Paintings etched by our distant ancestors onto walls deep within the Altamira caves in Northern Spain provide an invaluable glimpse into the earliest throws of human culture – and, as a result, the ongoing preservation of these ancient artworks is of crucial importance to those tasked with conserving the prehistoric site.

Now research published in Scientific Reports has drawn on data from ESA’s SMOS mission and other Earth observing satellites to better understand changes to the microclimate of the Altamira caves and how this could affect the artefacts they contain in the next decades.

This study represents another unexpected and novel application for SMOS, which has continued to demonstrate its versality during its 15 years of operation – a milestone that will be celebrated next month at a science conference organised by ESA.

Discovered in 1879, the Altamira paintings include vibrant, colourful illustrations and engravings of bison, deer and horses, dating as far back as 14,000 years.

Located deep within the labyrinth of the Altamira cave system, these artforms remained undisturbed for millennia, with the low light levels and constant humidity and temperature of the caves helping to prevent the paintings from deteriorating.

This stable microclimate is key to the preservation of the paintings, but it is under threat from atmospheric changes caused by visitors, human-made modifications to the caves, and the long-term environmental changes impacting the planet.

Cave art is known to be particularly sensitive to variations in carbon dioxide levels and temperature that initiate corrosion and reprecipitation of rock, as well as causing imbalances to air exchange patterns that have the potential to introduce microorganisms and nutrients linked to the deterioration of pigments.

As part of the study, researchers developed a modelling approach to predict annual fluctuations of carbon dioxide levels within the caves, representing the first time such a methodology was employed for this application.

Unexpectedly, the model showed that two factors external to the cave environment could be used to predict carbon dioxide levels within it; these drivers were the temperature of the outside atmosphere and soil water content of the land above the cave.

The model was then adapted so that it could incorporate satellite observations of the two predictors, before being tested with these data as inputs.

Soil moisture data were provided by ESA’s SMOS mission, which hosts one instrument – named the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) – that collects global information on soil moisture over land and salinity over oceans. Land surface temperature observations were provided by NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) satellite. Data used in the study were acquired over Altamira, covering from 2010 until the present day.

By feeding these data into the model, it was demonstrated that it could successfully estimate the dynamics of cave carbon dioxide levels, opening up the possibility of using satellite data to monitor the atmospheric conditions of cave systems in near real-time.

If it were made available, this information could help teams responsible for the site to manage the conditions within the caves and ensure the micro-climate remains constant, minimising potential deterioration of the historic artworks they contain.

By employing the equations underpinning the model, the researchers found it was also possible to estimate cave carbon dioxide concentrations in the past according to different external factors, such as numbers of visitors.

They used visitor logs from the 1950s to the present day to estimate carbon dioxide concentrations and found that an influx of visitors in the mid-1970s was likely to have fostered the introduction of microorganisms known to pose a threat to the cave paintings. After looking at past conditions, the research team employed the same mathematical model to investigate potential future changes in response to long-term shifts in the climate.

Products from the Intergovernmental Panel on Climate Change, which represented different scenarios for how climate change could affect soil temperature and land surface temperature, were used to investigate cave carbon dioxide levels up until 2100, showing that future climate changes could pose a risk to the cave paintings during this century.

Soledad Cuezva, researcher at the National Museum of Natural Sciences of the Spanish National Research Council (MNCN-CSIC) and study co-author, commented, “The study highlights significant future challenges, particularly in the context of climate change, which would lead to increased carbon dioxide concentrations, exacerbating the risks of corrosion, microbial proliferation and deterioration of artistic representations.”

In conclusion, Sylvain Mangiarotti, researcher at the Center for the Study of the Biosphere from Space (UMR CESBIO) and co-author, added, “Although the equations obtained are specific to Altamira and focus on carbon dioxide, the approach is general and opens up new perspectives for other caves whose paintings are also at risk.”

References:

Sáez, M., Benavente, D., Cuezva, S. et al. Scenarios for the Altamira cave CO2 concentration from 1950 to 2100. Sci Rep 14, 10359 (2024). https://doi.org/10.1038/s41598- 024-60149-9