Modelling the transfer of long-term temperature changes in the atmosphere to the vadose zone of karst regions: from glacial cycles to the ongoing global warming

Informations projet

MOTKA

N° de convention de subvention: 101030314

DOI

10.3030/101030314

Projet clôturé

Date de signature de la CE 24 Février 2021

Date de début 1 Juin 2021

Date de fin 31 Mai 2023

Financé au titre de

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Coût total

€ 160 932,48

Contribution de l’UE

€ 160 932,48

Coordonné par

UNIVERSIDAD DE SALAMANCA
Spain

Periodic Reporting for period 1 - MOTKA (Modelling the transfer of long-term temperature changes in the atmosphere to the vadose zone of karst regions: from glacial cycles to the ongoing global warming)

Période du rapport: 2021-06-01 au 2023-05-31

Résumé du contexte et des objectifs généraux du projet

This project focus on the impact of temperature in the underground of karst regions. It is well known that caves have a relatively stable temperature that is dominated by temperature above the cave. Thermal conduction is the main mechanism known to transfer the heat from the surface above the cave to the underground. However, in a context of global warming little is known on how current global warming will affect underground temperature in karst regions, affecting subterranean ecology, microclimate, hydrology, cave art conservation etc. This project has developed a series of models to transfer different scenarios of temperature changes to the underground at different scales, from the past glacial cycle to future global warming scenarios.


We are living a period of global warming that is noticeable at human scale. One of the unsuspected impacts of climate change will be warming up the deep underground of karst regions. Changes in the thermal regime will impact the underground environment, affecting not just the nature but the people using it as a resource. So, knowing future scenarios could help to plan accordingly.


The general aim of MOTKA project is to investigate the impact of climate changes in the karst underground (including caves), by using thermal conduction models implemented at different scales, from glacial cycles to the current global warming, as well as exploring the consequences of those changes in one of the multiple fields affected by those underground temperature changes.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

During this project, research efforts focused in conducting these experiments:

Experiment 1: Calibration of thermal conduction models in real caves and calculation of specific thermal diffusivity values.
This experiment uses 5-year temperature data from a Spanish cave. A detailed topography of the cave and the surface above the cave was conducted and a thermal diffusivity was calculated. Thermal diffusivity values for the same site slightly change through time. This was not a surprise, since karst is a medium composed not only of rock, but has a significant porosity that can have a variable proportion of air and water. The performed 1-D thermal conduction model has an excellent agreement with observations, confirming that thermal conduction dominates the temperature variability. This calibration experiment is a proof of concept that validates the model supporting its implementation to other time periods.

Experiment 2: Karst underground temperature during a full glacial cycle
This experiment transfers the surface atmosphere temperature during a full glacial cycle to several hypothetical caves located at different depths. Underground temperatures were simulated for a series of caves 10 to 1000 m deep, and a thermal decoupling was observed in all of them. Simulated underground temperature and thermal anomalies show significant thermal attenuation and signal delay that increases with depth. It is obvious that underground temperature records differ from surface atmospheric temperature records, and that time response of the underground microclimate reacts with delay to atmospheric climate changes. This magnitude and time differences in the thermal regimes between the surface and the underground causes a thermal decoupling that impacts cave microclimate. The impact of this decoupling is more obvious in deeper caves/sites. The implication of thermal decoupling was applied to the theoretical oxygen isotope speleothem records, resulting in outstanding variability that is most notorious at transitional periods.

Experiment 3: Considering different global warming scenarios, model how the ongoing global warming will be recorded in caves at different depths until the year 2300.
This experiment simulates the temperature of caves at five different depths based on five predicted scenarios of global warming according to the AR6 of the IPCC. Simulations suggest that the impact of global warming will be negligible at depths below 250 m, independently of the scenarios. The simulations show that all caves at depths of 10 m already record increases of temperature related to global warming, whereas none of the caves located at depths of 100 m has recorded yet the onset of the global warming. Since the underground temperature will imply a delay at depth, the return to conditions similar to 1850-1900 will occur during the mid part of the XXII century for the most optimistic climate scenario at a depth of 10 m and will extend during the XXIII century and beyond for deeper caves and less optimistic scenarios. Underground thermal anomalies up to 7 centigrade degrees could be recorded in shallow caves in the more pessimistic scenario.

Results of the work performed are accessible at : https://motka-usal.es.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

State of the art clearly states that thermal conduction dominates cave temperature and underground karst temperature in most cases, although most professionals working in karst still ignore the important implications of this thermal transfer mechanism and its consequences. So, experiment 1, focus on the observation and model of thermal regime in a paradigmatic cave, to illustrate as an “educational” case used to help divulgating the implications of thermal diffusion in karst areas. In addition, we highlighted the role of the three phases of karst (rock, water and air) that help to conceal laboratory and field measurements of thermal diffusivities.

Our second experiment focus on the model of climate changes in the past during a full glacial cycle. Beyond thermal diffusivity, two main parameters control the transfer of heat: depth and period of the thermal anomaly. Thermal signals are transferred as waves and climate changes will reach a certain depth at different times depending on the period of the different waves in which the climate change is decoposed. We developed a model that for the first time quantifies the thermal amplitude and delay of the single waves being transferred. The results suggest that delay times for glacial anomalies lasted thousands of years and caused notorious thermal decoupling between the surface and the underground environments. The implications of the thermal decoupling impacted the oxygen isotope sginature of speleothems, the most popular proxy in speleothem paleoclimatology. So, thermal decoupling should not be ignored in speleothem paleoclimate interpretions.

In our third experiment, we simulated the evolution of underground temperature during the subsequent century considering five different scenarios of global warming. The transfer of global warming depends on the period of the thermal anomaly, so periods of thermal anomaly diverge among different scenarios. Since there is a delay between the signal at the surface and its record underground, the models show that all caves shallower than 10 m already record the global warming, whereas caves deeper than 100 m still do not record any thermal anomaly related to the global warming. These thermal changes will represent a major challenge to underground ecosystems, will change the geochemistry of groundwater and the conservation of cave art among other impacts. The underground thermal warming projections will affect industries such as tourist caves, wine cellars, mines, etc. These simulations might be useful to plan the future of their business, and if necessary, to start investing in research and development programs or in known methods to ameliorate the impact of global warming.

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