this estimate in- This might be caused by distance, the use of another type of volve the fact, that the dates of full grape maturity in Switzer- proxy, different time windows of reconstructed temperatures land were obtained from observations of grape-vine devel- (AMJJ vs. JJA or MAMJJAS etc.) as well as by differing opment made by several vine-growers, for which, of course, climatic conditions which taken together may aggregate pos- uncertainties cannot be quantified. Moreover, it remains to sible errors. An in depth analysis of the reason for the low be determined by further research whether and how far this correlations would be interesting but is beyond the scope of result obtained from local analyses can be spatially extrap- this paper. olated. A further uncertainty relates to effects of drought The key point of this analysis involves comparing spring– stress. It cannot be excluded that the outstanding conditions summer temperatures in 2003 and 1540. It should be borne in in 1540 slowed down the process of grape maturity. Pierre mind that the comparison involves AMJJ temperatures only. de Teysseulh being a capitular of the church of Limoges As the 35 to 40 days preceding the harvest are known not (central France) notes that “this year there was such a great to be subject to temperature induced modifications (Daux et drought that grapes were harvested in August. The grapes al., 2012), it can be excluded that August temperatures mat- were like roasted and the leaves of the vines had fallen to the tered at all for the Dijon GHD of 15 August 2003. Then ground like after a severe frost” (Table S1, Sc6, in the Sup- we need to consider that the GHD of 3 September, 1540 plement). Grapes rate of net photosynthesis decreases signif- ◦ contained in the corrected Dijon series (Labbe´ and Gaveau, icantly above 35 C. The absolute limit of CO2 absorption is 2010) does not represent full grape maturity, as many grape reached if 40 ◦C are achieved. In such cases the plant stops harvests were postponed beyond this stage until the next rain its vegetative activity. Low water drainage soils in combina- spell due to severe drought impacts. The likely time of full tion with dry periods may furthermore have the same effect grape maturity in Switzerland was assessed to be between on the plants growth phase if temperatures for a longer time 12 and 24 August. This estimate refers to vineyards situated period do not fall below 30 ◦C (Currle et al., 1983). somewhat above towns in the Swiss Plateau at altitudes of Trees like vines suffered from drought stress. According 400 to 410 m a.s.l. Thus, they are compatible with the mean to chronicler Sebastian Fischer from Ulm (south Germany), altitude (436 m a.s.l.) of the main Swiss series. Estimating leaves on the trees withered (at the peak of the worst heat the 1540 date of full grape maturity for Dijon, situated at wave) in early August and fell to the ground “as if it had 245 m a.s.l. in the context of this information is more spec- been in late autumn” (Table S1, Sc5, in the Supplement). ulative. It would involve adjusting the date for Switzerland A comparison of MXD values for 1540 between the series to Dijon by subtracting 18 days according to the average dif- from Lotschental¨ (Canton Valais), Lauenen (Bernese Ober- ference of GHD for Dijon (DOY 271) and the Swiss series land) and Tyrol (Battipaglia et al., 2010) reveals striking dif- (DOY 289). This leads to dates between 25 July and 6 Au- ferences between the series. Whereas mean MXD of trees in gust, 1540 for the likely full grape maturity in Dijon which at Lotschental¨ situated in the rain shadow between two Alpine least does not contradict the main argument that AMJJ tem- mountain chains is negative, the values from Lauenen and peratures in 1540 were likely higher than in 2003. Tyrol both situated at the windward side of the Alps range www.clim-past.net/9/41/2013/ Clim. Past, 9, 41–56, 2013 50 O. Wetter and C. Pfister: An underestimated record breaking event A Grape-Temp-18-5 12.12.2012 17

6 1540 5 1540

4 1822 1865 1718 2003 3 1556 1616 1868 1473 1538 1794 18341846 1945 1471 1599 1638 1726 2 1684 1807 1934 1 0 -1 1843 1805 1816 1879 1485 1529 1600 1720 anomalies (1901-2000) (°C) (1901-2000) anomalies 1939

Reconstructed temperature temperature Reconstructed 1642 1980 -2 1527 1573 1587 1481 1692 1628 1675 -3 1542 1698 -4 1444 1464 1484 1504 1524 1544 1564 1584 1604 1624 1644 1664 1684 1704 1724 1744 1764 1784 1804 1824 1844 1864 1884 1904 1924 1944 1964 1984 2004

Year 11-yr mov average grape harvest mean Apr-Jul temp.rec Upper 11-yr mov confidence bound; 2 x σ 1774-1824 verification Lower 11-yr mov confidence bound; 2 x σ 1774-1824 verification Y_temp=27.453-0.094*DOY_GHD

Fig. 4. Temperature reconstructionFig. 4. Temperature based reconstruction on the homogenised based on the Swiss homogenised GHD series Swiss covering GHD series the periodcovering from the period 1444–2011. from A Grape-Temp-18-51444-2011. 12.12.2012 19

1.00

Figure0.90 4 displays an 11-year high-pass filter of reconstructed temperature anomalies. It is noticeable0.80 that the curve after 1990 does not fully represent the lengthening of the average growing0.70 season usually observed in the context of Global Warming. This reflects the 0.60 previously mentioned fact established by Menzel et al. (2006) that responses of wild plants to 0.50 Global Warming are larger than that of phases of crops which are also subject to management 0.40

practice (pearson) coefficient 0.30 alterations. Moreover it has to be kept in mind that the reconstructed temperature 31-year moving correlation 31-year variability0.20 is likely to be suppressed as a result of the regression method. The underestimation of the0.10 low-frequency variability typically amounts to 20%-50%. Nevertheless low-frequency

0.00 shapes are1444 1464 generally1484 1504 1524 well1544 1564 reconstructed1584 1604 1624 1644 1664 (C1684 hristiansen1704 1724 1744 1764 et1784 1804 al.,1824 2009).1844 1864 1884 The1904 1924 GHD1944 1964 based1984 2004

temperature reconstruction indicates that 1540 April-JulyYear mean temperature was between 4.7 31 yr mov corr with Garnier et al. (2011), Besancon (France); overall corr. Pearson r = 0.82; p = 0.01 °C and 6.8 °C31 yr(+/- mov corr0,5 with °C Dobrovolny SEE) higher et al. (2010) than basing the on meanSwiss mean 1901-2000 AMJJ Pfister HISTALPIndices; overall corrtemperature r = 0.76, p = 0.01 (Auer 31 yr mov corr with Labbé and Gaveau (2011), corrected Dijon series (France); overall corr r = 0.73, p = 0.01 31 yr mov corr with Chuine et al. (2004), uncorrected Dijon series (France); overall corr.r = 0.67,th p = 0.01 th et al., 2007)31 dependingyr mov corr with Battipagliaon the etassumed al. (2010), Swiss date and ofAustrian full Alps; grape overall maturity corr r = 0.55, p(12 = 0.01 vs. 24 August). 31 yr mov corr with Kiss et al. (2011); Köszeg (Hungary); overall corr r = 0.53, p = 0.01 31 yr mov corr with Maurer et al. (2009), Vienna (Austria), overall corr r = 0.51, p = 0.01 According to this approach 1540 was by far the warmest April to July temperature anomaly in Fig. 5. 31-yr moving correlation between Swiss compiled GHD and Central European temperature proxy series. Fig. 5. 31-yr moving correlation the last 566 between years. SwissThe estimated compiled record GHD andbreaking Central value European for 1540 temperature is followed proxy in descending series. 31-yearorder by movingthat for 1822correlations (+3.51 °C)with and the for uncor 2003rected (+2.86 Dijon °C). TheGHD differe compilationnce between series 1540 (green and curve) reveal that the low correlation is due to the period 1516 to 1555, where the values drop in the 95- (Lauenen)2003 and is thequite 98- large (Tyrol) consideri percentile,ng that measured albeit AMJJthe Supplement)temperature anomalies and Biel-Bienne from the (Table1901- S1, Sc8, in the Sup- not quite indicatingto2000 record-breaking r = average 0.19. This in 2003 istemperatures. a wereconsequence 2.9 ºC in Accord- ofHISTALP wrong values (Auerplement). etin al.,1522 2007) Heat and a1523 becamend 2.7 and ºCunbearable inthe Basel questionable (Begert from early June, consid- ing to the botanist andvalueet al., politician 2005).for 1540 Fourth Renward (Fig. in5). the The Cysat ranking overall in Lucerne,is correlatithe year on1865 ofering (+2.830.76 theobtained °C), fact followed that with quarrymen averaged by 1868 AMJJ(+2.76 in Besançon Swiss °C), (France) got time dew was abundant enough north of the Alps to substantially off from hard physical work. (Table S1, Sc9, in the Supple- documentary temperature indices (Dobrovolny et al., 2010) is relatively low which might be dampen drought effects1718 (Table (+2.67 S1,°C), Sc7, 1473 in (+2.54 the Supplement). °C), 1556 (+2.5 °C),ment). 1616 Four(+2.26 independent °C), 1794 (+2.22 contemporary °C), 1834 chroniclers describ- Detailed qualitativedue(+2.21 descriptionsto an°C), accumulation 1536 and of weather1945 of errors (+2.17 patterns involved °C) as andwell with as estimating 1846ing (+2.1 the temperatures situation °C) and 1471 in for vine-growing (+2.07 individual °C). Coldestmonths. areas of Switzerland and their impacts on human,Correlationsmean ecological March-July with and temperaturestree physical ring (MXD) systems,appeared series be-in are 1542 all Alsace(-2.63significant, °C) agree and albeit that1698 on it (-2. didvery59 not °C).different rain They a droplevels. were between 23 June and ing widespread in a European scale, provide the most con- 6 August (Table S1, Sc2, Sc10, Sc11, Sc18, in the Supple- Thefollowed series by presented 1628 (-2.39 by Battipaglia°C), 1675 et(-2.32 al. (2010) °C) andaverages 1692 MXD(-2.02 data°C). forReconstructed Larch from vincing arguments supporting the temperature estimates ob- ment). The heat wave probably peaked in late July and early tained from the newLötschental Swiss series. (Canton The Valais) drought as andwell heatas for in Pine fromAugust. Lauenen In Besançon (Bernese Oberland) people used and taking Tyrol refuge in cellars af- 1540 began earlier(Austria). than in 2003,Considering it was the more inclusion intense of Tyrol and situatedter 09:00some a.m.distance LT from because the Western they could Swiss not stand the heat in it lasted much longer,Alps, namely the correlation more than of 0.55 10 months. is surprisingly High high.the On streets the other during hand, the correlations day (Table with S1, the Sc12, in the Supple- temperatures already prevailed in the long rainless period ment). On 2 August, the town council of Ulm ordered the in spring 1540 accordingother two to series a report from about the Swiss full flowering Alps are surprisinglyparsons low preaching which “aboutwould warrant the hot further and dry weather, begging of cherry trees oninspection around 10 (see April Table in 2). Ancy-sur-Moselle Correlations with tree ringGod based for rain” temperature (Table series S1, Sc5, derived in thefrom Supplement). At that (265 m a.s.l.) and cherriesother regions being in already Europe ripe far fr aom month Switzerland later aretime, very widespreadclose to zero (self-)ignition(e.g. Finland, Sweden; of forests and grassland is at the same place (de Bouteiller, 1881). All vines had fin- indicated by Swiss, Alsatian and German chroniclers (Ta- Esper et al., 2012; or Albania; Seim et al., 2012 etc.). This might be caused by distance-, the ished blossom on 10 June in Winterthur (Table S1, Sc3, in ble S1, Sc11, Sc13, Sc14, Sc15, in the Supplement), which is use of another type of proxy-, different time windows of reconstructed temperatures (AMJJ

Clim. Past, 9, 41–56vs., 2013JJA or MAMJJAS etc.) as well as by differing climatic conditions which taken www.clim-past.net/9/41/2013/ together may aggregate possible errors. An in depth analysis of the reason for the low correlations would be interesting but is beyond the scope of this paper. The key point of this analysis involves comparing spring-summer temperatures in 2003 and 1540. It should be borne in mind that the comparison involves AMJJ temperatures only. As A Grape-Temp-18-5 12.12.2012 23 therein, Fischer et al., 2007). There is consensus that soil moisture- temperature interactions were a key driver in the sequence of events that led to the exceptional heat-wave in early August, 2003. In temperate climates, a considerable part of incoming shortwave radiation is generally used for evapotranspiration, i.e. for driving the so-called latent heat flux. The remaining sensible heat flux ultimately impacts air temperature. In case of an initial strong soil moisture anomaly which may occur after a dry spring, the share of sensible heat increases O. Wetter and C. Pfister: An underestimated record breakingwith event the higher position of the sun in early summer leading to higher air temperatures 51 (see relationship A in Figure 6). not known for Western and Central Europe in 2003. In central Portugal, where forest fires were rampant at this time in 2003, the temperature anomalies reached values higher than 9 ◦C (Garc´ıa-Herrera et al., 2010). These observations suggest that the temperature excess estimated for 1540 in comparison to 2003 mainly needs to be credited to heat waves in April–May and July, which, considering the positive temperature anomalies of 0 ◦C in April, Fig. 6. Processes contributing to soil moisture–temperature coupling and feedback loop (Seneviratne et al., 2010) Fig. 6. Processes contributing to soil moisture–temperature cou- 1.1 ◦C in May and 2.9 ◦C in July from the 1901–2000 average pling and feedback loop (Seneviratne et al., 2010). measured in Basel (in 2003), were probably more extreme Relationship (B) relates to the link between evapotranspiration and sensible heat flux. in 1540. A valid comparison with June (6.8 ◦C) in 2003 is Decreased evapotranspiration leads to an increase in sensible heat flux and thus to an increase not possible. The fact that not a drop of rain fell during the in air temperature. Relationship (C) relates to a potential positive feedback leading to a further temperature increase: Increased temperature leads to a higher evaporative demand, and thus to entire month in 1540 suggests that temperatures may have autumn 2006 and winter 2007 were likely the highest for a potential increase in evapotranspiration despite the dry conditions, possibly leading to a been at about the same level as in 2003 (all values according more than 500 yr (Luterbacher et al., 2007). According to the further decrease in soil moisture. The feedback loop inducing land-atmosphere coupling can to Begert et al., 2005). As previously mentioned, tempera- continuedata until underlying the total drying the of reconstructionthe soil, when temperature of monthly increases cannot temperatures be dampened by tures in August cannot be assessed from GHD. Chronicler anyin further central increases Europe in eva onpotranspiration. the basis of(Seneviratne Pfister indices,et al., 2010). the In year2011, 1540the pre- Malachias Tschamser notices that the longest and most se- conditionswas the for warmest record breaking since temperature 1500 (Dobrovoln in summer werey´ etmetal., after 2010). an intense spring vere heat wave in Alsace occurred in the 32 days from 10 July drought,The but conditionsseveral waves of under heavy whichrainfall intemperatures June may have inhibited rise tothe recordtrigger of to 10 August (Table S1, Sc15, in the Supplement). Chroni- feedbacksbreaking (Quesada levels et al.,were 2012). In intensively 2003, spring drought investigated was again inte afternse with the precipitation 2003 cler Hans Salat confirms this observation mentioning that a betweenevent February (see theand May review being below by Seneviratne 50%. But this time, et the al., spring 2010, drought and gave refer- way to rain spell began on 11 August in Lucerne (Table S1, Sc16, consecutiveences therein, episodes of Fischer intensive anticyclonic et al., 2007). anomalies There in the summer is consensus months associated that in the Supplement). These observations suggest that the ex- withsoil stationary moisture–temperature blocking, clear skies, high interactions temperatures and were high aevaporation key driver amplifying in treme heat spell in 1540 culminated a couple of days earlier temperaturesthe sequence through ofthe feedb eventsack processes that led described to the above exceptional (Garcia Herrera heat et al., wave 2010). than in 2003. Temperatures from 11 to 31 August 1540 might Sensitivityin early analyses August, suggest 2003. that Ingiven temperate climatologic climates, mean soil amoisture considerable and similar have been at about the same level as in 2003 considering the continental-scalepart of incoming circulation, shortwavethe 2003 JJA surface radiation temperature is generallyanomalies would used havefor been observation of a second bloom of fruit trees in early Septem- evapotranspiration, i.e. for driving the so-called latent heat ber 1540 in Guebwiller (Alsace) (Stolz, 1979) which reflects flux. The remaining sensible heat flux ultimately impacts similar observations being made in Munich at the same time air temperature. In case of an initial strong soil moisture in 2003 (http://de.wikipedia.org/wiki/Hitzewelle 2003; last anomaly which may occur after a dry spring, the share of access: 3 September 2012). Based on observations by vine- sensible heat increases with the higher position of the sun in growers of a second flowering of vines on 9 October and early summer leading to higher air temperatures (see rela- cherries reaching maturity for a second time in Lindau on the tionship A in Fig. 6). shore of Lake Constance (Burmeister, 2008), it can be con- Relationship B relates to the link between evapotranspi- cluded that September and October were probably warmer ration and sensible heat flux. Decreased evapotranspiration than in 2003. The reports by several chroniclers (Table S1, leads to an increase in sensible heat flux and thus to an in- e.g. Sc2, Sc11, etc., in the Supplement) agree that weather crease in air temperature. Relationship C relates to a poten- was sunny and warm “like in April” until Christmas (Julian tial positive feedback leading to a further temperature in- Style), i.e. 4 January 1541, without any frost and snow cov- crease: increased temperature leads to a higher evaporative ering the ground (Table S1, Sc1, in the Supplement). At that demand, and thus to a potential increase in evapotranspira- time, several people demonstratively swam across the Rhine tion despite the dry conditions, possibly leading to a further at Schaffhausen (Canton Schaffhausen) (Table S1, Sc3, in the decrease in soil moisture. The feedback loop inducing land– Supplement). Chroniclers were eager to include such physi- atmosphere coupling can continue until the total drying of cal evidence in their narratives to demonstrate how extraordi- the soil, when temperature increases cannot be dampened narily warm it still was at the beginning of winter 1540/1541. by any further increases in evapotranspiration (Seneviratne Taking into account the preceding extreme spring–summer et al., 2010). In 2011, the pre-conditions for record break- temperature anomaly and the outstandingly warm conditions ing temperature in summer were met after an intense spring in autumn (SON) until December 1540, we assume that water drought, but several waves of heavy rainfall in June may have temperatures might have been at about 15 ◦C which is con- inhibited the trigger of feedbacks (Quesada et al., 2012). In siderably below comfortable water-temperatures for swim- 2003, spring drought was again intense with precipitation be- ming. Maximum water temperatures of the Rhine measured tween February and May being below 50 %. But this time, at this time of the year within the period 1978 to 2011 the spring drought gave way to consecutive episodes of in- were about 11 ◦C in December 2006 and about 9 ◦C in Jan- tensive anticyclonic anomalies in the summer months associ- uary 2007 (Data from Swiss Federal Office for the Environ- ated with stationary blocking, clear skies, high temperatures ment FOEN). Updated European averaged autumn and win- and high evaporation amplifying temperatures through the ter air temperature time-series indicate that temperatures for feedback processes described above (Garc´ıa-Herrera et al., www.clim-past.net/9/41/2013/ Clim. Past, 9, 41–56, 2013 52 O. Wetter and C. Pfister: An underestimated record breaking event

2010). Sensitivity analyses suggest that given climatologic The summer (JJA) 2003 was claimed to be unprecedented. mean soil moisture and similar continental-scale circulation, Evidence from GHD and tree-rings led to the conclusion that the 2003 JJA surface temperature anomalies would have been it was likely warmer than any other year since the Middle reduced by around 40 %. Thus in absence of soil moisture Ages. Although it is not possible to simply extrapolate our re- feedbacks, summer 2003 would still have been warm, but it sults for Switzerland from GHD to a wider domain, it is con- would not have been such a devastating event as it turned out cluded from a great number of coherent qualitative documen- to be (Fischer et al., 2007). tary evidence about the outstanding drought in 1540 that tem- Rainfall observations from Swiss and Alsatian chroniclers peratures were likely more extreme in large parts of Western living in the core region of the record breaking anomaly and Central Europe than in 2003. The persistent temperature in 1540 provide some clues for assessing soil moisture and precipitation anomaly in that year, described in an abun- deficits (Della-Marta et al., 2007). Some observers did not dant and coherent body of qualitative documentary evidence, only specify when, but often also how long and how in- may have severely affected the reliability of GHD and MXD tensively it rained. For example, chronicler Oswald Huber measurements on tree-rings as proxy-indicators for temper- from Schaffhausen reports just one abundant rain spell from atures. Due to the crossing of a poorly understood drought 12 February to early June (about 10 June) (Table S1, Sc3, in stress threshold it was widely observed that grapes were des- the Supplement). Hans Stolz from Guebwiller (Alsace) con- iccated at the climax of the heat wave in early August, which firms Huber’s observations specifying that between Febru- led many vine-growers to interrupt or postpone the harvest ary and 10 June it only rained for three days in mid-March, despite full grape maturity until the next rain spell. Likewise, whereas April and May were throughout sunny and very many trees were under extreme drought stress concluding warm (Stolz, 1979). It is concluded from these reports that from observations that leaves withered and fell to the ground spring drought in 1540 was far more severe than in 2003. Ob- as would typically observed in late autumn. It remains to be servations about extreme soil desiccation (Stolz, 1979) and determined by further research whether and how far this re- soil cracking (Table S1; Sc7, in the Supplement) confirm the sult obtained from local analyses can be spatially extrapo- hypothesis of a record-breaking soil moisture deficit. Some lated. cracks were so wide that people could put their feet into them Fundamental considerations regarding the estimate of (Table S1, Sc17, in the Supplement). Consecutive episodes of record-breaking extreme events in the pre-instrumental past intensive anticyclonic anomalies following the 1540 spring deal with four issues, namely (a) the critical evaluation of drought may have activated the previously described positive sources, (b) the approach to deal with past extreme events, feedback loop of rising temperatures and evaporation leading (c) the role to be played by documentary sources laid down to record-breaking temperatures within the last 500 yr. by individuals in assessing extremely rare events in the past and d) the interpretation of the 1540 extreme event in the context of global warming. 6 Summary and conclusion a. All proxies have their strengths and limitations and only Firstly, the main results of the study are briefly reviewed. if we can find a similar signal in different types of prox- Subsequently, fundamental issues regarding the approach to ies a robust assessment of past record breaking tem- be applied for reconstructing record breaking extreme events peratures can be made. GHD are a valuable documen- from documentary data are addressed. tary proxy for past summer (AMJJ) temperatures. How- A new long Swiss GHD series (1444 to 2011) was set up ever, their interpretation “should be seen as a delicate to assess spring–summer (AMJJ) temperatures; August tem- task requiring a lot of endurance and accurateness (sic)” peratures were excluded as these are not significant for the (Labbe´ and Gaveau, 2011), similar to data analysis in date of grape maturity. The calibration-verification approach the sciences. Drawing on uncritical compilations in us- using the HISTALP temperature series (Auer et al., 2007) for ing documentary evidence involves a risk of obtaining the north-western part of the Greater Alpine Area yielded the flawed results. Moreover, model building using docu- result that spring–summer (AMJJ) temperature anomalies in mentary evidence should be complemented by an in ◦ 1540 from the 20th century mean were between +4.7 C and depth interpretation of historical decision making over ◦ ◦ +6.8 C(±0.5 SEE) higher than those measured in 2003. time. From observations of a second bloom of fruit trees in early September, a second flowering of vines in October and the absence of cold spells in conjunction with extreme drought b. Extreme events are rare, which means there are few until the end of the year it is concluded that autumn (SON) data available to make assessments regarding changes was likewise warmer than in 2003. Considering the signif- in their frequency or intensity (Field et al., 2012). They icance of soil moisture deficits for the generation of record involve situations in which both human and ecological breaking heat waves, these results still need to be validated systems behave non-linearly outside the normal range with estimated seasonal precipitation. of biological and probability laws.

Clim. Past, 9, 41–56, 2013 www.clim-past.net/9/41/2013/ O. Wetter and C. Pﬁster: An underestimated record breaking event 53

c. Detailed observations provided by contemporary chron- Lorrain d’Histoire, Universite´ de Lorraine, Nancy are acknowl- iclers describing both, (bio-) physical proxy data as well edged for providing documentary evidence. Jan Esper, Depart- as the underlying meteorological conditions and the re- ment of Geography, Johannes Gutenberg University, Mainz and lated human decision making should be used to assess Ulf Buntgen,¨ Swiss Federal Research Institute for Forest, Snow the severity of record breaking extreme events and their and Landscape, WSL are acknowledged for providing dendro ev- impacts on human, ecological and physical systems. Be- idence. Thomas Herren, Bio- und umweltmeteorologische Anwen- dungen, MeteoSchweiz, Zurich¨ and Jean Laurent Spring, Agro- sides the example of 1540, this conclusion also refers to scope Changins-Wadenswil,¨ ACW, Centre de Recherche Pully are tree-ring based studies by Battipaglia et al. (2010) and acknowledged for providing grape phenological network observa- Buntgen¨ et al. (2011). As the latter authors put it, doc- tions. Sonia Seneviratne, Institut fur¨ Atmosphare¨ und Klima, ETH umentary evidence independently confirmed many of Zurich¨ is acknowledged for the allowance to include a figure. The the dendro-signals over the past millennium, and further State Archive of Basel is acknowledged for granting access to origi- provided insight on causes and consequences of ambi- nal sources and the immense willingness of the staff to help. The ex- ent weather conditions related to the reconstructed ex- pert advice of Werner Siegfried, Agroscope Changins-Wadenswil,¨ tremes. We must not play the statistical and the narrative ACW, was indispensable to properly assess the extraordinary situa- approach against each other. Rather, the two approaches tion of the vines in 1540. The Swiss Federal Office for the Environ- are complementary accounting both for the ordinary and ment (BAFU) is credited for providing monthly water temperature for the extraordinary. Subsequent analyses should fo- maxima of the Rhine at Rheinfelden from 1978 to 2011. Urs Neu, ProClim, Bern, Jurg¨ Luterbacher, Department of Geography, Justus- cus on assessing precipitation and drought severity to Liebig University Gießen and Thomas Stocker, IPCC WGI Tech- make the worst case event of 1540 and its devastating nical Support Unit University of Bern are acknowledged for their impacts more plausible and comprehensive (Wetter et important and constructive critique which helped to improve this al., 2013). Further record breaking extreme events are paper significantly. Last not least we want to thank Kathleen Pribyl, to be expected for the period prior to 1500 (Luterbacher University of Brighton, for style corrections. et al., 2007) which has so far not be systematically in- We are grateful for the many suggestions and thought-provoking vestigated. impulses of the four anonymous reviewers and the editor as well as d. The result presented in this paper that spring–summer for the comments of Pascal Yiou that – taken together – allowed a (AMJJ) temperatures in 1540 in Western Europe likely significant improvement of our paper. exceeded the amplitude of the previous hottest summer of 2003 does not challenge the notion that summer Edited by: H. Goosse 2003 can be partly related to global warming (Stott et al., 2004). It shows that even more extreme events than 2003 are documented from the pre-instrumental period References under cooler climate conditions than today. 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