Assessing the Future Risk of Damage to European Cultural Heritage Due to Climate Change

Abstract

This study presents an integrated approach for assessing the risk of damage to cultural heritage due to climate change, applied to five selected European cultural sites. Future changes in climate parameters and indices associated with climate pressure on cultural heritage sites are analyzed using a set of EURO-CORDEX high-resolution Regional Climate Model (RCM) simulations under three different future emission scenarios (RCP2.6, RCP4.5, and RCP8.5). Our results are reported for three different time periods, 1971–2000 (reference period), and two future periods, 2021–2050 and 2071–2100. The aim of this study is to apply the Heritage Outdoor Microclimate (HMR_out) and Predicted Risk of Damage (PRD) indices to quantify the risk of damage to inorganic materials resulting from future long-term changes in temperature and relative humidity (RH). Projections show a significant rise in temperature leading to increased HMR_out values and higher heat stress across CH sites. The projected temperature increase results in a rise in HMR_out index across all studied cultural heritage sites, with the rate of increase varying by period and scenario. Overall, as a result of the rising HMR_out index, the predicted risk of damage to monuments made from inorganic materials due to heat stress is expected to increase. The PRD index is projected to increase by up to 87% in certain CH sites by the end of the 21st century according to the RCP8.5 scenario. Conversely, as the climate becomes warmer, the RH and the associated risk are expected to decrease. This work highlights the necessity for continuous monitoring of future climate and the timely detection of the predicted risk of damage for monuments, to take immediate protective measures to preserve cultural heritage in the face of climate change.

1. Introduction

Cultural heritage (CH) is continuously exposed to the environment that hosts it, and any changes in this may have significant consequences. According to UNESCO [1], climate change is already affecting many CH sites and is expected to affect them even more in the future leading to significant deterioration, loss of historical artifacts, and extinction of traditional knowledge and cultural practices. The Intergovernmental Panel on Climate Change (IPCC) [2] reports that climate change poses serious threats to both tangible and intangible CH.

Changes in the frequency and intensity of extreme weather events, such as droughts, floods, landslides, fires, variations in wind intensity, and rising sea levels, are some of the impacts of climate change that are already affecting many CH sites [2,3]. Additionally, long-term variations in meteorological parameters, such as temperature and atmospheric humidity, are also influenced by climate change. For example, increasing temperatures threaten to melt the upper layers of permafrost or induce freeze–thaw cycles, which can disrupt the structure of soils or middens, through frost-heave, subsidence, and thaw-settling [4]. Similarly, heavy rainfall can lead to erosion and mass movement events [5], while changes in humidity can increase the frequency of salt crystallization events or mold growth [6]. Furthermore, these changes in temperature and humidity will significantly affect the chemical composition and physical properties of materials in cultural heritage sites. Porous materials such as limestone are particularly vulnerable to freeze–thaw cycles, where water expansion can cause internal stress and result in material disintegration [6]. Similarly, thermoclastism caused by thermal variations can cause surface erosion, micro-cracking, and exfoliation of stones, especially during seasonal and diurnal changes in temperature and direct insolation [7]. Increased precipitation could further exacerbate the deterioration of historical materials through saturation of soils, corrosion of metals, and surface recession of carbonate stones like marble and limestone [8]. Additionally, rising humidity could accelerate microbial growth on stone and wood materials, further contributing to material degradation [8]. These processes are essential for understanding the long-term risks faced by heritage materials, particularly as climate conditions continue to evolve and intensify.

According to the IPCC [2], Europe has experienced an average temperature increase of approximately 2.0 °C since the pre-industrial era (1850–1900). Regarding the future projections, climate models show significant agreement for all emission scenarios in warming throughout Europe. The strongest warming is projected for Southern Europe during the summer period and for Northern Europe during the winter period. As reported by the IPCC [2], Europe is projected to undergo a substantial temperature increase, estimated between 2.2 °C and 5.1 °C, by the end of the 21st century. Apart from temperature, climate change also affects other climate-related parameters such as precipitation. The IPCC [2] suggests that climate change will lead to increased amounts of winter precipitation in Northern Europe and decreased summer precipitation in Southern Europe and the Mediterranean. Furthermore, strong and extreme rainfall is expected to intensify across most of Europe throughout the year [9,10]. In general, future projections indicate a notable increase in extreme events in Europe, particularly heatwaves, hot days and tropical nights, droughts, and floods [11,12,13,14]. Due to the increase in extreme events, Giglio et al. [15] argue that a thorough understanding of future climate conditions is required to protect and preserve the CH.

Researchers have acknowledged the significant risks posed by climate change and extreme weather events to cultural sites, and various approaches have been introduced to assess these risks. Grossi et al. [4] predicted the risks of freeze–thaw cycles on built CH and archaeological sites across Europe, while Gomez et al. [16] mapped the impact of climate change on biomass accumulation. Similarly, Chiantelli et al. [17] evaluated potential surface recession, biomass accumulation, and deterioration due to salt crystallization cycles in UNESCO monuments in Panama. Bonazza and Sardella [18] examined the hydrometeorological risks to CH due to climate change in Central Europe, and Kapsomenakis et al. [19] assessed the threats to UNESCO cultural and natural heritage sites in the Mediterranean, showing an increase in risk as global warming progresses. Additionally, Tringa and Tolika [20] studied the Heritage Outdoor Microclimate Risk and the Predicted Risk of Damage for two Greek cultural regions, showing that the risk of damage is expected to increase for the inorganic materials due to the rise in mean seasonal temperature and its fluctuations. In parallel, Esteban-Cantillo et al. [21] quantified the combined effects of climate change and air pollution on building materials in cultural heritage sites in Europe and Mexico, revealing how damage patterns vary depending on regional climatic and pollution conditions. Recently, Brandano et al. [22] proposed a method for identifying cultural heritage sites at risk due to climate change, considering both natural hazards and the socio-economic and environmental vulnerabilities that may increase or mitigate potential damage. Overall, all these (and other) studies emphasized that despite the increasing number of publications on integrating CH into adaptation and mitigation efforts, they still remain much fewer compared to those addressing the physical impacts on individual buildings or sites [23]. This knowledge is crucial for establishing adaptive conservation strategies and appropriate measures for CH sites. Bonazza and Sardella [18] highlighted the urgent need for policymakers and decision-makers to have access to quantitative data and projected impacts of climate change on CH to formulate effective adaptation and mitigation strategies. In this context, Graham et al. [24] emphasize the immediate necessity for action in heritage management, stressing the importance of incorporating climate change considerations into protection strategies. Understanding the importance of further advanced research and protective measures, the European Union has funded related research projects over the past 15 years aiming at CH adaptation to climate change. Indicatively, some of the most successful projects are as follows: NOAH’s Ark (Global Climate Change Impact on Built Heritage and Cultural Landscapes) (2003–2007) [25], Climate for Culture (2009–2015) [26], STORM (2016–2019) [27], HERACLES (2016–2019) [28], ProteCHt2save (2016–2019) [29], Arch (2019–2022) [30], and the SHELTER Project (2019–2023) [31]. Projects initiated after 2020 include STRENCH (2020–2022) [32], ICCROM NetZero (2022–2023) [33], and PROCULTHER-NET (2022–2023) [34]. These projects aim to develop strategies and tools for protecting and preserving cultural monuments against the impacts of climate change. Within the framework of the TRIQUETRA project [35], a repository is being developed to document the impact of climate change and natural hazards on cultural heritage sites across Europe. The project’s main goal is to develop novel technologies that allow efficient and accurate quantification of threats to CH, promote systematic approaches for identifying risks, and increase public awareness, making individuals part of the solution. A previous work by Ioannidis et al. [36] on TRIQUETRA’s pilot CH sites from 1970 to 2020 indicated a pronounced warming trend, with temperature being projected to increase in the future. Additionally, the microclimate analysis highlighted increased heat stress, particularly on TRIQUETRA’s pilot CH sites crafted from stone and marble, thereby increasing the risk of damage.

The present study, implemented within the framework of TRIQUETRA project, focuses on advancing and quantifying the risk due to climate change in five pilot CH sites (the sanctuary at Kalopodi, the archaeological site of Kolona on Aegina Island, Ancient Epidaurus in Greece, the Choirokoitia archaeological site in Cyprus, and Villa di Giulia in Ventotene Island in Italy) (Figure 1). Although several projects have addressed the impacts of climate change on CH, this study applies an integrated approach specifically focusing on the quantification of the future predicted risk of damage at different European CH sites. Future changes in climate parameters (essential climate variables also denoted as ECVs) and indicators associated with climate pressure on CH sites are analyzed using a set of high-resolution Regional Climate Model (RCM) simulations under three different greenhouse gas and aerosol emission scenarios (RCP2.6, RCP4.5, and RCP8.5). The Heritage Outdoor Microclimate (HMR_out) and Predicted Risk of Damage (PRD) indices are applied here to quantify the risk of damage to inorganic materials (i.e., stone, mortar, and bricks) due to future long-term changes in temperature and relative humidity. Variations in these parameters can lead to phenomena such as corrosion, salt crystallization, and mechanical stress of building materials [37]. The selection of the HMR_out and PRD indices was based on their ability to accurately assess risks related to the outdoor microclimate, considering both environmental parameters and their interaction with cultural heritage [20]. Furthermore, to the best of the authors’ knowledge, the PRD index is the only index that quantifies the predicted risk of damage to the construction materials of monuments. The term “risk of damage” refers to the potential deterioration of construction materials due to changes in environmental parameters. In contrast to the previous study of Tringa and Tolika [20], which applied the HMR_out and PRD indices to cultural heritage sites in Greece, this study adopts a more comprehensive approach. By incorporating climate parameters and indicators associated with climate pressure on CH sites, it offers an enhanced understanding of the potential threats posed by future climate change, marking a significant advancement compared to previous studies. Additionally, the climate simulations in this study are derived from an ensemble model under three distinct emission scenarios (RCP2.6, RCP4.5, and RCP8.5), providing a more robust and reliable assessment across a range of possible future conditions.

The main question addressed by this study is as follows: as climate changes, how are climatic parameters associated with CH deterioration expected to be affected, and how will these changes impact the risk of damage to monuments? This research provides a detailed response to the above questions, knowledge that is essential for developing effective protection and conservation strategies. The CH sites, the data, as well as the developed methodology are presented in Section 2. Section 3 covers our results concerning future climatic conditions, climate-related risks, and the quantification of risk based on the construction material of CH. A detailed discussion of our results is given in Section 4, and our conclusions are summarized in Section 5.

2. Materials and Methods

2.1. Cultural Heritage Sites

This research focuses on five different CH sites located in Southern Europe (Figure 1). From South to North, the CH sites are Choirokoitia in Cyprus, Aegina, Epidaurus, and Kalapodi in Greece, and Ventotene in Italy. Information on the exact location of the selected CH sites is presented in Figure 1 and

Table 1. Location (coordinates) of the selected CH sites and the model grid cell elevation (EURO-CORDEX) corresponding to each site.

CH Site	Country	Longitude (°N)	Latitude (°E)	Elevation EURO-CORDEX (m)
Choirokoitia	Cyprus	33.3439	34.7964	115
Aegina	Greece	23.4236	37.7501	30
Epidaurus	Greece	23.1576	37.6259	251
Kalapodi	Greece	22.8954	38.6365	362
Ventotene	Italy	13.4300	40.8013	0

. These locations represent different environmental contexts (coastal, island, inland) and various types of CH typologies, structures, and materials. In addition, sites face distinct natural hazards related to their environment, such as coastal erosion, bioerosion, frost damage, and geological instability (landslides, marine erosion). This diversity allows for a comparative analysis of climate-related vulnerabilities across different typologies, materials, and environmental settings.

First, Choirokoitia, located 6 km from Cyprus’s southern coast in the Larnaka District, nestled at the foot of the Troodos Mountain range within the Agios Minas River valley, is susceptible to various natural hazards such as floods and erosion due to climate change. It is one of the best-preserved prehistoric settlements in Cyprus and one of the most significant prehistoric sites in the Eastern Mediterranean [38]. Archaeological excavations indicate that the settlement was composed of round houses of mudbrick and stone, with flat roofs and enclosed by a series of protective walls. The archaeological site remains well-preserved and retains all the elements that reflect its Outstanding Universal Value.

Aegina is an island in the Saronic Gulf in Greece in close proximity to Athens, capital of Greece. The significant archaeological site of Kolona is located in the north-western part of the island and was one of the most important cultural centers during the Aegean Bronze Age [39]. One of the most significant structures in the archaeological site of Kolona has been undoubtedly the Temple of Apollo, from which only one column remains intact today, characterizing the entire monument. The Temple of Apollo was of Doric order, mainly constructed from Athenian limestone and Parian marble.

Epidaurus is an archaeological site in the Peloponnese region of Greece. It is an important coastal and underwater UNESCO site which dates back to the 12th century BC [40]. It is primarily known for its ancient theater, which is one of the best-preserved and most significant theaters in ancient Greece. Epidaurus also includes other important archaeological gems, such as the Sanctuary of Asclepius and archaeological sites that provide a complete picture of the ancient city. It should be noted that stone is the predominant material used in the construction of the monuments here as well. The fluctuations of temperature, coupled with high humidity levels, constantly pose risks to structural materials in the region, contributing to issues such as crystallization cycles, cracks, and mold growth.

The sanctuary of Kalapodi, located in Ancient Phocis, Central Greece, exhibits a diverse range of materials and techniques [41]. These include natural stone (such as soft limestone and sandstone), Roman cement, and various integrated metals like bronze, iron, and copper, as well as plaster fragments within tis its monumental structures. Excavations carried out by the German Archaeological Institute (DAI) since 1974 have uncovered two temple complexes along with surrounding structures (dating from ca. 1300 BC to 700 AD). Frost poses a constant danger to the site’s materials, contributing to decay issues alongside the vulnerable structural components.

Ventotene, an island in the Pontine Islands cluster in the Tyrrhenian Sea, in western Italy, is recognized as a historical hub of European moral and intellectual heritage. It is a volcanic island characterized by rough morphology and rocky terrain. The archaeological site of Villa di Giulia at Capo Eolo is an ancient Roman imperial villa located in the northern part of the island. This archaeological site features fresco-decorated walls needing reinforcement against wind and sea impact, and therefore they are sensitive to climatic change.

2.2. Data

To assess the potential future effects of climate change on the five selected CH sites, meteorological data from weather stations (observations) and simulations from regional climate models (RCMs) were analyzed. The station data were sourced from various networks, including stations with long-term meteorological records near the selected CH sites (no data availability for Ventotene). For Choirokoitia, we use temperature data from the Larnaca weather station (28 km away) obtained from the European Climate Assessment and Dataset (ECA&D) network [42]. For Aegina and Epidaurus, temperature data from the Hellinikon weather station (33 and 60 km away, respectively) were obtained from the Global Historical Climatology Network (GHCN) [43]. Finally, for Kalapodi, temperature data from the Thissio weather station (103 km away) were obtained from the National Observatory of Athens (NOA).

For the purposes of this study, a multi-model ensemble was created using simulations from various RCMs driven by various Global Climate Models (GCMs) was produced. The ensemble consists of 11 sets of high-resolution (~12.5 km) RCM simulations from the EURO-CORDEX (Coordinated Downscaling Experiment—European Domain) covering the historical period from 1950 to 2005 and the future period from 2006 to 2100 under three distinct future scenarios (a total of 44 simulations) [14,44]. The examined scenarios correspond to three different Representative Concentration Pathways (RCPs) of the IPCC. These pathways reflect differing levels of future mitigation measures of greenhouse (GHGs), namely the RCP2.6 (strong mitigation), RCP4.5 (medium mitigation), and RCP8.5 (no further mitigation). The grid cell closest to each CH site is selected. To assess the model’s ability to quantify the future impact of climate change on the CH sites under study, whisker plots, statistical indicators, and estimation of the uncertainty range were used to compare with observational data.

2.3. Towards Climate Risk Identification

To assess climate change-related effects and risks in CH sites, it is essential to establish a clear framework linking climate parameters and associated climate change risks to processes impacting CH. In this study, we employed a systematic approach to identify and evaluate potential climate risks at CH sites. Initially, relevant climate stressors, parameters and indices associated with climate pressure on CH sites were defined and selected based on a comprehensive review of the existing literature, particularly focusing on the work of Colette [45] and Sesana et al. [37]. Subsequently, based on these criteria, and considering also the availability of model data, eight climate-related parameters (

Table 2. Description of the climate parameters examined.

Climatic Variable	Description	Units
PR	Precipitation	mmday−1
TAS	Near-Surface Air Temperature	°C
TASMAX	Daily Maximum Near-Surface Air Temperature	°C
TASMIN	Daily Minimum Near-Surface Air Temperature	°C
HUSS	Near-Surface Specific Humidity	gkg−1
RH	Near-surface Relative Humidity	%
RSDS	Radiation	Wms−1
WS	Wind Speed	ms−1

) and six climate indices (

Table 3. Description of the climate indices examined.

Index	Index Full Name	Description	Units
HD	Hot Days	Number of days within a year with TASMAX > 35 °C	days
FD	Frost Days	Number of days with TASMIN < 0 °C	days
DTR	Diurnal temperature range	Annual mean of the daily differences between TASMAX and TASMIN	°C
PD20	Very heavy precipitation days	Number of days within a year with PR > 20 mm	days
RX1DAY	Highest 1-day precipitation amount	The PR for the day with the highest precipitation in a year	mmday−1
CDD	Consecutive dry days	The largest number of consecutive days within a year with PR < 1 mm	days

) were examined for the assessment of the future climatic information associated with CH.

2.4. Heritage Outdoor Microclimate Risk and Predicted Risk of Damage (PRD) Indices

The Heritage Outdoor Microclimate (HMR_out) and the Predicted Risk of Damage (PRD) indices were employed to assess the suitability of climates hosting CH and to quantify the risk of damage due to climatic conditions, following the approach of Tringa and Tolika [20] for outdoor environments. The HMR_out index represents the risk posed by the outdoor microclimate to cultural heritage. It enables assessments of potential threats associated with local microclimatic conditions.

HMR_out is calculated according to the formula:

$$\mathrm{H}\mathrm{M}{\mathrm{R}}_{\mathrm{o}\mathrm{u}\mathrm{t}}={\displaystyle \frac{{\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}}+{\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{c}}}{2}}$$

(1)

where:

$$\mathrm{H}\mathrm{M}{\mathrm{R}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}}=1-\left[\left({\displaystyle \frac{{\mathrm{H}\_\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}}-{\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}.\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{a}}}{{\mathrm{H}\_\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}}-{\mathrm{L}\_\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}}}}\right)\ast 2\right]$$

(2)

where:

$${\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}.\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{a}}={\displaystyle \frac{{\mathrm{M}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}.\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{a}}}{\mathrm{N}}}$$

(3)

M_env.out.data represents the total sum of the outdoor time series data, for each considered variable (e.g., temperature and RH). N is the total of the values of the outdoor time series data for each variable considered.

For the higher and lower thresholds (H_HMR_env.out and L_HMR_env.out), we consider that the cultural objects have adapted to the historical microclimate, and these thresholds are derived from the maximum and minimum values of the outdoor time series data (temperature, RH) [20]. In this study, to examine the change in HMR_out and PRD indices in the future under three emission scenarios (RCP2.6, RCP4.5, RCP 8.5), the thresholds were derived from the emission scenario RCP 2.6 for the period 1971–2100. The RCP2.6 scenario was chosen because it is considered a very stringent mitigation scenario. Based on the H_HMR_env.out and L_HMR_env.out derived from the RCP2.6 scenario for the period 1971–2100, the indices were applied to the other two scenarios.

The “high” and “low” limit values of the HMR_out are defined as follows: L_HMR_env.out = −1 (the risk condition relative to the “lower threshold”) and H_HMR_env.out = +1 (the risk condition relative to the “higher threshold”).

Therefore, in this study the H_HMR_env.out (L_HMR_env.out) is the maximum (minimum) value from the outdoor data series from the RCP2.6 scenario for the period 1971–2100 (excluding the “scattered values”) [46].

$${\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{c}}=1-\left[\left({\displaystyle \frac{{\mathrm{H}\_\mathsf{\Delta }}_{\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{c}}-{\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{c}.\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{a}}}{{\mathsf{H}\_\mathsf{\Delta }}_{\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{c}}-{\mathrm{L}\_\mathsf{\Delta }}_{\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{c}}}}\right)\ast 2\right]$$

(4)

H_Δ_fluc (L_Δ_fluc) is the maximum (minimum) value of hourly fluctuation from the outdoor data series from the RCP2.6 scenario for the period 1971–2100 (excluding the “scattered values”) [46].

ΔΜ_fluc.data is defined as:

$${\mathsf{\Delta }\mathsf{{\rm M}}}_{\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{c}.\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{a}}={\sum }_{\mathrm{k}=1}^{\mathrm{n}}{\sum }_{\mathrm{j}=1}^{24}\left[{\mathrm{X}}_{\mathrm{d}\mathrm{a}\mathrm{y},\mathrm{k},\mathrm{h}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{j}}-{\mathrm{X}}_{\mathrm{d}\mathrm{a}\mathrm{y},\mathrm{k},\mathrm{h}\mathrm{o}\mathrm{u}\mathrm{r}(\mathrm{j}+1)}\right]$$

(5)

According to the literature, if HMR_env.out > 0, then the sign of the HMR_out index will be “+” (positive), while if the HMR_env.out < 0, the sign will be “−” (negative), with HMR_env.out as the predominant risk factor between HMR_env.out and HMR_fluc [46]. Ιn this study, when the index takes a value lower than 0, we interpret this as indicating no change compared to the lower threshold, signifying minimal risk, which is defined as HMR_out = 0. Accordingly, when the HMR_out exceeds the value 1, it indicates high risk and is defined as HMR_out = 1. This is based on maintaining the thresholds set for the RCP2.6 scenario and applying them also to the RCP4.6 and RCP8.5 scenarios.

Respectively, PRD index is defined as the “forecast” of damage, assessing the ability of damage due to the microclimatic conditions [46]. It mainly depends on the microclimate, the HMR index, and the type of material. The values of “a” and “b” are exponents that define the risk of damage. Specifically, “a” defines the risk of damage due to the persistence of the cultural asset in environments where maximum or minimum HMR values are present (higher risk), while “b” defines the absence of risk due to the persistence of the cultural asset in environments where average or zero HMR values are present (lower or no risk). Their values have been empirically determined based on the standards reported by UNI 10829:1999 [47] and EN 15757:2010 [48] and differ by the type of material (inorganic, organic, etc.).

For the outdoor environment, the PRD index is calculated according to the formula:

$$\mathrm{P}\mathrm{R}\mathrm{D}=1-0.95\times {\mathrm{e}}^{(-\mathrm{a}\times {\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}}^4-\mathrm{b}\times {\mathrm{H}\mathrm{M}\mathrm{R}}_{\mathrm{e}\mathrm{n}\mathrm{v}.\mathrm{o}\mathrm{u}\mathrm{t}}^2)}$$

(6)

Further details and information about HMR_out and PRD indices can be found in the works of Tringa et Tolika [20] and Fabbri and Bonora [46].

3. Results

3.1. Projected Climate Change at Pilot CH Sites

To determine the future climate change and identify the related risks and impact on the selected CH sites, we first looked into changes in climate indices based on the future projections (see

Table 4. Changes in ECVs and climate indices based on future projections for the near-future period (2021–2050) and the end-of-the-century period (2071–2100), relative to the reference period (1971–2000), under the RCP2.6, RCP4.5, and RCP8.5 scenarios. Symbols: “+” indicates statistically significant positive change, “−” indicates statistically significant negative change, and “○” indicates no statistically significant change.

ECV/Index	2021–2050 (RCP2.6)					2071–2100 (RCP2.6)
PR (mmday−1)	○	○	○	○	○	○	○	○	○	+
TAS (°C)	+	+	+	+	+	+	+	+	+	+
TASMAX (°C)	+	+	+	+	+	+	+	+	+	+
TASMIN (°C)	+	+	+	+	+	+	+	+	+	+
HUSS (gkg−1)	+	+	+	+	+	+	+	+	+	+
RH (%)	○	○	○	○	○	○	○	○	○	○
RSDS (Wms−1)	+	+	+	+	+	+	+	+	+	+
WS (ms−1)	○	○	○	○	○	○	○	○	○	○
HD (days)	+	+	+	+	○	+	+	+	+	○
FD (days)	−	−	−	−	−	−	−	−	−	−
DTR (°C)	○	○	○	○	○	○	○	○	○	○
PD20 (days)	○	○	○	○	+	○	○	○	+	+
RX1DAY (mmday−1)	○	○	○	○	○	○	○	○	○	+
CDD (days)	○	○	+	○	○	○	○	○	○	○
	2021–2050 (RCP4.5)					2071–2100 (RCP4.5)
PR (mmday−1)	−	○	−	−	○	−	−	−	−	○
TAS (°C)	+	+	+	+	+	+	+	+	+	+
TASMAX (°C)	+	+	+	+	+	+	+	+	+	+
TASMIN (°C)	+	+	+	+	+	+	+	+	+	+
HUSS (gkg−1)	+	+	+	+	+	+	+	+	+	+
RH (%)	○	○	○	○	○	○	○	○	○	○
RSDS (Wms−1)	+	+	+	+	+	+	+	+	+	+
WS (ms−1)	○	○	○	○	○	−	○	○	○	−
HD (days)	+	+	+	+	○	+	+	+	+	○
FD (days)	−	−	−	−	−	−	−	−	−	−
DTR (°C)	○	○	○	+	−	○	○	○	○	−
PD20 (days)	○	○	○	○	+	−	○	−	○	+
RX1DAY (mmday−1)	○	○	○	○	○	○	○	○	○	+
CDD (days)	+	+	+	+	○	+	+	+	+	○
	2021–2050 (RCP8.5)					2071-2100 (RCP8.5)
PR (mmday−1)	−	○	○	○	○	−	−	−	−	−
TAS (°C)	+	+	+	+	+	+	+	+	+	+
TASMAX (°C)	+	+	+	+	+	+	+	+	+	+
TASMIN (°C)	+	+	+	+	+	+	+	+	+	+
HUSS (gkg−1)	+	+	+	+	+	+	+	+	+	+
RH (%)	○	−	−	○	○	○	−	−	−	+
RSDS (Wms−1)	+	+	+	+	+	+	+	+	+	+
WS (ms−1)	○	○	○	○	○	−	○	○	○	−
HD (days)	+	+	+	+	○	+	+	+	+	+
FD (days)	−	−	−	−	−	−	−	−	−	−
DTR (°C)	○	○	○	○	−	−	−	−	○	−
PD20 (days)	○	○	○	○	+	−	−	−	−	+
RX1DAY (mmday−1)	○	○	○	○	+	○	○	○	○	+
CDD (days)	+	○	+	+	○	+	+	+	+	+
	Choirokoitia	Aegina	Epidaurus	Kalapodi	Ventotene	Choirokoitia	Aegina	Epidaurus	Kalapodi	Ventotene

). These findings are based on heatmaps, which, following the work by Ioannidis et al. [36], were generated from the analysis of the ECVs and the related climate indices for the future. This analysis was conducted by comparing modeled data (multi-model means) from two future periods: the near-future (first) period 2021–2050 and the late 21st century (second) period 2071–2100, relative to the historical reference (historical) period 1971–2000 across three different RCPs. The statistical significance of the results was evaluated using a t-test at 95% confidence level.

Based on the EURO-CORDEX ensemble (multi-model mean) projections, TAS, TASMAX TASMIN are expected to increase across all the CH sites for all the RCPs and both future periods, indicating a statistically significant warming trend. An increase in HD is anticipated for Choirokoitia, Aegina, Epidaurus, and Kalapodi across all RCP scenarios and for both future periods, aggravating heat stress. Ventotene is expected to experience an increase in HD during the end of the century under RCP8.5 scenario. Conversely, a decrease in FD is predicted for all CH sites and RCPs for both future periods in association with the warming trend. There are no statistically significant changes in precipitation under the RCP2.6 scenario for both future periods. However, a reduction in precipitation (PR) is anticipated for both future periods under RCP4.5 and RCP8.5 almost everywhere, suggesting a drier climate. DTR generally tends to decrease under all scenarios, although the reduction is not statistically significant everywhere. The only case where an increase in DTR is expected is during the period 2021–2050 under the RCP4.5 scenario in Kalapodi. PD20 is generally projected to decrease over the examined sites in the south during the period 2071–2100 under RCP4.5 and RCP8.5 and to increase in Ventotene. An increase in RX1DAY is also projected mainly for Ventotene during the second future period 2071–2100. For CDDs, these are expected to increase mainly under the RCP4.5 and RCP8.5 scenarios. Overall, the climate analysis suggests that the examined CH sites will experience hotter and drier conditions in the future. It also highlights a strong warming trend and increased heat stress on organic materials at these CH sites under all the RCP scenarios.

To obtain an insight into the temperature changes which are projected for the 21st century, we examined the time series of TAS covering the period 1950–2100 under three different RCPs. Specifically, Figure 2 presents time series and box plot figures for each CH site, in order to assess the TAS changes for the period 1950–2100 compared to the reference period (1971–2000). A similar steady increase of up to 1.5–2 °C in the near future and up to 5 °C in the end of the century is projected for all the CH sites under RCP8.5. Under RCP4.5, TAS is projected to increase at a higher pace since the middle of the 21st century, reaching up to ~2 °C by 2050, with some high-altitude areas experiencing an increase of up to 2.5–3 °C, following the gradual decrease in GHG emissions (see also Georgoulias et al. [14]). Under RCP2.6, TAS is projected to increase since the middle of the 21st century and then stabilize (~1–1.5 °C). A similar change is projected for specific humidity (HUSS) for all the CH sites as a result of more evaporation in warmer conditions. In contrast, a decrease is projected in the future for Relative Humidity (RH) due basically to the higher capacity of air masses to hold more water vapor in warmer conditions. Furthermore, the intermodel range associated with temperature projections for the period 1971–2100 for the CH sites was assessed from the intermodel standard deviation (e.g., see the error bars in Figure 2 denoting the ±1σ from the 11 EURO-CORDEX simulations annual means). The standard deviation for the temperature ranged from 0.7 °C at Ventotene to 1.8 °C at Epidaurus, over this period, leading to a model uncertainty range between 4% and 11%, respectively, when compared to the average temperature. These uncertainty values indicate a relatively low level of variability in the temperature projections, suggesting that the models used provide a robust estimate of future temperature trends for the CH sites. The robustness of these temperature projections is further supported by the evaluation of the EURO-CORDEX ensemble against ground-based observations for the period 1971–2000, as presented in Appendix A.

3.2. Assessment of Heritage Microclimate Risk and Predicted Risk of Damage Indices

Based on the aforementioned factors, notable changes in future climate conditions are expected, aggravating heat stress in CH sites. To assess the suitability of climates hosting rich CH, and to quantify the risk of damage, due to temperature and relative humidity, the HMR_out and PRD indices were applied in this section. The indices were calculated based on simulated data from RCM simulations under three different future scenarios. The results of the HMR_out index for temperature and RH for the reference period (1971–2000), the near-future period (2021–2050), and the end-of-the-century period (2071–2100) for each CH site under the three emission scenarios are presented in

Table 5. Percentage of years characterized by “Minimum–Low”, “Moderate–Medium”, and “High–Maximum” risk due to temperature change and daily temperature range for each case study and emission scenario per period.

		Heritage Microclimate Risk
		1971–2000			2021–2050			2071–2100
Site	RCP	Minimum–Low	Moderate–Medium	High–Maximum	Minimum–Low	Moderate–Medium	High–Maximum	Minimum–Low	Moderate–Medium	High–Maximum
Choirokoitia	2.6	100%	0%	0%	40%	60%	0%	33%	60%	7%
	4.5	100%	0%	0%	20%	60%	20%	0%	17%	83%
	8.5	100%	0%	0%	3%	27%	70%	0%	0%	100%
Aegina	2.6	100%	0%	0%	17%	80%	3%	20%	80%	0%
	4.5	100%	0%	0%	37%	63%	0%	0%	20%	80%
	8.5	100%	0%	0%	16%	77%	7%	0%	0%	100%
Epidaurus	2.6	100%	0%	0%	73%	27%	0%	60%	37%	3%
	4.5	100%	0%	0%	53%	47%	0%	0%	60%	40%
	8.5	100%	0%	0%	40%	60%	0%	0%	0%	100%
Kalapodi	2.6	100%	0%	0%	60%	40%	0%	40%	57%	3%
	4.5	100%	0%	0%	37%	63%	0%	0%	47%	53%
	8.5	100%	0%	0%	33%	60%	7%	0%	0%	100%
Ventotene	2.6	100%	0%	0%	13%	50%	37%	0%	60%	40%
	4.5	100%	0%	0%	13%	40%	47%	0%	3%	97%
	8.5	100%	0%	0%	23%	30%	47%	0%	0%	100%

and

Table 6. Percentage of years characterized by “Minimum–Low”, “Moderate–Medium”, and “High–Maximum” risk due to relative humidity change and daily relative humidity range for each case study and emission scenario per period.

		Heritage Microclimate Risk
		1971–2000			2021–2050			2071–2100
Site	RCP	Minimum–Low	Moderate–Medium	High–Maximum	Minimum–Low	Moderate–Medium	High–Maximum	Minimum–Low	Moderate–Medium	High–Maximum
Choirokoitia	2.6	80%	20%	0%	50%	40%	10%	93%	7%	0%
	4.5	80%	20%	0%	80%	20%	0%	93%	7%	0%
	8.5	50%	40%	10%	80%	20%	0%	93%	7%	0%
Aegina	2.6	63%	27%	10%	100%	0%	0%	87%	13%	0%
	4.5	70%	20%	10%	93%	7%	0%	97%	3%	0%
	8.5	73%	17%	10%	97%	3%	0%	100%	0%	0%
Epidaurus	2.6	60%	30%	10%	93%	7%	0%	83%	17%	0%
	4.5	60%	30%	10%	93%	7%	0%	83%	17%	0%
	8.5	60%	30%	10%	93%	7%	0%	100%	0%	0%
Kalapodi	2.6	43%	43%	14%	83%	17%	0%	70%	23%	7%
	4.5	33%	40%	27%	83%	17%	0%	90%	10%	0%
	8.5	40%	35%	25%	77%	23%	0%	100%	0%	0%
Ventotene	2.6	67%	26%	7%	77%	23%	0%	77%	23%	0%
	4.5	67%	26%	7%	80%	13%	7%	70%	20%	10%
	8.5	67%	26%	7%	83%	13%	4%	67%	30%	3%

. Similarly,

Table 7. Percentage of Predicted Risk of damage for each CH and for each emission scenario per period (temperature).

Predicted Risk of Damage (PRD)
Site	RCP	1971–2000	2021–2050	2071–2100
Choirokoitia	2.6	5%	15%	17%
	4.5	5%	29%	68%
	8.5	5%	60%	87%
Aegina	2.6	5%	12%	16%
	4.5	5%	20%	60%
	8.5	5%	26%	87%
Epidaurus	2.6	5%	9%	13%
	4.5	5%	12%	48%
	8.5	5%	18%	87%
Kalapodi	2.6	5%	9%	15%
	4.5	5%	14%	51%
	8.5	5%	23%	87%
Ventotene	2.6	5%	42%	44%
	4.5	5%	47%	81%
	8.5	5%	44%	87%
Low Risk 0–20 (%)	Moderate Risk 20–40 (%)	Medium Risk 40–60 (%)	High Risk 60–80 (%)	Maximum Risk 80–100 (%)

and

Table 8. Percentage of Predicted Risk of damage for each case study and for each emission scenario per period (relative humidity).

Predicted Risk of Damage (PRD)
Site	RCP	1971–2000	2021–2050	2071–2100
Choirokoitia	2.6	7%	19%	6%
	4.5	7%	7%	6%
	8.5	19%	7%	6%
Aegina	2.6	16%	5%	7%
	4.5	15%	6%	6%
	8.5	15%	5%	5%
Choirokoitia	2.6	7%	19%	6%
	4.5	7%	7%	6%
	8.5	19%	7%	6%
Epidauros	2.6	17%	6%	8%
	4.5	17%	6%	8%
	8.5	17%	7%	5%
Kalapodi	2.6	21%	8%	14%
	4.5	32%	9%	6%
	8.5	25%	15%	5%
Ventotene	2.6	15%	9%	8%
	4.5	15%	12%	17%
	8.5	15%	8%	14%
Low Risk 0–20 (%)	Moderate Risk 20–40 (%)	Medium Risk 40–60 (%)	High Risk 60–80 (%)	Maximum Risk 80–100 (%)

display the results for the estimated PRD index. To enhance clarity, we categorized the results of the PRD index into five distinct classes. A sensitivity analysis was performed for the PRD index, examining the intermodel range uncertainty based on temperature projections discussed in Section 3.1. The estimated uncertainty for the PRD index ranges from ±0.1% to ±1.2%, indicating a relatively low level of uncertainty.

Generally, during the reference period across the five (5) CH sites, climate is characterized by “minimum–low” risk (due to temperature) in all emission scenarios over the study years. Consequently, this results in a minimum probability of damage (PRD index) as indicated in Table 7. However, analyzing the three time periods (1971–2000, 2021–2050, and 2071–2100), we observe significant fluctuations in the HMR_out index due to exacerbation of heat stress. Future projections indicate a decrease in the percentage of years characterized by “minimum–low” risk, while the percentage of years with “moderate–medium” and “high–maximum” risk are anticipated to increase in both future periods, leading to a more extreme climate. Therefore, the predicted risk of damage for the CH is expected to increase in both future periods for all CH sites. We should outline that the percentage of the increase varies depending on the scenario, period, and the specific region. For example, during the future period 2021–2050, the PRD index ranges from 9% to 44%, while during the future period of 2071–2100, it ranges from 16% to 87% (wider range). Regarding the regional differences, there are notable variations in risk across the different sites. For instance, Ventotene and Choirokoitia tend to exhibit higher percentage of years with high–maximum risk compared to the other sites under the same emission scenario, resulting in a higher PRD index. Specifically, Ventotene exhibits a large number of years with “high–maximum” risk during the second future period, with further increases expected until the end of the 21st century. During the period 2021–2050, 37%, 47%, and 47% of the years are projected to be characterized by “high–maximum” risk under scenarios RCP2.6, RCP4.5, and RCP8.5, respectively. Throughout the period 2071–2100, these percentages are expected to potentially rise to 40%, 97%, and 100%. This implies a significant predicted risk of damage to CH, ranging from 42% (period 2021–2050, RCP 2.5) to 87% (period 2071–2100, RCP 8.5).

Regarding RH, a different distribution of HMR_out index is seen. Throughout the historical period, it appears that “minimum–low” risk predominates (33–80%). However, there are also notable percentages of years characterized by “moderate–medium” risk (17–43%) and fewer years with “high–maximum” risk (0–27%). Indeed, at the Kalapodi site, the highest percentage of years with “high–maximum” risk is observed compared to the other stations for each emission scenario. Specifically, during the period 1971–2000 and based on the RCP 2.6, RCP4.5, and RCP8.5 scenarios, the percentages of years with “high–maximum” risk are 14%, 27%, and 25%, respectively, at Kalapodi. Regarding the PRD index (Table 8) during the historical period, it remains low in the sites of Aegina, Choirokoitia, Epidaurus, and Ventotene. However, there is an increase in the PRD index at the Kalapodi site due to a higher HMR_out index. In general, future projections indicate that the HMR_out will decrease due to the reduction in RH. Specifically, the years with “moderate–medium” and “high–maximum” risk are expected to decrease, while the years with “minimum–low” risk are expected to increase due to a drier climate. Consequently, the PRD index is expected to remain at low levels in all regions for both future periods.

4. Discussion

This study aims to assess the impacts of climate change on future heritage microclimate risk and to quantify the potential damage risk to construction materials of monuments at cultural sites across Europe. A set of RCM models and three different future scenarios (RCP2.6, RCP4.5, and RCP8.5) were used for future projections. To evaluate the EURO-CORDEX model’s ability to quantify the future impacts of climate change on the CH sites, a comparison with ground-based observations was performed. The results showed satisfactory performance of the models in capturing the observed climatic conditions on both an annual and seasonal basis. The discrepancies between observed and modeled values result from model uncertainties, such as analysis, parameterization, and site characteristics, like elevation and land type differences between the weather station and the model grid point.

Climate projections based on the EURO-CORDEX ensemble indicate a significant increase in temperature, with a rise of 1.5–2.0 °C in the near future and up to 5.0 °C by the end of the 21st century under the RCP8.5 scenario. Also, there was a significant rise in TASMAX, TASMIN, and hot days (HD) under all emission scenarios, highlighting a warming trend and increased heat stress across CH sites. These findings are consistent with those of Kapsomenakis et al. [19], who reported an increase in hot days, with the most pronounced changes projected for southern and inland Mediterranean heritage sites. Furthermore, the present study showed that the number of frost days (FD) is expected to decrease for all CH sites and RCPs for both future periods, resulting in a reduction in frost damage to CH. This aligns with the study by Grossi et al. [4], which assessed the potential for frost damage and showed that CH sites build from stone will suffer less damage in the future due to the decrease in frost. Precipitation reduction (PR) is expected for both future periods under RCP4.5 and RCP8.5 almost everywhere, indicating a drier climate.

The findings from the HMR_out and PRD indices reveal significant future changes in microclimate risk due to temperature increase, resulting in an increased predicted risk of damage to inorganic building materials. Specifically, the majority of years during the period 2021–2050 are projected to characterized by “moderate–medium” risk, while during the period 2071–2100, a “medium–maximum” risk is expected to prevail due to rising temperatures. The increase in the HMR_out index leads to a higher probability of damage to CH objects during future periods across all investigated locations, with percentages varying between 9% and 44% in the first future period and increasing to 16–87% in the second future period. The results of this study are consistent with those of Tringa and Tolika [20], who applied the HMR_out and PRD indices in two Greek cultural regions, revealing an increase in years characterized by “medium–maximum” risk due to the rise in mean seasonal temperature and its fluctuations. Regarding RH, future projections suggest a decrease in the HMR_out index due to reduced RH, leading to fewer years with “moderate–medium” and “high–maximum risk”, and an increase in years with “minimum–low” risk, reflecting a drier climate. Consequently, the PRD index is expected to remain at low levels across all regions for both future periods, with percentages ranging from 7% to 32% during 1971–2000, from 5 to 19% during 2021–2050, and from 5% to 17% during 2071–2100, reflecting a generally low predicted risk of damage to CH due to RH.

Overall, by tailoring the analysis to the specific climate conditions and construction materials of the monuments, the methodology can serve as a flexible tool for assessing risks across different cultural heritage locations. In addition, while the study’s methodology and results are robust, future research could improve the predictions by addressing potential limitations, such as the reliance on specific emission scenarios and the spatial resolution of the models. These adjustments could further enhance the accuracy of future climate risk assessments for CH sites.

5. Conclusions

This study employed a systematic, indicators-based approach to identify and assess climate-related risks for CH sites across Europe. A total of eight climatic parameters and six impact-related indices were selected based on their relevance to CH vulnerability and data accessibility. The HMR_out and PRD indices served as key tools in quantifying the predicted risks to monuments. HMR_out assessed the risk to CH due to changes in outdoor microclimatic conditions, while PRD incorporated material vulnerability, offering a comprehensive framework for damage risk assessment. The analysis is based on an ensemble of 11 high-resolution (~12.5 km) EURO-CORDEX RCM simulations, spanning both the historical period (1950–2005) and the future period (2006–2100). The projections are explored under three IPCC RCPs emission scenarios (RCP2.6—strong mitigation, RCP4.5—medium mitigation, and RCP8.5—no further mitigation). Our main findings are summarized below:

• Increase in Temperature: Temperature is expected to rise of 1.5–2.0 °C in the near future, and up to 5.0 °C by the end of the 21st century under the RCP8.5 scenario. TASMAX and TASMIN are expected to increase across all CH sites under all RCPs, indicating a statistically significant warming trend.
• Increase in Hot Days: A significant rise in the number of HD is expected across all emission scenarios for all CH sites.
• Decrease in Frost Days: A reduction in the number of FD is anticipated for all CH sites and RCPs in both future periods.
• Precipitation Reductions: PR is projected to decrease across all CH sites, under both RCP4.5 and RCP8.5 until the end of 21st century, signaling a shift towards a drier climate.
• Heritage Microclimate Risk: The HMR_out index indicates increasing thermal risk to CH due to rising temperatures. The period from 2021 to 2050 is expected to be characterized by “moderate–medium” risk, while the period from 2071 to 2100 is anticipated to experience a shift toward “medium–high” risk.
• Predicted Risk of Damage: PRD index indicates a growing risk of damage to inorganic materials with the most significant risks anticipated in the period 2071–2100.

The findings of this analysis highlight the expected increase in thermal stress due to a warmer and drier climate, particularly in Europe’s cultural heritage sites. Heritage Microclimate Risk (HMR_out index) due to temperature is expected to increase, varying by region and emission scenario. The importance of continuous monitoring of future climate projections and early identification of predicted risks to monuments is underlined. Heritage managers could use these indices to improve maintenance protocols by adjusting inspection frequencies based on the projected increase in risk levels during specific periods. To further support adaptive strategies in resource-limited locations, prioritizing CH sites with the highest predicted risks is necessary. The use of appropriate materials for the conservation of monuments which reduce moisture absorption, regular cleaning to remove dirt and salts, and vegetation control to avoid mechanical damage are essential in mitigating risks to cultural heritage sites [49]. Future research should explore improving risk assessment methodologies by considering additional environmental and material factors. In addition, collaboration with local stakeholders through targeted training and pilot applications would strengthen efforts to safeguard cultural heritage in the long term.