Besides the climatic shift in 1988 several warm (positive) and cold (negative) fluctuations stand out: GHDs were 6.5 d earlier between 1383 and 1435 than between 1354 and 1382. These fluctuations agree with those of glacier length. The Gorner Glacier (Canton Valais, Switzerland) advance since the 1340s culminated in 1385 on its first Little Ice Age maximum, which corresponds to the position of the glacier in 1859. Then the glacier melted back to a low level, which cannot exactly be established (Holzhauser, 2010). Likewise, the GHD curve mirrors the well-known 1520–1560 and 1720–1739 warm phases as well as the cold ca. 1600, ca. 1640 and 1820–1860 phases documented through the waxing and waning of Alpine glaciers (Nussbaumer and Zumbühl, 2018). In 1520–1560 grapes were on average harvested 4 d earlier (24 September) than the mean value prior to 1988 and 6 d earlier (22 September) in the period 1720–1739. Between 1820 and 1860, in contrast, GHD occurred 4 d later (2 October) than the mean value. This phase is strongly influenced by multiple volcanic eruptions from which the climate system only recovered slowly (Brönnimann et al., 2019).

The curve is clearly divided in two parts. Grapes were on average picked on 28 September from 1354 to 1987, comprising most of the Little Ice Age and the 20th century period of slow warming. In contrast, GHDs were 13 d earlier (15 September) during the last 31-year-long period of rapid warming from 1988 to 2018 (Table 3). The main phenological phases in the development of grapes ( Vitis vinifera ) – bud break, flowering, veraison (colour change and softening of the berries) – were in step with the harvest dates (Jüstrich, 2018).

Pearson correlation coefficients are expected to be negative because the warmer a growing season is, the earlier the harvest date and the thicker the tree ring or the denser the latewood. Correlation coefficients for both sites are high and clearly significant ( p <0.05), through they are obviously not as high as with instrumental temperature because tree-ring proxies include additional noise from non-climatic influences. Correlation coefficients are robust and remain constant throughout all the tested sub-periods (Table 4).

Tree-ring-based temperature reconstructions were used to investigate the similarity with the grape harvest dates. Both have annual resolution and are influenced by the summer growing season. The closest tree-ring reconstructions are found at a distance of a few hundred kilometres from Beaune, similar to the long instrumental measurements from Paris. Nevertheless, seasonal average temperatures should be highly correlated over regions of several hundred kilometres.

Correlations between documentary-based proxy series and the Beaune GHD series turned out to be significant (Fig. 7). We focussed on the period prior to 1850, and significant Pearson correlation was found between wheat harvest dates in Norfolk and grape harvest dates in Beaune despite the considerable distance between the two locations and the different nature of the proxy. The GHDs available for Metz from 1420 to 1537 are well correlated with Beaune GHDs as well ( r =0.60). On the other hand, the coefficient is surprisingly high between the GHDs from the Czech lands and the Beaune series despite the distance of 900 km between Beaune and the region northwest of Prague.

The autocorrelation structure of the Beaune GHD-based temperature reconstruction (black in Fig. S2; models B linear and B weigthed.simulations lead to equal results) is very similar to the tree-ring reconstruction from the Pyrenees (blue) but has clearly less autocorrelation than the tree-ring reconstruction from the Lötschental, Switzerland (red). The Lötschental time series experiences much more low-frequency variability than the Beaune and Pyrenees records. However, it is hard to argue that one of them should be the correct one. All have uncertainties with regard to their low-frequency behaviour, which could be due to age detrending, temporally inconsistent tree-age distributions and other factors. On the other hand, documentary data for GHDs are also not free of issues concerning low-frequency variability. In the case of vines there may be adaptation, breeding and/or gene (de)activation processes over the decades that may dampen low-frequency variability. There may also be changes in taste altering harvest dates in both directions.

GHDs can be well reconstructed from temperature using either model F linear or F transformed (see Table 5, Fig. 8; scatterplots are shown in Fig. S1 in the Supplement). Interestingly, both models produce a better correlation (>0.8) in the evaluation period (1851–2018) than in the calibration period (1659–1850), probably due to the strong and well-reproduced trend of GHDs in the evaluation period. Given the fact that the observed temperature refers to a location more than 300 km away and is based on early instruments with presumably substantial errors, a correlation coefficient of r =0.8 over the entire period is indeed surprising.

Early GHDs are related to high-pressure anomalies centred over western or northern central Europe. High-pressure situations are accompanied by increased radiation and high temperatures. With respect to blocking, anomalies are weak over the study area (central–western Europe). Rather, blocking during early GHD years occurred more frequently over Denmark (less frequently over northern Scandinavia). In such situations the study area lies to the southwest of the block and receives dry and warm continental air masses. A similar blocking pattern is also found for the 10 earliest GHD years in the 20CR reanalysis. The year 2018 follows a similar pattern. Early harvest dates are thus related to blocking over Denmark. Late harvest dates (not shown) do not imprint significantly onto blocking.

What atmospheric conditions are conducive to early GHDs? Using the analogue approach we can analyse the April-to-July averaged 500 GPH and blocking statistics over the North Atlantic–European region for past years. Figure 9 shows GPH (anomalies in contours) and blocking (anomalies in colour, climatology in contours) fields that are consistent with the GHD of 1556, the second earliest on record after 2003. For comparison, we also show a composite of summer blocking for the 10 earliest GHDs in the period 1851 to 1980 from the 20CRv2c reanalysis (we excluded the last decades due the strong anthropogenic warming effect) relative to the average over that period. Finally, we also show blocking anomalies for summer 2018 relative to the average for 2000–2018 from the ERA5 reanalysis.

4.5 Grape ripening in extremely hot and dry years

The 33 extremely warm events comprising the fifth percentile bracket of GHDs are unevenly distributed over time (Fig. 10); 21 of them occurred between 1393 and 1719, i.e. one out of the 15 years included a hot spring–summer period. In contrast, this is the case for just 5 years between 1720 and 2002, i.e. one out of 56. Under those circumstances, the memory of outstandingly warm years faded. No wonder that the hot summer of 2003 came as a surprise. Since then 8 out of 16 spring–summer periods were outstanding according the statistics of the last 664 years, no less than 5 among them within the last 8 years. This implies that the extremes in the past have now become normal. The acceleration of extreme temperatures in the last decade went along with an increased melting back or decay of Alpine glaciers, which lost about 20 % of their remaining volume (Swiss Glaciers, 2017).

Subsequently, conditions in two outstanding years – 1540 and 1556 – are considered in more detail. First of all, it is puzzling that the exceptional heat and drought in 1540 ranks only 19th in the statistics of Beaune GHDs. Possible reasons to de-emphasise this event based on tree-ring evidence were brought forward by Büntgen et al. (2015). However, their arguments are thought to be questionable (Pfister et al., 2015). An interpretation of this paradox is attempted using vine phenological evidence available from vineyards around Biel–Bienne, Zürich and Schaffhausen (Switzerland) in 1540 and 1556 in comparison with the Beaune evidence. Source references are provided on the Euro-Climhist data platform (https://echdb.unibe.ch/, last access: 15 July 2019). Elevation matters for the comparison: the three Swiss vine-growing areas are located at altitudes of about 430 m a.s.l., i.e. about 130 to 200 m higher than those of Beaune region vineyards. Vine cultivars (CVs), i.e. varieties, need to be considered in addition to altitude. In Biel–Bienne CV Chasselas used to be grown, while in Zürich and Schaffhausen CV Räuschling was cultivated, which survives in a few vineyards today. The maturity of the Chasselas and Räuschling cultivars is 10 to 14 d earlier than that of Pinot noir grown in the Beaune area (Altwegg, 2018). Table 6 lists the main phenological grapevine stages – end of flowering, beginning of veraison (colour change and softening of the berries), and harvest dates for the years 1540, 1556, 2003 and 2018. Gladstones (2011) refers to the widely observed fact that “the date of flowering, can usually predict quite closely the dates of veraison (colour change and softening of the berries) and maturity to follow […]. The later phenological intervals show little response to temperature and tend to be constant from year to year”. His assessment is confirmed by Chuine et al. (2004) and Jüstrich (2018). In 1540, however, the interval between veraison and harvest took 67 d in the Zürich area, i.e. 27 d longer than usual, while this interval was somewhat shorter in 1556. In both years the grapes were harvested some time after full ripening. For 1540 this assumption is confirmed by the sources. Vine growers in Schaffhausen (Switzerland) were “long waiting for rain to begin the harvest”, as chronicler Oswald Huber relates. He writes, however, that they “finally tackled the work nevertheless, because the plants withered.” Likewise, vine growers at the shores of Lake Constance and in the Upper Alsace interrupted the vintage after picking a few juicy grapes because the remaining ones were somewhat dried out. The vintage was then resumed after a 2 d spell of rain. At harvest time grapes in many vineyards had become raisins. They yielded a sweet sherry-like wine which made people more rapidly drunk than usual (Wetter and Pfister, 2013). It needs to be stressed that meteorological conditions were almost matchless in 1540. This mega-drought described in documentary data offers a broad spectrum of evidence on past weather and climate and their societal impacts. It is described in a sample of 312 first-hand documentary weather reports originating from continental Europe (Wetter and Pfister, 2013). The hot and dry period in 1540 lasted from April to the end of the year. The heatwave peaked at the end of a 46 d long rainless period between 23 June and 7 August during which many forests and settlements in a large area from the Ardennes to Poland went up in flames (Pfister, 2018). Maximum temperatures from late July may have exceeded 40 ∘C (Orth et al., 2016). Vagrants and homeless people were hunted down for starting town and forest fires (Pfister, 2017). The heatwave of 1540 undoubtedly struck the region of Burgundy. The church of Notre-Dame of Beaune organised eight processions to call for rains from the beginning of May to the end of August (Labbé and Gaveau, 2011). In the near city of Besançon (ca. 100 km eastwards) a chronicler wrote that warm temperatures lasted from April to November and that the heatwave was hardly bearable by humans during summer (Wetter et al., 2014). The possibility cannot be excluded that even the date of veraison was also somewhat delayed due to drought stress. Research in grapevine biology has established that under conditions of extreme heat and drought the development of grapes is slowed down or stopped (Keller, 2016). When occurring before veraison, extreme hydric stress alters grape quality and the onset of ripening, possibility because it induces leaf defoliation and therefore carbon assimilation limitation (Basile et al., 2012; Girona et al., 2009; Ollat and Gaudillere, 1998). In fact, this phenomenon was observed during the temperature peak in the hot summer of 1947 in Schaffhausen (Amtsblatt, 1947).

In 1556 grapes in Beaune were harvested on 16 August. In central France, the 1556 heat and drought began in mid-April, i.e. about 45 d later than in 1540, following a wet winter. The heatwave to mid-June spurred vegetation growth. In the Loir-et-Cher region, a variety of red Pinot noir cultivar called Auvernat was already blossoming around 25 April (Nouel, 1878, 235). Not a drop of rain fell during this period. On 14 June, it poured down for 3 to 4 h, which visibly refreshed the vegetation. Before 10 July the first grapes were ripe. In July the ground became so hot that it burnt people's feet walking barefoot. Like in 1540 the heat and drought peaked in early August (Bourquelot, 1857, 30–31; Hiver, 1867, 81 and 90; Nouel, 1878, 235–237). Following the deliberation protocols of the chapter of Notre-Dame of Beaune, seven processions for obtaining rain were held in the city from 15 June to 15 August (Arch. Dep. Côte d'Or, G 2499).

The 1556 harvest of Chasselas grapes in Biel–Bienne was estimated to have occurred between 25 and 30 August, i.e. at about the same time as that of Pinot noir in Beaune, considering the chronicler's remark that the abundant harvest had already ended on 10 September (Gregorian style). Like in the Beaune area, the preceding winter of 1556 had been very wet considering the daily weather observations by Wolfgang Haller in Zürich. Disregarding June, which was completely rainless, the spring–summer period included seven precipitation days in April, three in May and five in July (Haller in Pfister, Rohr; https://echdb.unibe.ch/, last access: 15 October 2018). A hot and almost rainless period began on 29 July and lasted to 11 September, i.e. more than a month later than in 1540.

The heatwave in summer 2003 was reassessed by Pfister (2018). The water deficit was not sufficiently strong to “block” grape ripening through extremely limited photosynthesis. Obviously, ripening made its course quickly. Additionally, winegrowers harvested soon to maintain sufficient acidity in wines (and in some cases avoid excessively high alcohol content in wines).

Since the early 21st century, higher temperatures combined with increasing control of grape sanitary status (grey mould disease mostly) has made the ripening duration (i.e. the lag during veraison and harvest) more winemaker dependent. Winemaker choices depend on both cultivar and the style of wine. For instance, the number of days between veraison and harvest for CV Cabernet–Sauvignon has nearly been doubled in a famous Château in the appellation Margaux near Bordeaux (Van Leeuwen and Destrac-Irvine, 2017)

In sum, the decline by 13 d of the average date of GHD since 1988 went along with a large increase in the number of extreme spring to summer seasons. These include situations such as in 1540 when both human and ecological systems behaved non-linearly outside the normal range of biological and probability laws. Documentary data may be helpful to describe such conditions in the necessary detail.