LAUSR

The impacts of climate change are not limited to current society and infrastructure. Cultural heritage sites, along with their associated intangible values, are also impacted by the unpredictability, extreme events and longer-term trends that come with climate change. Even under present climate conditions, the conservation of heritage sites is necessary as many sites are vulnerable to weathering and degradation from environmental processes, though it is not always clear what factors pose the most risk and how conservation can minimize the impact. Climate change adds an additional consideration to the implementation of longer-term conservation strategies, creating a moving target as risks change and develop over time. There is a “pressing need for a research-driven approach to identify effective conservation strategies under both current and potential future climates”, explains Jenny Richards from the University of Oxford, the lead author on a new study published in Scientific Reports that aims to address exactly that. In their study, the author group uses a new model developed using field data to investigate deterioration of an earthen heritage site at Suoyang Ancient City in semi-arid north-western China to understand which conservation strategies might be most effective. Suoyang was built over a thousand years ago in the Han and Tang dynasties and is part of the Silk Roads World Heritage sites, which lie along the Silk Road and show the timing and extent of the exchange of goods and ideas along this road between Asia and Europe. Despite their age, earthen heritage sites like Suoyang are vulnerable to environmental degradation through wind and rain. Deterioration can take the form of polishing (when the surface of a structure is smoothed by wind), pitting (when small holes form in the structure) and slurries (which form when rain loosens material, causing it to slide down a vertical structure and form a crust) (Fig. 1). Conservation strategies therefore often involve the creation of a windbreak to protect the structure from the elements. Conservation of the site is further complicated due to uncertainty around future climatic changes in north-western China, with projections from Coupled Model Intercomparison Project Phase 5 and regional models showing various changes in rainfall and wind speed. The Vegetation and Sediment TrAnsport model for Heritage Deterioration (ViSTA-HD) model used by the researchers for the Suoyang site recreates environmental conditions and calculates the risk of deterioration based on these conditions under a variety of climate and conservation scenarios over 100 years, much longer than can be measured in the field. Exposed parts of the site have a fairly high risk of deterioration under future wind and precipitation changes, while areas covered by dunes show a lower risk. Increases in wind speed had a marked impact on the overall risk of deterioration but changes in precipitation did not, although these changed the outward appearance of the site due to increased formation of slurry crusts. Richards notes that the finding showing the importance of wind over rain is surprising because rainfall is a known threat to earthen heritage sites, and it shows the importance of low-magnitude, high-frequency events. It is also exciting “as it suggests that conservation strategies that minimize on-site wind speeds could be effective mitigators of deterioration under a range of future climate scenarios.” The authors then investigate the impact of different conservation strategies and find that natural vegetation cover to 80% had the greatest impact in reducing deterioration under future climate change and was a more effective strategy than building windbreaks. The impact of wind speed elucidated in this study helps to inform future research directions, as changes in wind speed at the local scale are not well understood or represented in climate models. To preserve cultural heritage, and earthen sites in particular, Richards says “collaborations between climate and heritage scientists are vital. We need to work together to understand local-scale climate changes and how these will impact heritage sites.” The importance of local processes is also true for other types of heritage sites. For example, sea level rise is a known threat to coastal sites, particularly those at low elevation. In another recent study, published in Environmental Research Letters, Salma Sabour and colleagues mapped shoreline change from 1984–2016 for the boundaries of 67 Natural World Heritage Sites and found a correlation between shoreline change and sea level rise for the Wadden Sea (Environ. Res. Lett. 15, 10; 2020). Although impacts of sea level rise were not observed globally for this time period, underlining the importance of local variation and drivers, rising seas are expected to play a more dominant role in the future. Taken together, these studies highlight the potential impacts from the many facets of climate change on heritage sites that have and continue to shape the understanding of human societies.