1. Introduction
There is global scientific consensus on the anthropogenic phenomenon of climate change (therefore CC) [
1] and of the related altered bioclimate by focusing on its vital impacts on humans [
2,
3], fauna [
4,
5,
6], flora [
7,
8,
9] natural resources, ecosystem-climate services and management policies [
10,
11,
12,
13].
The Greek Peninsula located in Southeastern Europe appears as a future climatically threatened territory due its rapidly changing bioclimatic conditions [
14,
15,
16,
17]. Projections on the country’s changing climate reveal significant warming trends, under the RCP8.5, with a near surface temperature average rise of 4.3 °C by the end of the 21st century and therefore a significant increase of the annual number of hot days and tropical nights, night frosts, continuous dry spell days and length of the growing season and the decrease of frost days. The reduction of precipitation by 16% along with an increase of the annual number of consecutive dry days by 30% (15.4 days) pinpoint to a future drier environmental evolution [
18,
19].
Furthermore, the directly linked to CC present and future extreme weather events appear modified in their frequency and intensity e.g., increase of extreme wind speeds [
20], more frequent flash floods and drought episodes [
21,
22,
23], more often, more extreme and longer heatwaves [
24,
25] and a lengthened hot extremes' season [
26].
Predictions on Greece’s future climatic parameters’ evolution conjointly with its present more xerothermic climatic footprint may justify the country’s increasing vulnerability to CC capable of forming significant impacts on its extensive natural vegetation and its highly heterogeneous agricultural ecosystems [
8,
27,
28].
The present and projected impacts of CC on the Greek natural ecosystems involve increased wildfire and flood risks [
29,
30], altered fire behaviour in natural landscapes [
31], increase of high fire danger days [
32], reduction of water availability [
33], declining tree growth and productivity [
34,
35], dieback of tree species [
36], higher extinction risk of the endemic flora [
37,
38,
39], reduction in habitat-suitable areas [
37,
40], elevational and altitudinal shifts of dominant species, changes in forest cover [
41,
41], occurrence and spread of alien plant taxa [
42] and loss of biodiversity [
43].
As for the agricultural areas, respective impacts include alterations of frost agroclimatic indicators, increase in the growing season duration [
44], crop yield and product quality reduction [
45,
46], agricultural soil losses [
47], degradation of surface and groundwater resources, declining of water availability [
48], crop phenology modifications [
49], changes in area suitability for cultivation [
50], impacts on crops’ adaptive capacity [
51], declining variety suitability [
15], cultivations’ expansion to higher altitudes and northern areas, increased frequency of crops’ vulnerability (elevated heat injuries) [
44,
52] andrural areas’ negative socioeconomic evolution [
53].
A thorough understanding and better capture of the phenomenon of bioclimatic change is justifiably demonstrated by the very common utilisation of bioclimatic indices (mathematical formulas based on fundamental climatic parameters such as temperature and precipitation) as tools for the climate’s characterisation in various surveys of different scientific fields (e.g., climatology, bioclimatology, forestry, agricultural surveys, investigations on climate/bioclimate change) [
15,
54,
55,
56,
57,
58,
59,
60,
61,
62,
63,
64,
65,
66,
67,
68,
69,
70].
Temperature and precipitation are decisive climatic inputs for the exploration of CC, but the utilisation of bioclimatic indices allows a more thorough understanding and better capture of the phenomenon's dimension in bioclimatic terms [
71]. By exploiting the indices’ values the assessment of the effects of climate on vegetation and the environment’s correlation with the predominating vegetation types is more feasible [
72,
73].
Based on fundamental climatic parameters (temperature, precipitation and evaporation) the Emberger index (IEMB), commonly termed as the pluviothermic quotient (Q), classifies the bioclimate zones in the Mediterranean area according to a scheme extending from the “Per-Humid” to “Per-Arid” characterization (or bioclimatic type, bioclimatic category). For the index’s estimation the temperature is represented, on an annual basis, by the average value of the maximum temperatures of the hottest month (M) and the average value of the minimum temperatures of the coldest month (m), given that vegetation development is strictly associated with these thermal limits. The precipitation (P) is expressed by its annual values and evaporation is indirectly represented by (M – m), considering the parameter’s common increase with the latter difference [
54,
73,
74]. Concomitantly, for the phytoclimatic classification in bioclimatic subtypes (or Q2 classes), Emberger also utilized a simplified algorithm on the basis of the minimum winter temperature (m) extending from the “Very Hot” to the “Very Cold” temperature characterization [
74,
75]. As such, the phytoclimatic conditions’ mapping is conducted through the combination of the characterisations of the bioclimatic types as obtained from the estimates of the Q values and the temperature conditions corresponding to the estimates of the m values, which results in the Q2 bioclimatic subtypes (e.g., a Q2 subtype described as “sub-humid with mild winter”).
In Europe, the IEMB has been applied for the classification of the bioclimate [
76,
77] , for investigations on the vegetation’s dissemination [
75,
78], for surveys on the preservation (conservation, restoration, habitat suitability) of landscapes [
10,
63,
73,
79,
80,
81,
82], for researches on the risk of desertification [
10] and on changes of the bioclimatic regime [
83,
84].
For Greece in particular, very limited investigations on the changes of the bioclimate have been conducted based on applications of the IEMB. Up to present, researchers have conducted bioclimatic classifications at a very local scale involving surveys on fires and wildfire risk assessment [
85,
86,
87], the bioclimatic classification of natural vegetation environments [
88], reforestation potential, restoration and conservation of natural landscapes [
10,
88,
89,
90,
91], plant diversity [
92,
93,
94,
95], land cover change [
96], environmental monitoring and management of water resources and water quality parameters [
97,
98]. The ΙΕΜΒ has also been applied in investigations concerning the effects of the climatic factors on the fuel complex characteristics of the pine forests and on the fire activity patterns in natural vegetation formations in Greece [
31,
87].
Within the changing climate’s research framework, the overview on previous investigations, demonstrates the very limited applications of the IEMB as a tool for bioclimatic classification (mostly conducted at the local scale) and for predictions on the future Greece’s bioclimatic regime under the influence of CC.
By accounting the aforementioned limited scientific outcomes related to the applications of the IEMB, the paramount importance of the preservation of the cultivated and natural plant formations and their susceptible nature to the change of bioclimatic conditions (particularly to those connected with temperature and precipitation), it was considered imperative to investigate the evolution of the bioclimate in Greece, an area with particularly variable, extensive and distinguished natural and agricultural areas. Thus, in the current study, the changes of the spatiotemporal distribution of the Emberger bioclimatic index (mathematical expression of temperature and precipitation parameters) are investigated for the entire Greek Peninsula. The variability of the IEMB is studied over the reference period (Ref: 1970–2000) and two future time periods (p1: 2021–2040; ; p2: 2041–2060), computed at a high resolution of approximately 300 m under the RCP4.5 and RCP8.5 emission scenarios. The short-term and long-term bioclimatic change trends are being targeted by approaching the main differences between the present and the future bioclimate regime.
The originality of the current research lies in the high-resolution computation (approximately 300 m) of the Emberger index’s Q2 classes of bioclimatic characterisation which are analysed and illustrated for the first timein order to capture the present bioclimatic regime and its future evolution in a particularly extensive area represented by the entire country of Greece.
The more detailed outcomes on the Country’s bioclimatic alteration drawn from this study, might serve as exhortation to implement advanced technologies for their successful preservation under the future status of an altered climate. Additionally, results for the phytogeographical regions of Greece [
99] , will support decision making for targeted future research and resources allocation to enhance conservation management for habitats and species (in natural and anthropogenic, human induced, cultural ecosystems) found in these areas.
4. Conclusions
Bioclimate change, driven primarily by human activities, has significant and far-reaching impacts on the floristic diversity of various regions. Τhese impacts are particularly pronounced in Greece, a country characterized by rich biodiversity and distinct ecological zones..
The innovative findings of the present study may be summarized by the following conclusions:
Overall, Greece is projected to face drier and warmer conditions under both examined scenarios (RCP4.5 and RCP8.5).
It also appears that for both emission scenarios the bioclimate of Greece temporarily becomes more xerothermic given the resulting gradual drier and warmer transitions between the studied timeframes (reference period: 1970–2000, 1st time period: 2021–2040 and 2nd time period: 2041–2060).
Classifications of the Q1 type under the RCP4.5 scenario reveal distributions among four (4) bioclimate types (PeHu, Hu, SuHu and SeAr) independently of the examined time frame. Classifications of the Q2 subtypes (20 in total), however, demonstrate the appearance of more xerothermic categories (Ar-Ho, SeAr-VHo, HuHo and SuHu-VHo), the spatial distributions of which are mostly evident during the investigated long-term time period (2041–2060).
Under the RCP4.5, the phytogeographical regions which appear as mostly dryer and warmer in the more distant future (2041–2060) are almost entirely located in the right half of the Country. By 2060 under the RCP4.5 scenario, the SeAr-Te bioclimate type is expected to dominate the NAe and WAe phytogeographical regions, while the more xerothermic SeAr-Hot will influence most of the KiK region and parts of the WAe, StE, PE, KK, EC and EAe regions. Additionally, it is underlined that, even under the less influential RCP4.5, the even more adverse conditions of the SeAr-VHot class may possibly be present in the KK and EC regions.
The same Emberger Q1 classifications in the 2021–2040 period of the extreme RCP8.5 (4 bioclimate types: PeHu, Hu, SuHu and SeAr) are demonstrated and exhibit similar spatial distributions with the respective ones for the 2041–2060 period of the RCP4.5 scenario. The xerothermic trends’ similarity between these cases highlights the crucial role of the more extreme RCP8.5 owing to the pronounced advancement of its impacts on the investigated area’s bioclimate. Classifications of the Q2 fall within the aforenamed Ar-Ho, SeAr-VHo, HuHo and SuHu-VHo, forming once more the same types (also 20 in total) and relatively similar spatial distributions per phytogeographical region with some exceptions concerning the SeAr categories.
The most impactful is the 2nd period (2041–2060) of the RCP8.5 scenario which is projected to induce substantial drying and warming over the investigated area. Three (3) Hu, SuHu and SeAr subtypes are determined, while an increase of the latter’s % coverage (most xerothermic SeAr) is exhibited over all geographical zones. With reference to the 1970–2000 period, the expected as most impacted by the SeAr conditions are the NC, NE, NAe, WAe, KiK, EC and the StE phytogeographical regions. As for the % spatial coverage of the Q2 types, a small expansion of the Ar-VHo and the absence of the PeHu-Te both justify more intense dry-thermal trends. The latter trends are clear given the significant spatial evolution mainly of the Ar-Ho, SeAr-VHo, SeAr-Ho, SeAr-Te classes, respectively over the KiK, KK, WAe, NE–NC regions which is projected to occur by 2060 under the RCP8.5.
It is evident that the long-term time period (2041–2060) of the extreme RCP8.5 scenario exhibits the strongest dry-thermal trends over the eastern half of the Greek territory.
The alteration of the bioclimate can be characterized as one of the most tenacious and pressing threats to the ecosystems over the Greek territory. Its impacts are expected to pose substantial threats to the phytogeographical regions of Greece, impacting agriculture, forest health, national parks and protected areas, urban environments, and phenological events, related to the until now established flora and vegetation. By this, our study highlights the need for comprehensive strategies to mitigate and adapt to CC, urging for decision makers efforts to focus on sustainable agricultural practices, forest management, urban planning, and conservation of biodiversity to safeguard Greece's rich floristic heritage and natural capital in the face of a changing climate.