THESAN Project: the role of a changing climate in Mediterranean cultural hypogea

Natural or man-made hypogea (such as tombs, catacombs and crypts) are underground environments with a stable microclimate (temperatures rarely above 20°C and a consistently high relative humidity, often close to water vapour saturation). These features result from the combined effect of the high heat capacity of the structure and limited air exchange between the indoors and outdoors. Despite the stable microclimate conditions, the monitoring in such environments requires careful consideration of instrument choice and deployment due to prolonged near-saturation conditions. The THESAN project has been funded by Sapienza University of Rome to investigate the impact of a changing microclimate and microbial dynamics on the preservation of tombs in the Etruscan necropolis of Tarquinia, a UNESCO World Heritage Site in the Mediterranean basin. This contribution describes the microclimate investigation undertaken in selected tombs, where an environmental monitoring has been implemented by the authors since 2024 and it is still ongoing. Particular attention was paid to selecting a sensor to measure relative humidity, which is challenging in such environments. Additionally, the contribution investigates the potential impact of a changing climate on related indoor microclimate conditions. Heat and moisture transfer functions were applied to indoor and outdoor microclimate observations collected in 2024-2026 in order to model indoor temperature and specific humidity based on past and future outdoor conditions. In this way, the evolution of the indoor microclimate from 1940 to 2100 under the Shared Socioeconomic Pathway (SSP) 2-4.5 scenario was analysed. Both microclimate variables increase slowly but steadily in line with the outdoor variables, with their intensity differing only according to the depth of the structures. This is a significant factor when assessing the risk to the preservation of these unique sites, as most degradation, including the spread of microorganisms colonising mural paintings, is induced by microclimate conditions. The investigation enabled the evaluation of the impact of outdoor climate and informs possible future pathways for the management of these sites in a constructive dialogue with microclimatologists, microbiologists, restorers, and the site manager.