Investigation of Climate Change Impacts on the Building Materials of Archeological Monuments ^†

Abstract

Weather conditions affect the microclimate of architectural monuments. The alteration of microclimate conditions may create risks for monuments, accelerating their weathering process. For Greece, hosting numerous monuments, the identification of the risks that climate change possess is essential for planning mitigation actions. The main soluble salts that affect archaeological materials are halite and the system of thenardite/mirabilite. The thermodynamics of the salts’ equilibrium are affected by atmospheric conditions. We study the climatology of these conditions, adopting modeled data produced by high-resolution simulations. Possible climate change impacts are investigated, aiming at mapping monuments’ vulnerability in Greece.

1. Introduction

Greece is a country with numerous important archaeological sites, architectural monuments, and traditional buildings constructed over a period of thousands of years spanning from the Paleolithic Era to the modern day. Due to their significance, the scientific community is actively engaged in understanding the impact of climate conditions occurring over the centuries and those of the imminent climate change on monuments and archaeological sites [1,2]. Weather conditions have an impact on the preservation of monuments since they affect the condition of their building materials and control the initiation of several weathering mechanisms on their surface [3,4]. Most of the archaeological sites in Greece are in coastal regions or at a short distance from the coastline. In these sites, the main weathering mechanisms are initiated by the transportation, deposition, and crystallization of soluble salts caused by marine aerosols [5]. Therefore, an important goal is the study of the crystallization conditions and weathering mechanisms that can lead to the development of mitigation measures for the preservation of archeological buildings, taking into consideration also possible effects of Climate Change. In this work, we study on a daily basis the occurrence of relative humidity (RH) and temperature (T) conditions that affect the crystallization of salt ions, adopting daily gridded modeled climate data produced by dynamically downscaled high-resolution simulations of 5 km [6]. The Weather Research and Forecasting (WRF) regional model driven by the Global Circulation Model EC-EARTH has provided extensively validated data used in the current study, for the historical (1980–2004) and future periods (2025–2049 and 2075–2099), under Representative Concentration Pathways 4.5 and 8.5 [7,8,9]. This paper focuses on two different salt transition processes (sodium chloride and sodium sulfate) and their occurrence on an annual and seasonal basis. The interpretation of raw data is presented in figures that exhibit the annual number of events. Additional figures that exhibit the seasonal results have been included in the Supplementary section (Figures S1–S34). The first transition process concerns the crystallization of Halite (NaCl), while the second is the crystallization of Thernadite (Na₂(SO₄)) and its hydration to Mirabilite (Na₂SO₄·10H₂O). Moreover, it was examined the annual occurrences of these transitions for the 13 archaeological sites shown and described in Figure 1 the supplementary material. These results have also been included in the Supplementary section.

2. Methodology

RH is the main factor at both salt transitions, while temperature affects only the transition of Thernadite to Mirabilite. The gridded dataset adopted contains 4 values in a span of 24 h, i.e., every 6 h. For our calculations, we have obtained the minimum (RHmin), mean (RHmean), and maximum (RHmax) of RH, and similarly, the minimum (TN), mean (TM), and maximum (TX) of T at 2 m above ground level. For Halite, we sum the number of days that RHmin is lower than 75.3% and RHmax is higher than 75.3% [10]. An increase in that number of days could potentially lead to an increased risk for the porous materials on the surface of the buildings. The transitions of Thernadite to Mirabilite follow a more complicated rule since TX must be lower than 32.38 °C and TN higher than 0 °C, while RH must be between two non-constant thresholds as proven experimentally by Steiger and Asmussen [11]. From the performed analysis, we have extracted the equations describing the upper and lower thresholds by fitting a polynomial regression for the upper threshold and a linear regression for the lower threshold, as described in the following equations:

RHup = −0.0122 × TM² + 0.1218 × TM + 98.2847

(1)

RHlow = 0.73 × TM + 60.58

(2)

RHup in Equation (1) and RHlow in Equation (2) are the upper and lower thresholds for the daily values of RHmax and RHmin, respectively. Figure 2 shows the stability diagram (RH vs. T) for the sodium sulfate system as calculated by adopting Equations (1) and (2). The region between the blue and the red line satisfies the conditions for the crystallization of Mirabilite and, consequently, the potential damage initiation on the building surface of building materials.

3. Results

Each period we examine contains 25 years of daily values; thus, 9132 daily values for the historic period (1980–2004) and 9131 values have been studied for each of the two future periods (2025–2049 and 2075–2099). For our analysis, we have used R coding. In this section, we present the number of events we have computed following the methodology described in the previous section.

3.1. Crystallization of Halite

The past period (Figure 3A) yields the highest values that exceed 200 events per year in the Aegean Islands, the Evros region (Northeast of Greece), and most areas of Western and Northwestern parts of Greece. Most regions yield below 150 events per year. In the future periods (Figure 3B–E), most regions yield a lower number of events than in the past, with the lowest result in RCP8.5 and the 2075–2099 period (Figure 3E). These results imply a lower future damage risk due to halite crystallization. The exception to the above trend is the mountainous areas that yield a higher number of events in all future periods. In Figure S2A, we observe that M/A/M is the season that higher values distributed in lower altitudes and the season that yields the highest positive variances in the future, reaching 17 days in 2075–2099 and RCP8.5 (Figure S1E), mostly in mountainous regions, where we also observe the highest negative variances in values, mainly in Eastern mainland Greece. D/J/F period (Figure S1) exhibits a similar number and distribution of events to M/A/M, while S/O/N (Figure S4) yields the highest values in the Aegean Islands and exhibits positive variance values in RCP8.5 in the 2025–2049 period in Eastern Greece, and negative for the same scenario in most parts of Greece for the 2075–2099 period. The season with the lower number of events is J/J/A (Figure S3A), which exhibits zero or close to zero anomaly values in all future scenarios, apart from RCP8.5 and 2075–2099 (Figure S3E), where we observe the highest negative anomaly values (up to 12 in Northeastern Greece). From the monuments’ list under consideration, Delos (Figure S11) is the place with the highest number of events reaching approximately 300 per year while yielding a minor negative trend in the future. The monument that is affected by the fewest number of events is the Apollon Theater in Patras (Figure S10). The number of events ranges between circa 50 and 110 per year in all periods, with no significant changes between them.

3.2. Crystallization of Thernadite to Mirabilite

Crystallization of Thernadite to Mirabilite yields a lower number of events compared to the crystallization of halite, reaching the highest values (up to 75) in the central Aegean Islands and Northeastern mainland of Greece. Most parts of the country yield negative anomalies in the future, reaching 33 fewer events in the 2075–2099 period and RCP8.5 (Figure 3E), implying a decrease in future risk similar to halite crystallization. The exception to the above trend is the mountainous areas that yield zero or slightly positive anomalies in all scenarios. D/J/F is the season with the highest number of events in most parts of Greece in the past period (Figure S5). The rest of the seasons (Figures S6–S8) yield lower than 10 events per season. Also, D/J/F yields the greatest negative change in values in the coastal regions around Aegean, while on the contrary, the mountainous areas yield the greatest positive anomalies. The rest of the seasons exhibit mostly close to zero negative anomaly values in future scenarios.

All monuments yield lower than 80 events per year in the past period, and the majority show a negative trend in the future. Such an example is Panagia Kechria in Skiathos (Figure S31), where the number of events in the past period range between 30 and 80, while in the future, it ranges between 10 and 60 for the RCP4.5 scenario and between 5 and 50 for RCP8.5. Other monuments, like the Medieval City of Rhodes (Figure S29), are affected by less than 20 events per year, yielding no significant trend in the future for both scenarios.

4. Conclusions

This work attempted to quantify the daily events that satisfy the conditions necessary for the transitions of sodium chloride and sodium sulfate soluble salts. The process for the crystallization of Halite demands simpler criteria to be satisfied than the process for the crystallization of Thernadite and its hydration to Mirabilite. This is justified by the results presented, where a much higher number of events occurred for the first compared to the second process. Both processes yield negative anomaly values in the future period scenarios in most parts of the country. The exception to this is mainly the most mountainous areas of Greece, which yield zero or positive anomaly values. However, the number of events related to these processes does not describe the damage caused but the potential for damage to occur. The results discussed are related to the conditions created on the surface of the building materials, which are affected straightforwardly by the weather conditions. The prediction of possible damage mechanisms below the surface of materials requires a more thorough analysis of additional parameters, such as mineralogy, pore-space characteristics, and mechanical properties of the building materials. Moreover, Greece is covered with monuments constructed across all historic periods, and the exposure of each monument to weather conditions varies in time and intensity, as we observed in Figure 3 and Figure 4. Therefore, future work could focus on monuments located in the areas where the highest number of salt transition events are observed, in order to examine the impacts of the salt transitions in the actual materials of the specific monuments.