Structure and Trends in Climate Parameters of Wine-Growing Regions in Slovenia

Abstract

This study examined the structure and trends of climate parameters important for grape production from 1952 to 2022 in the wine-growing regions of Podravje, Posavje, and Primorska in Slovenia. Average and extreme temperature and precipitation data from six meteorological stations in three wine-growing regions were divided into annual and growing seasons. The results show that in the period 1991–2022, there was a warming in the growing season in all regions by 1.4–1.7 °C, except the southern part of Primorska (Koper station) 0.6 °C, compared to the reference period 1961–1990. The heat accumulation indices (GDDs and HI) increased significantly, which is mainly due to the increase in the maximum temperature in the growing season temperature (GST max) and the number of days with Tmax > 30 °C (NDT30). The NDT30 increased the most, by a factor of more than four. In the reference period (1961–1990), however, the trend in the number of hot days was even slightly negative. The mean seasonal temperature rose to around 17 °C in regions with a continental climate and to around 19 °C in the Mediterranean region, which could be reflected in the earlier ripening of the grapes. The trends show a decrease in total annual precipitation (AP) after 1991, but this was significant only at one inland location (Maribor), while the total precipitation during the growing season (GSP) decreased significantly at three locations (Maribor, Bilje, and Koper).

1. Introduction

Climate is one of the most important limiting factors for agriculture production. Climate change is also challenging viticultural production everywhere, especially in regions with warm and dry climates. Frost and drought risk during the growing period are common problems in viticulture. Therefore, maintaining viticulture also requires adaptation to climate change, and the assessment of adaptation strategies needs to be more precise and multidisciplinary, tailored to local conditions [1].

For Western and Northern Europe, a temperature increase of 2.5 to 4.5 °C is predicted by the end of the 21st century and most climate models predict an increase in winter precipitation [2]. In recent years, warming trends have been observed in all seasons [3]. In the northern hemisphere, the warming will be stronger in the colder half of the year. It is expected that viticulture in many regions with cool climates will benefit from the higher temperatures in the growing season. Milder winters and warmer summers are expected, and extreme high temperatures will occur more frequently. The risk of low winter temperatures and unfavorable conditions during the flowering period is expected to be lower, while the risk of late frosts in spring will increase due to early budbreak [4,5,6].

Many researchers have studied the impact of climate change on the viability of viticulture production, examining changes in overwintering tolerance potential, frost incidence, growing season length, and heat and drought stress during the growing season [7,8,9,10,11]. The need to adapt to climate change is even greater for grapevines than for other crops, as the composition of berries, which is a key factor in fruit and wine quality, typicality, and market value, depends strongly on “terroir” (the particularities of the culture area) [12]. Spatial modeling research has shown that wine-growing regions will expand as parts of southern Europe become too hot to produce high-quality wines, and northern regions will acquire vineyard potential, as in the Middle Ages, from the 9th to the 13th century AD [13,14]. However, despite these short-term benefits, the predicted rise in global temperatures over the next half century could require major changes in the wine industry. Slight changes in temperatures during the growing season could lead to shifts in varietal suitability in many regions [15] or require costly adaptation measures both in the vineyard and in the winery.

Unfavorable weather conditions could lead to varietal changes in some wine-growing regions. Due to climate change, the growing season has been extended, which also enables longer favorable conditions for the development of the two most persistent diseases (downy and powdery mildew). In Slovenia, there is usually greater pressure from downy mildew in the continental part (Podravje and Posavje) and powdery mildew in the Mediterranean (Primorska). The earlier appearance of downy mildew in the continental part may be a direct result of more favorable temperature conditions in May and June. In the worst-case climate scenario, two more fungicide sprays will be needed compared to the current management regimes, as reported by Salinari et al. in 2006 [16]. Therefore, from this point of view, sustainable varieties (PiWi, tolerant to downy and powdery mildew) are more adaptable to these conditions than traditional ones [17]. Adaptations to higher temperatures include also changes in plant material (e.g., rootstocks, varieties, and clones) and viticultural techniques (e.g., changes in trunk height, leaf area to fruit weight ratio, and timing of pruning) so that harvest dates fall within the optimal period of late September or early October in the northern hemisphere [18].

In some regions, projections show that an increase in precipitation can in turn affect soil development by increasing the amount of water flowing through the soil [18]. Climate change would therefore have a significant impact on soil development as it would lead to the loss of very fine particles of organic matter. The tendency towards more extreme weather events (more intense rainfall), as a consequence of climate change, can increase soil erosion, especially in vineyards on steep slopes [19,20,21,22].

The climatic factors are summarized in a series of climatic indices, of which the average temperature in the growing season (GSTavg), the Huglin Index (HI), and the growing degree days (GDDs) or Winkler Index are the most used [15]. The minimum heat requirement for the growth of vines is expressed as a value of the heat sum index, i.e., as growing degree days (GDDs) from April to October in the northern hemisphere, at a base temperature of 10 °C [15]. Becker [23] established a minimum GDDs of 1000 °C units, but later studies found a minimum value of 850 °C units [24,25]. After 1990, the developmental stages of the grapevine, such as budbreak, flowering, and ripening, occurred earlier on average than in the 1980s [26,27,28,29].

Slovenia is a very small wine-growing region in Central Europe with different climates (Mediterranean, Continental, and Pannonian climates). Despite the fact that Slovenia is not a globally important wine-growing country, most of the globally important grape varieties for quality wines are grown there. The aim of this research was to investigate the changes in temperature and precipitation as well as some bioclimatic indices based on which adaptation strategies to climate change can be recommended in the future, even for neighboring wine-growing regions with a similar climate.

2. Data and Methods

2.1. Study Area and Climate Data

For this study, the longest available data series from 6 meteorological stations (Maribor, Murska Sobota, Novo Mesto, Črnomelj, Bilje, and Koper) in three wine-growing regions (Podravje, Posavje, and Primorska) in Slovenia were used (Figure 1). The Podravje wine-growing region lies between the river Sava (SW) and the Hungarian border (NE). Geologically, the area is part of the former basin of the Pannonian Sea, which consists of folded and poorly interlocked marine sediments from the Neogene and has a Pannonian continental transitional climate [30]. The continental climate characteristics increase with increasing distance from the Alps. The vineyards (6000 ha) are predominantly planted with white grape varieties and are located on steep slopes with a gradient of 15–50% and at altitudes of 250 to 350 m [31]. The climate in the Posavje wine-growing region (SE) is also continental, with large seasonal temperature differences, cold winters, and moderately warm summers. The vineyards (3000 ha) are planted 50% with white and 50% with red grape varieties. In both wine-growing regions, half of the vineyards are on steep slopes with a gradient of 15 to 30%, and a quarter of the area has a gradient of more than 30% [31]. The Primorska (SW) wine-growing region lies along the Italian border, from the Adriatic to the Alps, and the general climate is sub-Mediterranean [30], characterized by an average annual minimum temperature of over 0 °C and a temperature in the warmest month of over 20 °C. Half of the vineyards are located on slopes with a gradient of up to 15%. The long-term average (1952–2022) of precipitation during the growing season (1 April–31 October) varies between 612 mm (Koper) and 870 mm (Bilje), and precipitation is very unevenly distributed throughout the year. The vineyards (6500 ha) are planted with 60% white and 40% red grape varieties [31].

The daily precipitation and temperature values (mean, maximum, and minimum) from six meteorological stations (1952–2022) were used for the analysis. The data were taken from the archives of the Slovenian Environment Agency (ARSO) [32]. All stations had sufficient records for a long-term analysis. The data provide a good reference for the general structure and trends of temperature and precipitation.

2.2. Climate Parameters and Bioclimatic Indices

An analysis of the observed climate was carried out for the periods 1952–2022, 1961–1990 (reference period for the 20th century with a minor change in climate parameters), and 1991–2022. The data from the individual stations were categorized according to growing seasons or important grapevine growing seasons and used to derive bioclimatic indices and extreme climate indices that are important for wine grape production (

Table 1. Analyzed bioclimatic parameters.

Parameter	Parameter Description
Tavg	Average annual temperature, °C
Tmax	Average annual maximum temperature, °C
Tmin	Average annual minimum temperature, °C
GSTavg	Average growing season temperature (April to October), °C
GSTmax	Average growing season maximum temperature (April–October), °C
GSTmin	Average growing season minimum temperature (April–October), °C
HI	Huglin Index (April to September), °C units
GDDs	Growing degree days (sum of temperature above 10 °C), °C units
TMJ	Average temperature in May and June, °C
NDTN20	Tropical nights: number of days with TN > 20 °C days
NDT25	Number of days with maximum temperature > 25 °C
NDT30	Number of days with maximum temperature > 30 °C
NDT35	Number of days with maximum temperature > 35 °C
NDF	Number of days with minimum temperature <0 °C (frost occurrence)
NDFF	Number of days between last and first frost (frost-free period length)
NDTN-2.5	Moderate cold days: number of days with TN < −2.5 °C days
NDTN-10	Extreme cold days: number of days with TN < −10 °C days
AP	Total annual precipitation, mm/m2
GSP	Total growing season precipitation (April to October), mm/m2

). For the growing season (April–October), precipitation and temperature (average, minimum, and maximum) of each station were summarized, as the averages of the growing season usually correlate significantly with wine varieties and type of wine production [15]. To assess the signs of heat stress, the number of days with temperatures above 30 °C was determined [9]. This temperature leads to premature ripening of the grapes (shorter growing season), lower total acidity, and lower aroma compounds [33].

To obtain more information about the wine region and to determine general guidelines for the potential quality and style of the wine, the GDDs [34] and the Huglin Index (HI) [35] were calculated. These two bioclimatic indices make it possible to classify wine-growing regions according to the sum of the temperatures required for the development of the vines and the ripening of the grapes [36].

For the northern hemisphere, the Huglin Index [35] is calculated using the following formula:

$$HI=\sum _{01.04}^{30.09}d\cdot \left[{\displaystyle \frac{\left(Tavg-10\right)+\left(Tmax-10\right)}{2}}\right],$$

where Tavg is the daily mean air temperature (°C), Tmax is the daily maximum air temperature (°C), and d is the day length coefficient, ranging from 1.02 and 1.06 between the 40° and 50° of northern latitude. Baseline temperature = 10 °C. This index enables viticultural regions to be classified in terms of the sum of the temperatures required for the development of the vines and the ripening of the grapes. HI climatic ripening groups: very cold—HI-3 (HI ≤ 1500), cold—HI-2 (1500 < HI ≤ 1800), temperate—HI-1 (1800 < HI ≤ 2100), temperate–warm—HI+1 (2100 < HI ≤ 2400), warm—HI+2 (2400 < HI ≤ 3000), and very warm—HI+3 (3000 < HI) [35].

Growing degree days (GDD) [34] were calculated for 1 April to 31 October by summing the daily average temperatures (Tavg) above a base value of 10 °C, where values below 10 °C are set to zero:

$$\mathrm{G}\mathrm{D}\mathrm{D}=\sum _{01.04}^{30.10}\mathrm{m}\mathrm{a}\mathrm{x}\left[(\mathrm{T}\mathrm{a}\mathrm{v}\mathrm{g}-10)\right].$$

The GDD climatic ripening groups are as follows: Region I—very cold (≤1390), Region II—cold (1391–1670), Region III—warm (1671–1940), Region IV—hot (1941–2220), and Region V—very hot (≥2220).

Some indices for climate extremes were also calculated to determine changes in extreme temperatures. These indices are recommended by the WMO (World Meteorological Organization) and are currently being investigated by researchers [37]. The average temperatures in the period May–June (TMJ) were also calculated by Vršič et al., 2024 [38]. This parameter is important for predicting an increase in disease pressure (e.g., downy mildew), as more severe epidemics can be a direct result of more favorable temperature conditions in May and June [16]. Of the other extreme indices in Table 1, NDTN20, NDT25, NDT35, NDTN-2.5, and NDTN-10 were calculated for the first time for our wine-growing regions. NDTN20 and NDF indices were estimated annually as recommended by the ETCCDI (Expert Team on Climate Change Detection and Indices).

The variables were analyzed using descriptive statistics and trend analyses. As some of the parameters examined in this study were not normally distributed, a more stringent non-parametric Mann–Kendall trend test (MK test) with a significance level of 95% was used for all series [39]. The Mann–Kendall test, like other distribution-free or parametric tests, is very sensitive to an autocorrelation effect (persistence).

3. Results and Discussion

3.1. Climatic Structure in the Wine-Growing Regions of SLOVENIA

The general climate for the period 1952–2022 for the inland wine-growing regions of Podravje and Posavje is temperate continental, characterized by significant seasonal temperature fluctuations, cold winters, and moderately hot summers with an average annual temperature of 10.3 °C (5.7 to 15.5 °C) for Maribor, 9.9 °C (4.8 to 15.3 °C) for Murska Sobota, 10.2 °C (5.4 to 15.7 °C) for Novo Mesto, and 10.8 °C (5.6 to 16.4 °C) for Črnomelj, with annual precipitation of 998 mm, 801 mm, 1130 mm, and 1281 mm, respectively (

Table 2. Mean and trend of bioclimatic parameters (standard and tested) for the 6 meteorological stations (in 3 wine-growing regions) Maribor and Murska Sobota (Podravje), Črnomelj and Novo Mesto (Posavje), and Bilje and Koper (Primorska) in Slovenia for the long-term period 1952–2022. Bold numbers indicate significant trends (p ≤ 0.05).

Wine-Growing Region/Station	Podravje/Maribor					Posavje/Novo Mesto					Primorska/Bilje
	Variable					Variable					Variable
Parameters	Mean	SD	Trend yr−1	MK-Test	p	Mean	SD	Trend yr−1	MK-Test	p	Mean	SD	Trend yr−1	MK-Test	p
Tavg	10.3	0.99	0.037	0.628	0.001	10.2	1.00	0.051	0.603	0.001	12.6	0.79	0.017	0.299	0.001
Tmax	15.5	1.14	0.038	0.542	0.001	15.7	1.19	0.051	0.440	0.001	18.5	0.82	0.021	0.367	0.060
Tmin	5.7	1.06	0.042	0.66	0.001	5.4	1.10	0.053	0.672	0.001	7.3	1.03	0.021	0.305	0.001
GSTavg	15.8	0.99	0.037	0.589	0.001	15.6	1.0	0.039	0.565	0.001	17.5	0.93	0.023	0.328	0.534
GSTmax	21.5	1.15	0.038	0.491	0.001	21.8	1.3	0.034	0.371	0.001	24.0	0.97	0.024	0.315	0.001
GSTmin	10.6	1.02	0.041	0.615	0.001	10.0	1.1	0.045	0.663	0.001	11.6	1.22	0.031	0.357	0.124
HI	1839	206	7.03	0.559	0.001	1849	218	8.76	0.462	0.001	2197	188	6.18	0.386	0.020
GDDs	1325	186	6.88	0.599	0.001	1291	192	8.35	0.573	0.001	1631	191	4.77	0.338	0.305
TMJ	17.0	1.28	0.038	0.48	0.001	16.73	1.31	0.040	0.478	0.001	18.26	1.25	0.031	0.365	0.001
NDTN20	1.55	2.54	0.063	0.444	0.001	0.45	1.04	0.023	0.412	0.001	3.04	3.75	0.069	0.297	0.001
NDT25	63.5	17.6	0.63	0.549	0.001	68.07	18.9	0.52	0.387	0.001	95.2	15.6	0.29	0.236	0.004
NDT30	13.3	11.8	0.57	0.502	0.001	16	14.2	0.39	0.390	0.021	31.0	15.6	0.45	0.383	0.030
NDT35	0.75	1.64	0.025	0.275	0.003	1.31	3.29	0.055	0.263	0.005	2.14	3.70	0.10	0.465	0.001
NDF	95	19	−0.56	−0.411	0.001	100	19	−0.81	−0.422	0.002	67	18.4	0.03	−0.008	0.927
NDFF	206	22	0.53	0.340	0.310	197	22	0.55	0.345	0.247	220	28.9	−0.20	−0.069	0.401
NDTN-2.5	54.9	16.1	−0.49	−0.411	0.001	54.51	16.1	−0.44	−0.394	0.001	36.2	14.7	0.045	0.0339	0.684
NDTN-10	8.2	7.77	−0.22	−0.447	0.001	9.06	7.76	−0.20	−0.448	0.001	1.39	2.39	0.014	−0.041	0.659
AP	998	150	−2.88	−0.252	0.002	1130	190	0.16	−0.058	0.481	1424	289	−2.78	−0.139	0.087
GSP	700	124	−1.68	−0.214	0.008	757	146	−0.18	−0.113	0.165	870	209	−2.83	−0.186	0.022
	Podravje/Murska Sobota					Posavje/Črnomelj					Primorska/Koper
Tavg	9.9	0.99	0.034	0.565	0.001	10.8	0.97	0.028	0.484	0.001	13.8	0.64	0.013	0.259	0.001
Tmax	15.3	1.16	0.036	0.520	0.001	16.4	1.13	0.030	0.400	0.010	18.1	1.31	0.040	0.406	0.006
Tmin	4.8	1.05	0.038	0.599	0.001	5.6	0.92	0.025	0.405	0.007	9.9	0.91	−0.008	−0.123	0.131
GSTavg	15.5	1.0	0.033	0.511	0.001	16.3	1.0	0.034	0.452	0.001	18.6	0.86	0.020	0.283	0.001
GSTmax	21.6	1.2	0.038	0.446	0.001	22.6	1.2	0.022	0.263	0.001	23.2	1.61	0.042	0.404	0.006
GSTmin	9.7	1.0	0.037	0.576	0.001	10.3	1.0	0.033	0.466	0.001	14.2	0.93	−0.007	−0.106	0.192
HI	1831	213	6.43	0.478	0.001	1985	209	9.37	0.366	0.064	2227	236	7.44	0.412	0.004
GDDs	1278	186	6.21	0.532	0.001	1420	198	5.80	0.464	0.001	1855	180	4.18	0.277	0.001
TMJ	16.8	1.25	0.037	0.475	0.001	17.44	1.33	0.035	0.388	0.001	18.99	1.23	0.025	0.35	0.001
NDTN20	0.49	0.88	0.021	0.429	0.001	1.13	2.25	0.061	0.446	0.001	17.8	12.1	0.033	0.586	0.558
NDT25	64.8	17.4	0.57	0.495	0.001	79.3	17.5	0.31	0.243	0.003	84.2	25.1	0.74	0.376	0.001
NDT30	14.1	12.1	0.37	0.447	0.001	22.2	13.9	0.35	0.282	0.001	22.2	20.7	0.70	0.437	0.001
NDT35	0.83	2.04	0.023	0.209	0.027	1.8	3.38	0.064	0.253	0.005	1.37	3.29	0.08	0.413	0.001
NDF	110	18	−0.43	−0.374	0.050	100	16.7	−0.76	−0.138	0.092	29	17.6	0.24	0.225	0.006
NDFF	188	18	0.41	0.359	0.111	193	20.3	0.41	0.265	0.001	264	31	−0.38	−0.181	0.039
NDTN-2.5	64.9	15.5	−0.364	−0.313	0.001	59.6	13.4	−0.15	−0.123	0.135	9.75	8.67	0.04	0.083	0.313
NDTN-10	13.0	10.0	−0.264	−0.417	0.001	10.9	7.43	−0.21	−0.272	0.001	0.11	0.43	−0.003	−0.151	0.117
AP	801	112	−0.17	0.021	0.800	1281	184	0.59	0.051	0.532	995	185	−1.83	−0.133	0.101
GSP	574	94	0.28	0.037	0.655	803	165	0.62	0.057	0.487	612	152	−1.85	−0.208	0.010

Parameter abbreviations are in Table 1.

). The Primorska climate is a sub-Mediterranean climate characterized by an average annual temperature of 12.6 °C (7.3 to 18.5 °C) for Bilje and 13.8 °C (9.9 to 18.1 °C) for Koper (near the Adriatic Sea) with a total annual precipitation of 1424 mm for Bilje and 995 mm for Koper.

For wine grape maturity potential, the location temperatures range between 15.5 and 15.8 °C (Podravje), 15.6 and 16.3 °C (Posavje), and 17.5 and 18.6 °C (Primorska) based on the long-term average temperatures of the growing season (GSTavg) (Table 2) [40]. The variability in temperature in the growing season (GSTavg and GSTmin) is similar at all locations, while GSTmax is more pronounced in the coastal location (Koper). The GSTmax temperatures are as follows: 21.5 °C for Maribor, 21.6 °C for Murska Sobota, 21.8 °C for Novo Mesto, 22.6 °C for Črnomelj, 24.0 °C for Bilje, and 23.2 °C for Koper; moreover, the GSTmin values are 10.6, 9.7, 10.0, 10.3, 11.6, and 14.2 °C, respectively.

The number of days with temperatures < 0 °C (NDF) was highest in Podravje (110 d in Murska Sobota) and the lowest in Primorska (29 d in Koper). The lowest number of days with moderately cold days (NDTN-2.5) and the number of days with extremely cold days (NDTN-10) was in the Podravje wine-growing region, followed by the Posavje, while the differences were not significant in the Primorska wine-growing region. The frost-free period (NDFF) was the longest for Primorska averaging 220 d (Bilje) to 264 d (Koper), followed by Podravje with 188-206 d, and Posavje with 193-197 d (Table 2). The number of days during the growing season with temperatures > 30 °C (NDT30) also follows the pattern from inland to the coast, with Podravje and Posavje having fewer days overall, around 13–14 days and 16–22 days, respectively, while Primorska has 22–31 days with maximum temperatures > 30 °C per year. The same pattern of increase is shown by the number of days with temperatures above 25 °C (NDT25), the days with a maximum temperature of more than 35 °C (NDT35), the number of tropical nights (NDTN20), and the average temperature in the period May to June (TMJ) (Table 2).

The values of growing degree days (GDDs) for Podravje, Posavje, and Primorska are between 1278 and 1325, between 1291 and 1420, and between 1631 and 1855 units, respectively (Table 2). These values place Podravje and Posavje in Winkler Region II (cool), which indicates a generally favorable climate for the production of quality wines [34]. The values for Primorska place it in Winkler Region II-III (cool to warm), which is also favorable for the production of quality wines but can be affected by excessive heat in this region, especially in the last two decades. In this region, the NDT30 value has doubled compared to the reference period 1961–1990 (

Table 3. Averages of bioclimatic parameters for the six meteorological stations (Maribor, Murska Sobota, Novo Mesto, Črnomelj, Bilje, and Koper) in three wine–growing regions (Podravje *, Posavje **, and Primorska ***) in Slovenia for the period 1952–2022, the reference period 1961–1990, the period 1991–2022, and by decades for the 1991–2022 period.

Periods/Parameters	GSTavg ± SD		GSP ± SD		GDDs ± SD		HI ± SD		T > 30 °C ± SD
MARIBOR *
1952–2022	15.8	± 0.99	700.5	± 124.3	1324.9	± 186.5	1839.0	± 206.6	13.2	± 11.8
1961–1990	15.2	± 0.59	725.4	± 116.5	1205.5	± 108.0	1704.5	± 120.0	5.8	± 3.8
1991–2022	16.6	± 0.80	669.2	± 128.2	1496.5	± 155.9	2017.1	± 199.8	21.9	± 12.7
1991–2000	16.2	± 0.75	738.9	± 142.1	1415.8	± 130.2	1914.0	± 163.4	13.5	± 9.9
2001–2010	16.7	± 0.48	692.9	± 97.4	1496.5	± 121.9	1996.9	± 169.2	21.8	± 13.2
2011–2022	17.0	± 0.57	591.3	± 128.8	1540.5	± 106.4	2119.7	± 145.5	28.5	± 8.9
MURSKA SOBOTA *
1952–2022	15.5	± 1.00	574.5	± 94.1	1277.9	± 186.3	1831.2	± 212.8	14.1	± 12.1
1961–1990	14.8	± 0.65	577.9	± 100.3	1142.9	± 111.1	1686.3	± 125.4	6.3	± 4.6
1991–2022	16.3	± 0.87	578.2	± 93.6	1432.3	± 168.5	2002.9	± 201.6	23.0	± 12.6
1991–2000	15.9	± 0.75	571.4	± 138.7	1364.3	± 125.7	1914.7	± 164.8	17.3	± 12.6
2001–2010	16.3	± 0.47	581.4	± 88.7	1414.4	± 119.3	1985.3	± 164.8	23.9	± 12.4
2011–2022	16.8	± 0.59	581.3	± 74.4	1518.5	± 107.1	2091.1	± 151.7	27.0	± 10.5
NOVO MESTO **
1952–2022	15.6	± 1.04	757.5	± 146.3	1291.2	± 193.8	1849.2	± 218.5	16.0	± 14.2
1961–1990	14.8	± 0.61	771.1	± 127.4	1146.4	± 109.2	1693.7	± 112.8	7.3	± 4.1
1991–2022	16.5	± 0.88	734.2	± 160.3	1450.2	± 161.4	2013.7	± 215.1	25.4	± 16.0
1991–2000	16.0	± 0.77	778.3	± 135.4	1374.4	± 127.2	1901.2	± 154.6	15.5	± 10.5
2001–2010	16.4	± 0.50	772.2	± 156.8	1443.4	± 122.8	1971.0	± 171.1	22.9	± 12.8
2011–2022	17.0	± 0.51	665.8	± 207.7	1538.9	± 90.0	2143.1	± 181.5	35.8	± 16.8
ČRNOMELJ **
1952–2022	16.3	± 1.03	802.6	± 165.5	1419.6	± 208.2	1984.6	± 208.7	22.2	± 13.9
1961–1990	15.6	± 0.60	793.9	± 181.8	1279.0	± 111.2	1834.5	± 127.3	12.9	± 7.2
1991–2022	17.1	± 1.07	789.5	± 205.5	1579.5	± 208.2	2137.8	± 223.8	30.5	± 15.5
1991–2000	16.5	± 10.8	816.6	± 129.0	1467.3	± 191.1	2003.5	± 189.6	17.9	± 11.9
2001–2010	17.1	± 059	781.9	± 153.7	1581.1	± 137.8	2133.7	± 172.6	30.8	± 11.8
2011–2022	17.7	± 0.58	773.3	± 323.8	1696.1	± 115.2	2253.3	± 133.8	40.7	± 11.1
BILJE ***
1952–2022	17.5	± 0.93	869.6	± 209.0	1631.3	± 190.8	2196.7	± 187.8	31.0	± 15.5
1961–1990	16.8	± 0.66	890.4	± 194.8	1479.0	± 133.9	2062.3	± 121.0	22.0	± 8.0
1991–2022	18.2	± 0.98	842.7	± 218.0	1753.2	± 199.0	2327.0	± 200.9	41.2	± 16.9
1991–2000	17.6	± 0.60	1026.9	± 220.2	1648.0	± 116.2	2190.6	± 123.3	29.6	± 9.8
2001–2010	18.1	± 0.47	758.4	± 219.4	1752.0	± 116.0	2313.9	± 162.3	38.6	± 14.6
2011–2022	18.7	± 0.64	759.3	± 195.5	1877.3	± 126.3	2451.5	± 118.3	53.1	± 14.6
KOPER ***
1952–2022	18.6	± 0.86	612.7	± 152.4	1854.6	± 181.4	2227.4	± 236.3	22.2	± 20.7
1961–1990	18.3	± 0.60	653.8	± 165.7	1794.1	± 125.1	2068.3	± 138.3	8.2	± 7.9
1991–2022	18.9	± 0.99	573.5	± 155.6	1904.7	± 205.4	2399.6	± 260.2	38.2	± 23.3
1991–2000	18.2	± 0.62	598.8	± 126.3	1774.5	± 124.2	2225.7	± 144.2	22.8	± 13.7
2001–2010	18.6	± 0.50	574.7	± 171.8	1854.1	± 112.4	2343.1	± 144.4	33.7	± 14.8
2011–2022	19.9	± 1.07	551.4	± 147.7	2115.1	± 221.2	2591.7	± 204.3	54.8	± 19.1

GSTavg—average growing season temperature; GSP—total growing season precipitation; GDD—growing degree days (1 April to 31 October) °C units; HI—Huglin Index °C units; T > 30 °C—number of days with maximum temperature > 30 °C; and SD—Pearson’s standard deviation.

).

The average values of the Huglin Index, which is possibly more suitable than the Winkler Index for European regions [36], were for Podravje, Posavje, and Primorska between 1831 and 1839, between 1849 and 1985 (HI-1), and between 2197 and 2227 (HI+1), respectively (Table 2). These values assign Podravje and Posavje to the cool climate type, which is suitable for Chardonnay, Sauvignon Blanc, and Pinot Noir, for example, while the values for Primorska are assigned to the warm climate type according to Huglin and are more suitable for Cabernet Sauvignon and Merlot [35].

Growing season precipitation (GSP) values generally show that rainfall amounts decreased slightly throughout the period (1952–2022), although the trends were not significant for all locations. GSP differed significantly between stations, increasing from Murska Sobota (Podravje), 574 mm, towards Bilje (Primorska), 870 mm, per vintage (Table 2). The variability in the GSP in this long-term period shows a variation of 16 to 25% between years at all locations. The Murska Sobota location was drier (influence of the Pannonian climate) with a total GSP amount of 574 mm than Maribor, 700 mm, and both locations in Posavje (Novo Mesto, 757 mm, and Črnomelj, 803 mm), and then Bilje (Primorska), 870 mm (Table 3). A similar amount of GSP as in the eastern part of the Podravje wine-growing region (Murska Sobota) was also measured in the coastal area of the Primorska wine-growing region (Koper 613 mm) (Table 3). However, drier conditions with more frequent and longer dry spells were more likely, as higher temperatures can probably lead to a higher evapotranspiration rate, as also noted by Ramos et al. [41]. This is particularly pronounced in the Primorska wine-growing region, especially at the Bilje location, which has the highest average annual precipitation of all six locations at 1424 mm. However, precipitation at this location falls in the form of very intense short-term showers (due to the mixture of Mediterranean and Alpine climate) with increasingly intense dry periods, as the highest number of days with maximum temperatures > 30 °C (NDT30) is recorded here. During the growing season, the annual precipitation falls around 70%, 67–77%, and 60% in Podravje, Posavje, and Primorska, respectively.

3.2. Temperature Parameter Trends

The annual trends of the individual temperature parameters for 71 years (1952–2022) for the wine-growing regions in Slovenia are shown in Table 2. An increase in the average annual temperature (Tavg) for the period 1952–2022 ranged from 0.13 (Koper) to 0.51 °C (Novo Mesto) per decade (Table 2), and the average growing season temperature (GSTavg) ranged from 0.20 (Koper) to 0.39 °C (Novo Mesto) per decade (Table 2 and Figure 2). Over the periods studied, this corresponds to a change in GSTavg of 2.6 °C in Maribor, 2.3 °C in Murska Sobota, 2.8 °C in Novo Mesto, 2.4 °C in Črnomelj, 1.6 °C in Bilje, and 1.4 in Koper (Table 2). These changes can have a major impact on wine production. Many studies have confirmed that viticulture is one of the sectors most sensitive to climate change [42,43,44,45]. Similar trends have been observed in other world’s wine-growing regions [15,46,47], and also in Central Europe. In Italy, the Venetian area experienced an increase in the average vegetation temperature (1964 to 2009) of up to 2.3 °C [40,48]. Lower warming (1.5 °C) was observed in moderate climate conditions of northern wine regions in Slovakia, which has not yet caused sufficient changes in the grapevine phenology to require serious adaptation measures [29]. Climate change led to the earlier development of phenological stages as reported by Ruml et al. (2016) [49] in Serbia and Prša et al. (2022) [50] and Omazić et al. (2024) [51] in Croatia. Some regions in Croatia are becoming less suitable for economically sustainable grape production [52]. In the Western part of the Carpathian basin (Hungary), climate change has several positive effects in the Sopron wine-growing region, this may result in the cultivation of more quality wine grapes and wines [53], which is also expected in other Hungarian regions [54]. The trend of earlier ripening of grapes was also found in northeastern Slovenia in the period 1980–2009 [55]. In Austria, based on temperature evolution, a doubling of the areas suitable for viticulture is predicted to occur by the 2050s [56].

The trends in minimum and maximum temperatures (annual and growing season) are similar at all locations, but the trends in minimum temperatures (Tmin and GSTmin) are slightly more pronounced, except at the Koper and Murska Sobota locations. This could be related to the lower humidity, as found in other wine-growing regions in the USA [57] and Europe [47]. These two locations have less rainfall than the other four. However, the average warming rates of the growing season for the six stations studied in Slovenia are determined by changes in maximum temperatures, with a significant increase in the number of days with a maximum temperature above 30 °C (NDT30), by an average of 3.5 (Črnomelj) to 7 (Koper) days per decade (Table 2). The temperature trends at the six studied locations have led to significant changes in the heat summation indices HI and GDD, whose values have increased on average from 6.2 (Bilje) to 9.4 (Črnomelj) and from 4.2 (Koper) to 8.3 (Novo Mesto) °C units per year, respectively, which for HI means from 440 to 667 and GDDs from 298 to 589 °C units in 71 years of the studied period. Heat accumulation has also increased in other European wine-growing regions, in Spain by 155–464 °C units [41] or 250–300 °C units in the last 30–50 years [47,58]. When comparing these data, two facts must be considered, namely the fact that our study period is almost two decades longer, and the fact that climate change trends are more pronounced in our area after 1990 [33], which is explained in more detail in the following paragraphs.

The trend in the number of days with temperatures < 0 °C (NDF) is decreasing in all locations in inland wine-growing regions. It is more pronounced in locations in Posavje (around 8 days per decade). The number of days between the last spring frost and the first fall frost (NDFF) is increasing (4.1 to 5.5 days per decade). At locations with a Mediterranean climate (Bilje, Koper), however, the trends are in the opposite direction, with the values for NDF increasing and for NDFF decreasing, although the NDFF trends for Bilje are not significant.

Other indices for temperature extremes (NDT25, NDT35, NDTN20, and TMJ) showed an increasing trend at all locations, with the exception of NDTN20 at the Koper location (Table 2). On average, the NDT25 increased from 2.9 d in Bilje to 7.4 d in Koper per decade and the NDT35 from 0.2 d in Murska Sobota to 1 d in Bilje per decade. The same trend can be observed in the number of days with tropical nights (NDTN20). The tendency towards an increase in the mean temperature in May and June (TMJ) was also significant for all locations (p = 0.001), namely the TMJ increased from 0.25 in Koper to 0.4 °C in Novo Mesto per decade (Table 2). More favorable temperature conditions in May and June may lead to higher disease pressure. In response to adaptation to future climate change, more attention will need to be paid to managing early downy mildew infections. Salinari et al. (2006) [16] reported that in response to adaptation to future climate change, more attention needs to be paid to the management of early powdery mildew infections. Their study found that under the most unfavorable climate scenario, two additional fungicide sprays are required compared to current management regimes. This could also be applied to neighboring wine-growing areas in the region.

A more detailed analysis of the individual periods within the long-term study period 1952–2022 shows even clearer changes. Minor changes in bioclimatic parameters were observed for the reference period (1961–1990) (Supplementary Figures S1–S3). The growing season average temperatures (GSTavg) were close to 15.0 °C for all stations in Podravje and Posavje, namely 14.8 °C for Murska Sobota and Novo Mesto, 15.2 °C for Maribor, and 15.6 °C for Črnomelj. The GSTavg values were higher in Primorska, 16.8 °C in Bilje, and 18.3 °C in Koper. The changes between 1991 and 2022 generally appear to be the most dramatic. The GSTavg values increased for all six stations and amounted to 16.6, 16.3, 16.5, 17.1, 18.2, and 18.9 °C (Table 3), with GSTavg trends for this period being 0.034, 0.040, 0.048, 0.057, 0.056, and 0.084 °C per year for Maribor, Murska Sobota, Novo Mesto, Črnomelj, Bilje, and Koper, respectively ( Supplementary Figures S1–S3). The warming was due to the changes in GSTmin and GSTmax at all locations, but the trends in GSTmax were higher, except at the Črnomelj station, where the trend in GSTmin was more pronounced than in GSTmax (Supplementary Figures S1–S3).

The number of days with maximum temperatures > 30 °C (NDT30) increased at all locations in the period 1991–2022. Compared to the reference period 1961–1990, the NDT30 increased from 3 to 15 days per decade (Supplementary Figures S1–S3). The NDT30 for Podravje, Posavje, and Primorska was 22 to 23, 25 to 30, and 38 to 41 days, respectively (Table 3), and was two to four times higher compared to the reference period (2.5 to 6.6 times higher after 2010). In the period 1991-2022, the NDT30 increased in the Primorska wine-growing region by 1.1 days per year in Bilje and by 1.5 days per year in Koer. The NDT30 increased from 30 and 23 days in the first decade of this period to 53 and 55 days after 2010 in Bilje and Koper, respectively. A similar trend can also be observed at the Črnomelj location (1 day per year). At the other three meteorological stations, Novo Mesto, Maribor, and Murska Sobota, the trend was less pronounced; in Murska Sobota it amounted to 0.3 days per year. In the reference period (1961–1990), the NDT30 trend was even slightly negative.

If the warming trend continues over the next 30 years at the same rate as it has since the 1990s, it is to be expected that the wine-growing regions of Podravje and Posavje will also move completely into the warm climate group. Daily maximum temperatures of 30 °C are critical for optimal grapevine development and can lead to plant stress, a decrease in photosynthesis, a greater lack of water, premature ripening of the grapes, and drying of the berries, even in early ripening varieties such as Bouvier in Slovenia [33]. However, a few days with temperatures above 30 °C during the ripening period can be beneficial [9,48], especially for late-ripening varieties [33]. At the Bilje location, there were already an average of 22 hot days in the reference period, which almost doubled in the period 1991–2022. At the Koper location, there were only eight such days during the reference period, and the number of NDT30 days was five times higher in the following thirty years (1991–2022).

The total warming of average temperatures in the growing season (GSTavg) was between 1.4 and 1.7 °C for all locations, except for the coastal location (Koper), where the warming was only 0.6 °C, over the respective periods (1991–2022) compared to the reference period (1961–1990). In addition, the GSTavg warming after 2010 was 1.8–2.0 °C, 2.1–2.2 °C, and 1.6–1.9 °C in Podravje, Posavje, and Primorska (Table 3). Similar results were also found in other European wine-growing regions [47] with an average warming of the growing seasons of 1.7 °C in the last 30–50 years.

The warming trends in the period between 1991 and 2022 are also confirmed by the increase in the heat sum indices. The growing degree days (GDDs), also known as the Winkler Index (WI) and Huglin Index (HI), are often used to assess the climatic suitability for specific grape varieties and/or wine styles and are variations of degree days (heat sum) or heat accumulation (Table 1). The trends for both parameters show significant changes at each location. However, changes were greater in relative terms, with HI giving more weight to maximum temperatures (Table 1) and GSTmax increasing more at all locations. The GDDs increased by 54, 67, 80, 103, 107, and 170 °C units, while HI increased by 82, 75, 106, 113, 121, and 183 °C units per decade for Maribor, Murska Sobota, Novo Mesto, Črnomelj, Bilje, and Koper (Supplementary Figures S1–S3). The average values of these two indices in this period were around 300 °C units higher for the GDDs (except Koper 110) and from 265 to 331 °C units higher for HI than in the reference period (Table 3). Heat accumulation has also increased by 250–300 °C units in other European wine-growing regions over the last 30–50 years [47], and increased heat accumulation (WI and HI) in inland Spanish wine-growing regions has been reported, but not in coastal regions.

HI values of 1700–1900 °C units [35] indicate that the Podravje and Posavje wine-growing region is suitable for medium-late varieties such as Chardonnay, Sauvignon Blanc, etc. In the reference period (1961–1990), the HI value exceeded the value of 1900 °C units at both stations in Podravje and in Novo Mesto in Posavje only once (1983) and in Črnomelj ten times, while in the period 1991–2021, the HI value exceeded this value in more than two-thirds of the years at all stations in Podravje and Posavje (in Črnomelj, the value of 1900 °C units was not exceeded in only three years in the 1991–2021 period). In addition, the HI also exceeded the value of 2100 °C units in this period, which was most pronounced after 2010, e.g., in Črnomelj in half of the years. In general, in the period 1991–2022, the values of the GDDs increased by 23–26% and HI by 16–18%, while after 2010, even the GDDs increased by 27–34% and HI by 23–26% compared to the reference period (1961–1990) (Table 3).

Based on the classification of wine-growing regions into climate maturity groups [24] and the increase in GSTavg in the last decade of the observation period, it can be concluded that this wine-growing region is suitable for growing some grapevine varieties from the warm climate maturity group [34]. Initial results from the process of Merlot introduction in these two regions (not published) confirm this prediction.

In the Primorska wine-growing region, the HI value did not exceed the value of 1900 °C units (first half of HI-1 ripening group) in the reference period (1961–1990) in only four years at either station. In Bilje and Koper, it averaged between 2062 and 2068 °C units (second half of HI-1), while the HI value exceeded the value of 2300 °C units (second half of HI+1) in the period 1991–2022. The HI value for Bilje was 2327 and for Koper 2400 °C (on the border of HI+2) units, which shows that this region belongs to the moderately warm ripening group for grapes, and based on the values in the last decade (2011–2022), Bilje with an average of 2451 °C units (HI+2) and Koper with 2591 °C units (HI+2), these locations are already in the warm ripening group [59] according to Jones et al. [25]. The result of this warming may be that some areas that are still suitable for growing certain varieties will no longer exist in the future, or it will no longer be possible to produce premium wines on them as reported by White et al. 2006 [60]. As Tate 2001 [61] also noted, pests and diseases that are currently restricted by the winter cold will extend their range northwards.

3.3. Precipitation Parameters Trends

The high inter- and intra-annual precipitation variability has weakened many of the trends in precipitation parameters (Figure 3). Annual precipitation (AP) did not change significantly at any of the locations in the long-term period (1952–2022), only the trend for Maribor (Podravje) showed a significant decrease in precipitation (−3 L m²yr⁻¹). The total growing season precipitation (GSP) showed significantly decreasing trends in Maribor (−1.7 mm m²yr⁻¹), Bilje (−2.8 mm m²yr⁻¹), and Koper (−1.8 mm m²yr⁻¹) (Table 2). The inter-annual variability in the climate makes it difficult to assess tendencies in precipitation distribution patterns and possible effects of climate change. Nevertheless, some recent studies indicate significant changes in extreme events, such as more frequent and more extreme droughts, an increase in precipitation in the cold season, and drying out in the warm season [62,63,64]. It is therefore to be expected that the frequency, intensity, and distribution of precipitation will change due to the increased speed of the water cycle, which will probably also have an impact on the water supply in agriculture. In Europe, decreasing precipitation trends or changes in the seasonality of precipitation have been observed for large parts of the Mediterranean region [63,65]. This also confirms our results for the growing season in the Mediterranean region of Primorska (both locations). In the continental part of Europe, these trends were exceptionally absent [47]. Many of these studies were conducted two decades ago, and in many places, the precipitation pattern has changed. The trends in annual and growing season precipitation for the Maribor location in the continental wine-growing region of Podravje confirmed our assumption.

The precipitation pattern is spreading from the east towards Maribor (the eastern part of the Alps), especially after 2010, and is becoming more and more similar to the Murska Sobota location (on the edge of the Pannonian climate) (Figure 3). The long-term average (1952–2022) of precipitation in the growing season (April–October) for Maribor is 700 mm m²yr⁻¹ and decreases to 591 mm m²yr⁻¹ after 2010. The average amount of precipitation after 2010 is 134 mm m²yr⁻¹ lower (−18%), compared to the 725 mm m²yr⁻¹ in the reference period 1961–1999 (Table 3). A similar pattern of decline in GSP after 2010 as in Maribor is also observed at both locations in Primorska, namely 131 mm m²yr⁻¹ (Bilje) and 103 mm m²yr⁻¹ (Koper). These decreases could be critical, as the vines should have sufficient access to soil moisture from flowering to véraison and should not be exposed to high drought stress.

The most stable amount of precipitation remains at the Črnomelj location with 773 mm m²yr⁻¹. The least changed amount of precipitation is recorded at the Murska Sobota location, which amounts to 575 mm over the entire long-term period (1952–2022) (Table 3). The highest precipitation amounts were recorded in Bilje (inland part of Primorska), where large-scale erosion events frequently occur. This is due to both the high rainfall and soil management system (soil tillage) in Primorska (in Podravje and Posavje the soil is green-covered), and the increasing extremes are likely to have additional negative impacts. Jones et al. [47] found that precipitation levels have not changed significantly in other European wine regions. However, higher temperatures lead to higher evapotranspiration. Moisture deficiency in the berry growth phase, especially in Koper (Primorska) and Murska Sobota (Podravje), could reduce cell division and lead to significant dehydration and sunburn, which in combination can lead to a reduction in berry size and yield, as also noted by Peacock [66]. In Murska Sobota, the amount of precipitation in the vegetation was already at a similar level in the reference period and in the last three decades as in the last decade in Maribor, which could indicate to a certain extent that the influence of the Pannonian climate on Maribor has become increasingly noticeable in the last decade.

Even though the trends of decreasing precipitation for Novo Mesto are not significant, on average 14% less precipitation was recorded after 2010 than in the reference period. In Črnomelj, the amount of precipitation in the vegetation was consistently at a similar level (over 770 mm m²yr⁻¹), although the distribution of precipitation and the intensity of individual weather phenomena must be taken into account. The average amount of precipitation in the coastal area (Koper) is only two-thirds of the amount of precipitation in Bilje. The distribution of precipitation in Primorska is the least favorable of all wine-growing regions. The distribution of precipitation and the intensity of individual weather phenomena must also be evaluated.

As far as the future climate is concerned, an average warming of 2.0 °C is predicted for most of the world’s best wine-growing regions by 2050 [67,68,69]. It is predicted that parts of southern Europe will become too hot to produce high-quality wines and that northern regions will acquire vineyard potential. Further warming could lead to grape varieties exceeding their climatic optimum, making current wine styles more difficult to produce.

4. Conclusions

The general conclusion of this study is that the greatest effects of climate change were felt in all three wine-growing regions after 1990. The greatest increase was in the number of hot days with a temperature above 30 °C (NDT30), which also had the greatest impact on other bioclimatic parameters, e.g., the average air temperature in the growing season, the sum of effective temperatures (GDDs) and the Huglin Index. In the period 1991–2022, the average growing season temperature (GSTavg) increased by about 1.5 °C or more in all wine-growing regions compared to the reference period, except in the coastal region of Koper, where the GSTavg temperature is 0.6 °C higher. In regions with a continental climate, the GSTavg temperature rose to around 17 °C and in the Mediterranean region to around 19 °C, which may be reflected in the earlier ripening of the grapes. If the warming trend continues in the next 30 years in a similar way as it has since the 1990s, it is expected that the wine-growing regions of Podravje and Posavje will completely transition to the warm climate ripening group. Another very important bioclimatic parameter at higher temperatures is the amount of precipitation during the growth of the vines. The total amount of precipitation in the growing season (GSP) shows a downward trend in all three wine-growing regions. The total amount of precipitation has decreased significantly in three locations, both in the maritime locations (Bilje and Koper) and in the inland locations (Maribor). As far as recent precipitation is concerned, it appears to be increasingly unevenly distributed, not only in the vegetation areas but also in the wine-growing regions. Therefore, the network of meteorological stations is even more important for the monitoring of weather phenomena and for the adoption and implementation of technological measures in the vineyards. For the network of meteorological stations to function well, its financing should continue to be the responsibility of the state and the municipalities.